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Post by isurffl on Feb 9, 2015 19:39:51 GMT -5
ok so I've been doing some browsing around and trying to get lots of confusion cleared up about purchasing the "right" bore kit. I'm doing this for myself and others so that it's all in one tight package. Keep in mind this is intended to help us make informed decisions on doing our own backyard work. The original post here may be edited many times over time to keep it up to date. I apologize if there is another thread covering this and please point me in the right direction and I'll drop this thread.
- what's up with valve length in relation to installation?
- if I want to install a different length, can I?
- some kits work with stock heads no matter what valve length? How?
-are these kits better than the ones that require a new head?
-to cam or not to cam?
-80cc or 100cc
-what do I only need for bare bones kit.
-how to tell a good quality kit?
-(more to add, as we come across other points)
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Post by isurffl on Feb 9, 2015 20:29:33 GMT -5
So I'm going to start putting my pennies here and hopefully someone will point out what I'm wrong on.
I'm pretty sure valve length is irrelevant if you're changing the rocker arms to match the vavles you're installing.
I imagine stock heads work with certain kits by changing the shape of the piston, but I've never installed a kit before so I'm not sure if this kit gains/lacks compression or performs like the others???
Performance cams work by changing the range of movement in the valves ultimately altering how your engine fires and how much fuel it lets in and exhaust out.
From what I'm reading 80cc is the way to go. You gotta think, these engines were only meant to be 50cc so they use parts to build respectively. 100cc should only be used if you at some point expected to break your scooter real good. I would only advise using a 100cc kit if you're replace the crank shaft or expect not to be home on time for dinner one day.
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Post by scooterpimp on Feb 9, 2015 20:48:20 GMT -5
47mm/72cc bbk , reuse your existing cyl. Head & valve length should not be an issue. Upjetting carb will be nesesary, read reviews on vendors to to find whats best for you as far as quality is concerened. Most important imo. Is do one upgrade at a time to eliminate headaches. Visit tech section , 90gtvert has great videos etc. Covering alot of different things. As well as forum members. This is general info do some research to see whats best for you. Keep it fun  |
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Post by 90GTVert on Feb 10, 2015 15:48:30 GMT -5
There are 64mm and 69mm valve lengths for the typical 139QMB/1P39QMB engines. You can use either style of head as long as you have the rocker arms or rocker assembly to match. Some kits come with rocker arms. Otherwise you can buy a whole assembly. Lots of big valve and aftermarket heads come with 64mm. 64mm heads even in stock form tend to be easier to find and cheaper. The smaller bores tend to be more reliable. 47mm (72cc or sold as 80cc) is the way to go for a big bore kit unless you aren't terribly concerned about engine life. Some say "de-tuning" a 50mm+ bore kit with heavier roller or slider weights so the revs stay lower works to increase longevity. If you want the most engine life then stick with a 49cc. If you want the most power a 50mm+ bore would be the way to go. 47mm bores are somewhere between. Aside from the weight of the piston and rings and potential balance issues, when you make more power you create more stress. The primary point of failure in the 50mm+ bore kits reported has been the big end connecting rod bearing. Replacing the bearing and conrod is possible, but it's not an easy job. Combined with the cost of a new rod, bearing, and main bearings the entire crankshaft is usually replaced. The camshaft changes valve timing and lift. Unfortunately shopping for a cam for a 139QMB is far from shopping for a cam for a small block Chevrolet or even a GY6 150. There are only a few of them out there. The "A9" cam is the most common. What exactly this means I'm not sure. I've read of cams with that designation actually being different depending who made it. Naraku and NCY offer cams as well. Depending how the cam is made you could have different results, but most of the stuff for these engines will provide the most noticeable gains at higher RPM. The easiest way to get an idea of a decent kit is to know where it's made. Taiwan made is usually better than Chinese made. The stock cylinder kits that many of us have are Chinese made though, and some of us find them to be quite reliable, so I'm not saying Chinese kits are necessarily bad. A simple and practical way to sort out potential junk may be to look at what others are using or ask about the kit you think you want before you hit the buy button. The necessities for a big bore kit are a cylinder, piston, piston rings, gaskets, and a selection of main jets. Some kits require a head with a larger combustion chamber or combustion chamber modifications. 47mm bore kits are more likely to get away with using a stock head. Compression would be way too much in most cases with anything larger than a 47mm bore and a stock head. You may also need to re-tune the CVT. |
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Post by humanshield on Feb 10, 2015 17:25:00 GMT -5
I recently installed a "100cc" (50mm piston, actual 83cc) kit on my scoot.
What I'm finding is that this piston probably just pushes too much air for the rest of the motor to handle. At the very least, I think the "100cc" kit works better with a big valve head and maybe even a 150cc intake manifold and one guy I spoke with today who does a lot of scooter work says I should go with a 22-24mm carburetor for the 50mm BBK.
If you're going to go with the 100cc kit, you may have to use the A9 cam and or big valve head, 150cc Intake, 22-24mm carb and a better flowing exhaust. Then keep in mind like 90GTVert said, the engine longevity may become an issue. Great performance for a month or who knows.
My motor does ok up to abut 25-30, then it seems to be choking on its own volume of air. I think my engine is doing what they call "4 stroking" because it can't handle the air that's being forced through it. It'll be going along at about 3/4 to full throttle and will suddenly take a momentary nose dive as if all the power is gone for an instant and it makes a "Wonking" kind of noise.....then resumes.
So, I think the 80cc (47mm piston) BBK might actually out perform the bigger kit without other mods because it may be at about the upper limit of what the stock motor can handle without additional mods. And as 90GTVert pointed out, the 50mm piston is a pretty heavy club to be swinging around in that "designed for a 39mm piston" crankcase.
I would probably go with a 47mm BBK next time unless I was going all out.
But again, this is from my limited recent experience. Others here may have been there, done that many times before I ever even owned a scooter.
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Post by isurffl on Feb 11, 2015 0:19:20 GMT -5
How important is ring gapping? And will we likely have to file one down in the process? I'm going to Fill the top post with answers eventually
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Post by 90GTVert on Feb 11, 2015 0:43:41 GMT -5
Piston ring end gaps should be checked. Too large of a gap and you won't get proper sealing. Too small and the ends can butt together when hot and create a lot of friction or potentially break. You may have to file them. Many people ignore checking end gaps and seem to do fine, but I think it's a mistake. It doesn't take long or require expensive tools. If you do have to file a ring, file only one side. File from the outside of the ring toward the inside. Keep the end square. A lot of people like to clamp a file in a vice to make filing rings flat/square easier. There are specialty ring filers, but they are not necessary.
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Post by niz76 on Feb 11, 2015 4:48:46 GMT -5
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Post by runningbuck on Feb 11, 2015 10:04:58 GMT -5
Ok I installed a 100cc bbk 50mm piston, a9 cam, blue racing CