How To : Install A Big Bore Kit On A 1E40QMB

[90GTVert]

This particular installation was performed on a 2009 Eton Beamer III. Some details will vary from one scooter to the next. This installation was done with the engine in the scooter, but scooters with smaller frames and/or shorter wheelbases may require the engine to be removed. Other scooter models may require more or different body panels to be removed to access certain parts. If you need to remove the engine from the scooter, take a look at THIS BASIC GUIDE. This installation was done keeping the stock oil injection system.

I was installing a Polini Sport (also called Contesta) 47mm big bore kit during this guide.



I have broken up the how-to into sections to make navigation faster. Simply scroll your way through, or choose an item from this list to skip to a section.

Section 1 : Kit Contents, Supplies, And Tools
Section 2 : Prep Work And Checks
Section 3 : Installation
Section 4 : Getting It Going And Finishing Up

[90GTVert]

Section 1 : Kit Contents, Supplies, And Tools

Before we go any farther, let's get to know the parts a typical big bore kit includes and some of the other things you may want to have around.

Cylinder. What a big bore kit is all about. A larger diameter cylinder to accommodate a larger diameter piston and create more displacement. The port locations and sizes determine how your engine will behave.



Piston. A big piston to go with a big bore. Most pistons are marked with an arrow that points toward the exhaust port. If you get a piston that is not marked with an arrow, pay attention to the location of the ports on one side of the piston. They will face up, toward the boost port in the cylinder.



Cylinder Head. This is where combustion occurs. Gases are pushed upwards and compressed by the piston. They get squeezed into the center section by the squish band around the outer edge of the combustion chamber so the spark plug can ignite them. Squish band and combustion chamber characteristics help to determine engine compression and other performance details.



Wrist Pin / Piston Pin / Gudgeon Pin. The wrist pin rides inside of a needle bearing inside one end of the connecting rod. The piston rides on the wrist pin. Minarelli/Jog/1E40QMB engines can come equipped with either a 10mm or 12mm diameter wrist pin. You must know which one your engine has before purchasing a big bore kit. Some scooter ears and models have known sizes that can be looked up on many retailer's websites. Others may require engine disassembly and measurement to determine wrist pin size. In mopst cases, you can look at the splines on the crankshaft, where the variator rides to get a good guess on what size your engine has. The splines on the crankshaft usually measure roughly 13mm or 16mm in diameter. 13mm cranks should have 17 splines and 16mm cranks should have 15 splines. 13mm versions typically have 10mm wrist pins and 16mm versions typically have 12mm wrist pins. I have heard that certain models may use 16mm splines but have 10mm wrist pins, so use this method at your own risk. MORE INFO HERE.



Circlips. Circlips are used to hold the wrist pin in place. These can come in a few different styles. I find that the style shown below with two "ears" to squeeze seem to the the least reliable. The type that looks more like a G or the ones with a ring and no center piece or "ear" that are more of a C shape have been the most reliable for me. Generally, it seems to me that the easier they are to install; the less I can trust them. I still use the G and C style, but normally replace the double-ear type after bad experiences with them on a few occasions. If a circlip comes out, it commonly damages the cylinder, piston, and cylinder head.



Piston Rings. These come in a few variations, but most are pretty similar. Generally piston rings for two strokes are made to close around a locator pin in the ring lands of the piston. Looking the piston rings and the piston over carefully should reveal any additional information needed. Some kits will include a single piston ring and many others include dual piston rings. Make sure you have a piston ring for each ring land, or groove, in your piston. Some also include a thin wavy expander ring. CLICK HERE for more information about those.



Base Gasket. Base gaskets are usually made of aluminum or a paper/poly gasket material. They fit between the cylinder's base and the engine cases. Some big bore kits may include multiple base gaskets. Varying thicknesses of base gaskets can be used to alter a cylinder's port position, timing, and duration as well as squish clearance and compression ratio. At times I use a very thin coating of Ultra Grey RTV or ThreeBond to coat both sides of base gaskets and assure a proper seal. Sealant is generally not necessary for the base gasket, but (when applied properly - very thin) it won't hurt anything. The negative side effect of applying a sealant here is the added cleanup upon disassembly. Do not apply a sealant to any surface if you are doing a mock-up to check clearance or other factors.



Head Gasket. These may be an o-ring style as shown in the first picture, or a flat gasket as shown in the second picture. O-rings are generally made of rubber or a poly material and fit into heads that have a receiver groove specifically for o-rings. Flat gaskets are usually constructed from copper or aluminum and can be used for most cylinder heads. When using an o-ring style head and gasket I like to coat the head with copper spray prior to installing the o-ring. This will help to keep the o-ring in it's groove while you install the cylinder head. When using flat gaskets, I often coat both sides of the gasket with copper spray to assure a good seal. If higher compression and/or tighter squish clearance is desired, a couple of coats of copper spray can often be used in place of any head gasket.







Exhaust Gasket. These usually come in one of two forms. A flat paper or other gasket material as shown in the first image, or a round ring as shown in the second image. Which gasket works best for you may depend on the exhaust you will be installing. Many stock pipes can be easier to seal using the ring style exhaust gaskets. They often have grooves machined into them specifically for this style of gasket. Some pipes, specifically flat flanges with no receiver groove for a ring gasket and no pipe protrusion through the flange, are easier to seal with flat cutout gaskets. I suggest using a thin coating of high temperature Ultra Copper RTV silicone on each side of flat gaskets to assure a good seal. In some cases you can use only copper high temp RTV in lieu of a gasket, but having something to compress a bit is generally desirable. NCY 0500-1008 metal flat gaskets are sometimes used for Minarelli engines. They are made for the GY6, but their stud spacing is compatible with typical cylinder setups and their metal construction makes them more durable. I often coat them with Ultra Copper RTV and reuse them, while many fiber/paper gaskets will fall apart or be burnt after the first use.







Exhaust Bolts or Studs. Studs thread into the cylinder's exhaust flange to allow the exhaust to be bolted up using a nut on each one. They are almost always 6mm x 1.0 studs and almost never include nuts or washers. I would recommend picking up four 6mm x 1.0 nuts or lock nuts and two 6mm washers. You can use two nuts on each stud, assuming the studs are long enough, to help lock the exhaust into place. You may also choose to use the stock or replacement exhaust bolts. The stock bolts usually work with aftermarket cylinders. If you decide to use new bolts, you will probably be looking for 6mm x 1.0 thread diameter and pitch bolts that are 20-25mm long. I suggest using a medium strength thread locking compound on any exhaust nuts, bolts, or studs. High temperatures and vibration causes exhaust pipes to rattle loose in many cases if precautions aren't taken.





Other Parts

Everything you need to do the job right isn't included with most kits. Here are some other parts that may be necessary.

Wrist Pin Bearing / Needle Bearing. Some kits include them and some do not. I strongly recommend using a new wrist pin bearing when you install your big bore kit. They aren't that expensive, and they could save your engine. I often use a Wiseco B1043 bearing for my 12mm pinned 70cc builds and I have used a B1042 for 90-100cc Minarelli builds. You can find full specs on those bearings to compare to what you need.
 


Main Jet. Your new 70cc big bore kit will probably need a 70-95 main jet in most cases. Aftermarket air filters, different elevations and climates, and other factors will make the necessary jet size vary. Shown here is a Mikuni VM11/22 main jet, which is a direct replacement for most carbs used on 1E40QMB engines. I suggest buying a main jet kit so you have a selection to tune with, such as part #114-50 from Parts For Scooters.



Spark Plug. Most 1E40QMB/Minarelli/Jog engines will use an NGK BR8HS or it's iridium equivalent the BR8HIX, though some more race oriented kits may require colder plugs such as an NGK BR9HS. Your cylinder kit instructions may suggest a different plug. If so, go with the cylinder manufacturer's recommendation. I suggest having a few of the standard plugs handy for tuning. If not used you'll have spares. Don't install the more expensive iridium until you are finished tuning.



Chemicals

Here are some chemicals, not mentioned above, that you should have handy when installing a big bore kit.

Two Stroke Oil. You can use your favorite two stroke oil as an assembly lubricant. Of course you also need to have this oil around to pre-mix and/or fill your oil tank.



Anti-Seize Compound. Use a touch of this on the threads of spark plugs to keep them from seizing in the threads of your cylinder head and make removal easier.



Carburetor Cleaner or Brake Parts Cleaner. These chemicals are good for cleaning certain parts before they are installed. I prefer brake parts cleaner, but I didn't have any around when I took this picture.



Tools

Ratchets, Sockets, Extensions. What can't you take apart with a good ratchet and socket set? In addition to a metric socket set, you'll need a 13/16" socket or a 13/16" / 20.6mm spark plug socket.



Hex / Allen Keys. Some scooters use metric hex/allen headed fasteners to hold on the airbox or other components.



Screwdrivers. You won't get far without a few screwdrivers. Medium sized phillips and flat head screwdrivers are a must. A small flat head screwdriver is good for carburetor adjustments and can be used to push out the piston pin. A large, #3 I think, is very handy for a lot of screws found on Chinese scooters. An impact driver isn't bad to have around either.





Wrenches. Sometimes you just can't reach a bolt with a socket. 8 and/or 10mm wrenches are often necessary to adjust cables as well. Having an adjustable wrench may be necessary if you don't have 2 wrenches of a certain size to adjust cables.



Rubber Mallet. Very useful for tapping the cylinder gently to free up the base gasket seal.



Pliers. Needle nose pliers are great for installing circlips.



Pick. A pick like this is good for popping out circlips.



Piston Ring Pliers. These aren't essential, but they make installing piston rings faster and easier.



Spark Plug Gap Gauge.



Feeler Gauges. Used for setting piston ring end gap.



Torque Wrench. Useful for many nuts and bolts, but it's especially important to tighten cylinder nuts evenly and to the proper specifications. Ideally, you should use a torque wrench where the values that you need fall in the center of it's capabilities. For example, it's not a best case scenario to use a torque wrench that is capable of 10 to 100 ft-lbs to torque fasteners to 10ft-lbs. I usually use a torque wrench with in-lb designations for low torque items like cylinder head nuts. Multiply desired torque in ft-lbs by 12 to get the value in in-lbs. Use a quality torque wrench. Bargain bin torque wrenches have been known to be way off. I personally would prefer guesstimating torque than using a $10 torque wrench after some past experience. Even quality torque wrenches need to be handled with great care at all times and calibrated from time to time.



Precision Files. These are perfect for chamfering your new cylinder's ports. Emery paper could also be used to knock off the worst of the sharp edges. Rotary tools can be used where they can reach, but must be used with great care.

[90GTVert]

Section 2 : Prep Work And Checks

You've got everything you need, now it's time to get your hands dirty. Don't crawl under that scoot just yet. I like to start off with a little prep work. This could actually be done well in advance if you so desire.

Some big bore cylinders are produced with rough port edges. Your piston rings have to pass over those rough edges, and the last thing you need is an opportunity for your piston ring to catch a rough edge. Often these rough edges will be worn down during the break-in process, but why take any risk of a piston ring getting snagged on something and being damaged or broken? Run your fingers up and down the cylinder's bore. If a port catches your finger or feels sharp, it may benefit from being chamfered. Here is a pic of a stock Polini Sport kit's exhaust port with rough edges.



Chamfering is simply beveling and smoothing the edges of a port. This can be done with small precision files or even emery paper. Some use rotary tools and burrs or stones, but you must be careful not to remove too much material or damage the cylinder walls.



You don't want to change the port's dimensions, just smooth the transition from the port to the cylinder wall. When you run your finger over the port it should feel relatively smooth and not try to catch your finger. Here's the same exhaust port shown above, after being chamfered quickly with small files. Exhaust port chamfering can be more aggressive than this to greater reduce the risk of snagging a piston ring and perhaps alter port flow characteristics, but generally you don't want the chamfers to go beyond about 1.5mm tall/wide and 0.5mm deep into the port. With transfer ports, the goal is to remove the sharp edges but not create bevels which could change how flow enters the cylinder.



Once you've chamfered all of the port edges to your satisfaction, you can move on to setting the piston ring end gaps. Piston ring end gap is the distance between the two ends of a piston ring while in the cylinder's bore. Improper settings can lead to poor sealing or excessive wear.

Piston ring end gap should be 0.10-0.12mm per inch of bore according to A. G. Bell's book, Two-Stroke Performance Tuning. Since most scooter big bore kits are measured in mm, we first need to determine the bore size in inches. This is done by dividing the bore diameter by 25.4.

47(mm) / 25.4 = 1.85039" Bore Diameter

i usually multiply the median number from Bell's equation (0.11), by the bore diameter.

1.85039 x 0.11 = 0.20354mm End Gap

If you wish to use SAE feeler gauges, you can convert the end gap figure in mm by 25.4 to get the end gap in inches.

0.20354 / 25.4 = 0.008" End Gap

There is a piston ring end gap calculator HERE to make it even easier for you.

Begin by placing a piston ring into the cylinder as shown.



Use the piston to make sure the ring is level inside the bore to get an accurate measurement.



Place a feeler gauge in the end gap. If the gap is smaller than the desired gap you calculated, remove the ring from the cylinder and lightly file the ends. Try to retain the original angle and only file in one direction from inside to outside. Carefully remove any burrs before checking the gap again. Re-install and re-check the end gap until it is correct. If the gap is too large you can either try to use a new ring or if it's very close, don't worry about it.



Repeat that process on the second ring, if your kit has 2 rings.

Once the rings are gapped, they can be installed on the new piston. Take a look at your piston. Most two strokes have a locator pin in each of the ring lands. These locator rings keep the end gaps in the appropriate spots. Take note of how the pins are located. Sometimes that are on the top of the ring land, or bottom, or middle. The piston ring may be manufactured to fit a certain way around these pins, so pay very close attention to detail to be sure your rings are being installed correctly.



You can use a pair of piston ring pliers to slightly open a piston ring and put it into position, or you can work the ring into the ring land using gentle pressure with your fingers.



Make sure the end gap is loated around the locator pin as shown.



Repeat this process with the second ring, if so equipped.



You're almost done with the prep work, but there's one more thing that can be taken care of early. You can install one of the circlips into the piston, but only one at this point. If one side of the scooter is going to be harder to work on, install the circlip that will be on that side. You can tell which side the circlip will be on by noting the location of the arrow on the piston's crown which should always point toward the exhaust port (down) or by where the ports are in the piston (if equipped) as they will face the inlet (up). Use a pair of needle nose pliers to squeeze the circlip and place it into it's groove inside the piston. You may need to take a small blunt object and apply light pressure to help push the circlip into position. This can be a tricky process at first, and frustrating. Take your time and be very careful not to lose the circlip.



Always install the circlip so the gap is facing the top or bottom of the piston as shown. This is because the force of the piston moving up and down is said to expand and collapse the circlips if the gap is facing the sides of the piston where it can be compressed.



Additional Checks

There are some additional checks that can smooth the installation process and potentially increase performance. Some can be performed without the piston rings installed, so you may want to hold off on the installation of the rings shown above until you have completed all of the checks that you wish to do.

One problem that some encounter on Minarelli clone big bore kit installations is the oil injection pump interfering with the cylinder's fit. Some cylinders have cooling fins or other features that will make contact with the oil pump, which will prevent the cylinder from seating against the engine cases. The images below show one member's experience with this problem.

 

Checking for this is simple. Slide the cylinder over the engine studs, making sure the exhaust port is facing downward. This is easiest with the piston not installed on the crankshaft. Make sure the cylinder can slide all the way down against the engine cases. This should not require any forcing. If it can't look around to figure out what is preventing a proper fit. If you find that it is interference with the oil pump, parts may be able to be notched to create clearance. Be certain of your actions before cutting anything. Make sure you won't be cutting into any ports or passages and remove as little as possible to gain the clearance needed. If you are uncertain of the right path to take, ask on the forum, preferably including pics of your problem in the post. The image below shows a simple solution to a pump interference issue by cutting a way a small section of the lowest cooling fin.



Another possibility is that the cylinder skirts are larger than the opening in the engine cases. This is not normally an issue with stock to 48mm bore cylinder kits, and most commonly occurs when someone tries to fit a 50mm or larger kit intended for the 90cc variant of the Minarelli clone onto a 49cc version. 90cc+ parts should not be installed onto an otherwise stock 49cc engine anyway, so there is no simple solution for that. If you have a 48mm or under bore and it still won't fit, you would need to have either the cases bored to fit the cylinder or the cylinder machined down to fit the cases.

Another simple check is to see how the ports align with the piston while at BDC. BDC stands for Bottom Dead Center, which is the lowest point in the piston's travel within the bore or the bottom of the stroke. You will need the piston on the crankshaft for this check, but you don't need the piston rings installed and you don't even need to install circlips to hold the piston in place yet. Once the piston is installed, put the base gasket that you intend to use in place over the studs and all the way up against the engine cases, then slide the cylinder in place as well. Do not use any sealant at this point, since this is just a mock up. For maximum accuracy, you should install spacers or washers over the cylinder studs and torque the nuts on, but you can do a quick check just by pushing down a little on the cylinder a little while you examine it.

Move the piston to the bottom of it's stroke by rotating the crankshaft until you find the point where the piston is at it's lowest in the bore. Put pressure on the cylinder if you haven't already, and look at how the top edges of the piston align with the bottoms of the transfer ports. What you're hoping to see is that the edges of the piston crown are level with the bottoms of the transfer ports. This will allow incoming mixture to better cool the piston and prevent a step in the transition that could cause the incoming mixture to tumble and change performance characteristics.



If the ports are too high above the piston, you can try using a thinner base gasket. Some companies supply a few gaskets of differing thicknesses to make this a simple task. Most do not, so you would need to buy one or make your own gasket from a thinner material. More info on gasket making can be found HERE and auto parts stores as well as many online retailers carry gasket making material. You could even use no gasket at all between the cylinder and cases if that gives you the result you need, but you will need to use a sealant of some type when you are ready to do the actual installation. I normally use ThreeBond 1194 or Permatex Ultra Grey, but there are many products suitable for the task on the market.

If the port floors sit below the piston's crown, you will need to use a thicker base gasket or add additional base gaskets to get the alignment that you're after.

You may also want to check squish clearance with a mock up, especially if you have lowered the cylinder with a thinner base gasket or none at all to change the port positions. Squish clearance is the distance between the piston crown and the cylinder head. This is generally smallest at the squish band if equipped. The image below points out the squish band. Some heads, usually stock, don't have a squish band but you should still check for clearance. If your new cylinder kit didn't come with a cylinder head and you are attempting to use a stock head with a smaller bore, then the flat mating surface could protrude into the bore and cause clearance issues.



Check for squish specifications from the cylinder kit manufacturer first. If you can't find a recommendation from the manufacturer, most consider 0.7mm-0.8mm to be a safe minimum clearance for a street engine. Insufficient clearance between the piston and head can lead to contact, which could cause a catastrophic failure. Keeping clearances tight helps to squeeze more of the mixture toward the center of the combustion chamber and decreases the likelihood of detonation. Squish clearance also affects the compression ratio, but if you are going to worry about the compression ratio you should work toward a proper squish clearance first and alter the combustion chamber to achieve a desired compression figure.

You will need to assemble the engine using the head and base gaskets that you intend to use for the final assembly to check the squish clearance. Again being a mock up, no sealants should be used on gaskets or mating surfaces. You can either tape a piece or pieces of solder across the piston crown or bend a piece of solder and insert it through the spark plug hole so that it is across the squish band area and touching the cylinder wall. Solder will need to be at least 1mm thick. You can also double over solder and twist it together to create something thicker to test with if 1mm isn't enough to find your squish clearance. I prefer to check the 12, 3, 6, and 9 o'clock positions on the piston to get a good idea of the minimum clearance, in case it varies due to surface irregularities or angles. Rotate the engine all the way around a couple of times. You may need to use a ratchet and socket on the variator or flywheel if it's too difficult to turn by hand as it tries to reach the top of the bore and squish the solder. If it's difficult with a ratchet, then don't force it. You may have no clearance between the head and the piston and forcing would only risk damaging parts. Check the thickness of the solder at it's thinnest points to see what the squish clearance is.

Using a different head gasket, stacking head gaskets, or removing the head gasket totally are the easiest options for altering squish. If the head gasket needs to be removed to achieve the squish clearance you're after, make sure you use a sealant like copper spray during final assembly. Machining the cylinder or head (usually the head) can also be done to change squish clearance. As long as you meet the minimum squish clearance, you may wish to consider compression before making any changes if you don't plan to work with the combustion chamber. It would be better to have a little more clearance than to raise compression to a level that could cause problems with detonation or overheating. Work toward a balance of squish and compression, again maintaining at least the minimum squish clearance, or plan to modify elsewhere as needed.
 
If you wish to work on the compression ratio, the best way to check is by taking a lot of measurements and doing some math. That method is detailed in THIS LINK. You can also get an idea of compression levels by checking cranking compression with a compression tester. A compression tester can't really tell you the exact compression ratio, it can only give you an idea of what's going on. 150-180psi has commonly been a good range for strong running street engines based on my own experience, but be aware that not all compression testers will read alike. Sealing issues can also cause discrepancies in readings. The not so easy way of measuring is the best way to do it if you really want to know what you have.

You could also check port duration and sizes during the mock up and pre-assembly phase, but I would advise most novice tuners to leave this alone during their first experience with a cylinder kit installation. You need to know about your setup and understand what changes are needed to improve it and properly execute those changes in order to get a positive result. A botched porting job can totally ruin performance. If you are interested in the topic though, HERE is a little more info and don't forget that you can ask for help on the forums. As with any of the process, it's better to ask than to guess and risk messing something up.

[90GTVert]

Section 3 : Installation

Everything is ready to go. Let's get to work.

Start by removing the two bolts that secure the header to the cylinder. These usually require a 10mm socket or wrench.





Remove the bolts holding the exhaust bracket and remove the pipe. These bolts are usually 12-14mm hex.



Remove the two screws holding this little access panel in place and then get it out of your way for a better view.





Remove the three bolts holding the cooling fan shroud. These are usually 8 or 10mm hex or phillips head as shown.



Rotate the shroud toward the ground and free this clip holding it to the other side of the shroud. Carefully remove the shroud. There are oil lines and cables running into the area behind the shroud, so take your time and make sure you don't pull anything loose.





You should see an adjuster on the cable that runs to the oil pump. I believe the majority of Chinese 1E40QMB engines have this adjuster, but I have heard that some genuine Yamaha scooters do not.



Pull back the covers to reveal the cable nuts.



I like to use a marker to mark one of side of the large cable nut as shown. This helps me keep track of how far I've turned the cable.



Loosen the cable nuts, in this case requiring two 8mm wrenches or adjustable wrenches, and turn the large nut 3-4 turns out. You should notice the tension being taken up on the oil pump cable. This adjustment is made to get the stock oil injection system to deliver more oil for your larger cylinder.



Tighten the nuts against each other to assure the cable stays in the same position and slide the covers back over the adjuster.



Move to the other side of the scooter and remove the two screws securing this access panel and remove it.



Remove the two bolts that secure the airbox to the CVT cover. These are commonly 5mm allen, phillips headed screws, or 8-10mm hex.



Open the seat and remove the 4 bolts shown here so you can lift the seat and storage bucket out of the scooter for better access.



Reach down and squeeze the handles of this clamp together and pull the airbox off of the carburetor.



Remove the spark plug wire.



Use a flat screwdriver to gently pry the cylinder shroud's tabs from their receptacles.



Move to the other side again and gently pop the oil hose out of it's groove in the cylinder shroud. Then the shround can be maneuvered off of the engine.



Remove the four cylinder nuts shown here. These are usually 10mm or 12mm hex nuts.



Slide the cylinder head off of the studs, being careful not to snag the oil hose that runs near it.



Remove the old head gasket.



Remove the cylinder by sliding it off of the cylinder studs. Sometimes the cylinder won't budge and needs a few light taps near the base with a small rubber mallet to free it up.



If you're lucky, the base gasket will come off with the cylinder and leave you a clean surface. If not, remove the base gasket from the cases. If the base gasket is stuck, pry it off gently. Be sure not to scratch or otherwise damage the cases.



Now you need to remove the circlip from one side of the piston.



You can try to use a pair of needle nose pliers to squeeze the clip and pull it out as shown in the first image, or use a small pick to pop the clip out gently as shown in the second image. It's a good idea to stuff a rag into the open crankcases to prevent the clip from finding it's way in there.









Move to the other side of the scooter and use a small screwdriver or other object to push the piston pin out of the piston.



You may not be able to push it all the way out easily. If not, move to the other side and pull the pin the rest of the way out by hand or with pliers.



Keep your hand on or under the piston as you remove the pin, because it should fall off of the crank. These cast pistons will often break if you let them hit the floor.



Remove the wrist pin bearing from the end of the connecting rod.



Now that the old top end is removed, it's a good time to place it next to your new parts and laugh at how small the stock bore was.



Alright, break's over. Crawl back under the scooter and clean any leftover base gasket material off of the cases where they meet the cylinder. Use brake parts cleaner on a lint free rag to assure there is no oil or residue left over.



Dip a new wrist pin bearing in your favorite 2T oil and then slide it into the connecting rod.





Coat the wrist pin with 2T oil as well, and slide it partially into the new piston.





Line the wrist pin up to go through the wrist pin bearing, and push the pin in gently until it rests against the circlip you installed earlier while doing prep work.



Make sure the arrow on the crown of the piston is facing down, toward the exhaust port. If the piston is not marked, the ports in the piston should be facing up, toward the boost port.



Make sure your crankcase's opening is filled with a rag and install the circlip into it's groove in the piston. It's a bit trickier when you're under the scooter, that's why I had you install the first one while it was out of the scooter. Just be patient and don't lose the circlip.



With the piston secured, you can slide the base gasket over the studs and into position against the cases. It may not be a bad idea to apply a very thin layer of sealant to the base gasket as I mentioned early in this article in the parts section.



Apply plenty of 2T oil to the inside of your new cylinder. Be careful not to get any on the base though, because it could prevent a proper seal.



Make sure your piston rings are positioned properly around their locator pins. Slide the cylinder into place, being sure the exhaust port is facing the ground. Be very gently and make sure you are moving the cylinder evenly. You will need to use your fingers to press the piston rings into their ring lands in the piston. Don't force anything here. If a lot of force is required, your piston rings are not aligned properly or something is wrong. Take another look and go slowly and gently.



Use a rag with some brake parts cleaner to remove any oil from the cylinder's deck, where it will mate with the head.



Insert the o-ring into the head's reciever groove or install the head gasket over the cylinder studs, depending which your kit is equipped with. As I mentioned in the parts section, it may be a good idea to put a thin coating of copper spray in the o-ring groove and mating surface of the cylinder head to assure the o-ring stays in place as you install it.



Slide the cylinder head over the studs and onto the cylinder. You should notice that one corner of the head is kind of cut away. This should match up to a similarly shaped corner of the cylinder.



Install the nuts on the cylinder studs finger tight. Then use a torque wrench to tighten them in a criss-cross pattern as indicated by the numbers in the first picture. Which nut you start on isn't actually important, as long as you use a criss-cross pattern. Torque specs are usually 10-15ft-lbs, depending on the source. I used to use 15ft-lbs regularly, but have seen threads in the cases snap before then so it may be best to refer to a service manual specifically for your model or use 10-12ft-lbs for generic Minarelli clones. I like to do this in increments. For example, begin at 8ft-lbs, then 10ft-lbs, and finally 12ft-lbs.





Many 1E40QMB/Minarelli/Jog engines will use an NGK BR8HS or it's iridium equivalent the BR8HIX.



Unless you have installed a different spark plug boot, you will need to remove the tip from the spark plug shown here. It should come off easily with pliers.



Use a gapping tool to set the spark plug gap to around .028". You cannot use a standard gapper if you are using an iridium plug. Many people suggest not gapping those. If you do wish to check and adjust the gap, you must use a tool that pulls the ground electrode rather than wedges in to widen the gap. The small iridum tip can be damaged with a standard gapper.



Apply a couple of dabs of an anti-seize compound to the spark plug's threads and install it into the cylinder head. It should only be tightened a couple of turns beyond finger tight until it is snug.





Slide the cylinder shroud back over the cylinder, being careful not to pinch or damage any wires or hoses nearby.



Insert the oil hose into it's groove in the cylinder shroud.



Begin installing the cooling fan shroud by connecting the tabs under the engine, being sure that the oil pump's cable is routed properly through the holes in the shroud.



Position the cooling fan shroud to line up with the cylinder shroud. Make sure you haven't pinched any hoses and install the 3 bolts to secure the shroud.



Push the spark plug boot onto the spark plug.



Pop the old exhaust gasket out of the header, and press in a fresh gasket.





Start one bolt in the exhaust bracket. Only thread it in enough to safely hold the weight of the exhaust while you line up the other connections.



Line up, and start both header bolts. Tighten them just beyond finger tight.



Start the second bolt in the exhaust bracket. Then tighten the header bolts and both bracket bolts.



In some cases, the exhaust won't align properly with the mounting holes when using a new cylinder. If the exhaust fit well beforehand, the cause is most likely that the exhaust flange on the new cylinder is clocked different than the original cylinder's flange. This usually causes the pipe to angle either outward away from the scooter or inward toward the wheel and tire. Some try to force the exhaust into position and then secure it, but that's a bad idea. The exhaust needs to be supported by the header and mount bolts, but should not be under any sort of stress or pressure as that will very likely result in future issues such as cracks forming in the exhaust or issues with the mounting points and fasteners.

If the exhaust needs to rotate in or out, you may be able to remedy the issue by slotting the bolt holes in the header flange so that the header can rotate in the direction that you need to move it toward or away from the wheel.



If there is still a gap between the mounts and the bracket, spacers or washers can be used to prevent putting tension on the exhaust.

Minor misalignment of the exhaust bracket and exhaust mounting holes on the engine case may be solved by enlarging the holes in the bracket. Large mismatches may require bracket modification or fabricating a new bracket or adapter.

[90GTVert]

Section 4 : Getting It Going And Finishing Up

Initial Tuning

Here I show a typical stock carburetor. If you have a different style or aftermarket carburetor, you should still do the jet swap as mentioned below, but the steps may be a bit different. You can take the carburetor all the way out if that's easiest for you or if you need to do other work to it. If the carburetor is old or has had old gas in it or if you've had any unresolved symptoms before the big bore installation, it may not be a bad idea to clean the carburetor thoroughly before you go any further. You can find cleaning help HERE.

Loosen the bolt or screw, usually 10mm hex head or phillips head, that clamps the carburetor into the intake.



Pull the carburetor out of the intake and let it hang down where you have easier access to it.





Place a container under the drain hose of the carburetor and loosen the bowl drain screw. It is not necessary to completely remove the screw in most cases. Just loosen it until gas flows out. If you have a manual fuel petcock, make sure it is turned off before this step. Once the gas has stopped flowing, tighten the drain screw.



Remove the four phillips head screws that hold the bowl on the carburetor and remove the bowl. The bowl gasket usually stays attached to the bowl or the body of the carb, but make sure you know where it is if it happens to fall off.





Use a flat screwdriver to unscrew the main jet from the carburetor and remove it.



I like to install a new main jet that is about 10% larger as a baseline. It's likely that the jet will need to be changed again for best results, but a 10% to 20% larger jet is a good start. Most stock carburetors use somewhere around a 70 main jet. A 77 main jet would be 10% larger and an 84 would be 20% larger, so 80-85 is a good start for most otherwise stock engines. I prefer to start big and work my way down, because it's safer to be rich than lean.

Screw the new main jet in. Make sure the bowl gasket is in good condition and reinstall the four screws that secure the bowl to the carburetor. These bolts must be snug, but don't over-tighten them. These bolts are notorious for stripping.



Insert the carburetor into the intake again, and tighten the bolt or screw to secure it.



Slide the airbox over the mouth of the carburetor and install the clamp to secure it.



Install the two bolts that secure the airbox to the CVT cover.



Now it's time to fire the engine up. If you used sealants on your engine during the installation, delay startup until after the cure time of the chemicals used. It may take a few cranks to get gas flowing if your scooter uses a vacuum operated petcock. Some people remove the vacuum supply hose from the petcock and apply suction to get fuel flowing, and then reattach it. THIS POST shows typical petcock connections. Some like to add one or two cap fulls of their favorite two stroke oil to the gas tank for the first tank with a new cylinder kit. I pre-mix the fuel and oil for all of my scooters instead of using the stock oil injection, and don't bother with adding anything extra. Any significant amount of oil added to the gas makes the mixture a little bit leaner (since some percentage of oil is now also passing through the jets).

Once the engine is running and warm, it's a good time to adjust the idle settings. Turn the mixture screw counterclockwise or clockwise until the engine idles at the highest RPM. After you have found the highest RPM, turn the mixture screw clockwise about 1/4 turn to err on the side of rich. Turn the idle screw in to raise the idle speed or out to lower it. Set the idle speed low, but above the point where the engine wants to stumble. If the engine struggles when you turn on the headlight(s), raise the idle speed slightly.

You should have adjusters near the throttle on the handlebars as well as on the carburetor. Adjust these until there is only a little slack. If the engine's idle increases as a result of your adjustment, you've taken up too much slack.





Engine Break-In

Now it's time to take a test ride/break-in pass. The seat and access panels should still be removed at this point. I don't suggest re-installing everything just yet, because you may need to get to the carburetor again soon. If your scooter has a seat like the one on this Eton, you can put it in place without the bolts and the latch will hold it well enough to make a quick pass. Whatever you decide to do, make sure the seat is secure and all wires and hoses are tied up before riding the scooter. Don't leave any lights disabled if you had to remove or disconnect them for the install. Use your best judgment here and be safe.

The following section is my take on engine break-in procedure for a scooter. There are many differing opinions on this matter, so do your research and decide what's right for you if you disagree with my methods.

With the scooter completely warmed up, take it out and make a 1/2 throttle pass for roughly 1/8 mile. If engine RPM is very high you'll probably need heavier roller weights for the new power, but for now apply less throttle to keep engine speed around 5,000-6,000RPM max. Let off the gas and coast until you feel the clutch disengage or until you come to a stop. Repeat this process one or two more times and take the scooter back to the shop, being sure to vary throttle position and keep RPM down.

Once you get back to the shop, let the engine cool down. When the engine is cool, remove the spark plug. If the spark plug looks white or very light, move up to the next size of main jet. If the spark plug looks like it's never been used, you should move up at least two main jet sizes. If the spark plug is dark, leave the jetting alone for now. If the spark plug is wet, you may want to swap to a size smaller main jet or if you didn't experience any sputtering, bogging, or other issues then leave it for now.

 If engine revs were off, you may want to swap roller weights now. Heavier weights lower RPM and lighter weights increase RPM.

Reinstall the spark plug and start the engine. Let it warm up and then go for another break-in run. Do a couple of 1/8 mile or so passes at 3/4 throttle and keep the revs to 7,000-8,000RPM max. Just like before, let off the gas and let the engine slow down the scooter until the clutch disengages or you come to a stop. Take the scooter back to the shop and check the spark plug again and adjust jetting as described earlier.

Warm the engine up again for one last set of break-in runs. This time, go wide open throttle for a short burst and then coast to a stop. Repeat that one or two more times and then head back to the shop. Remove the spark plug and check it's condition again. Adjust jet size accordingly.

Re-Torque And Other Checks

After the engine cools completely, it's a good idea to remove the shroud over the cylinder and head and re-torque the cylinder nuts. Many skip this step, but I find it very useful to ensure that everything seals well after heat cycling from break-in passes. Some gasket materials will compress quite a bit and you an end up with loose cylinder nuts at some point, which will most commonly lead to a head gasket leak and could end in a breakdown or even an engine failure.

To re-torque the nuts, back off the first nut in your tightening sequence by about 1/8 turn (remember how you criss-crossed them to tighten during the install?). Then torque again to the full spec that you are using. Repeat the process, following the criss-cross pattern, until you've completed all of the nuts.

It is also a good idea to make sure the exhaust fasteners are still tight at this point.

If you want to be really thorough and ensure that there are no leaks at all, you can build a leak tester and perform a pressure test on the engine at 5-7psi. It should hold the pressure for at least 30 minutes if it's sealed well and the tester has no leaks. If it's not sealing, you can listen for a hissing noise and use soapy water to spray areas that you believe sound is coming from. Common trouble spots are the head gasket, base gasket, intake/reed area, attachment point of the tester, and crankcase seals.

Tuning

Your engine should be in some rough state of tune by now, or even getting close, if you've been checking the spark plug and adjusting jetting accordingly as I've outlined above. Now you'll want to get it dialed in the rest of the way. Pay attention to symptoms. Watch plug color carefully. If your scooter is so equipped, watch cylinder head temperatures. Read THIS article about carburetor tuning for more in-depth info.

Once you've got your carburetor straight, adjust your CVT to work best with the power increase. Often, simply adding heavier roller weights works well. Some scooters do well with their stock roller weights. HERE is more info on choosing roller weights.

Finishing Up

Once you've got everything in tune, reinstall the seat bolts and the access panels on both sides.





All that's left now is to clean up the mess of tools you've probably left in the floor.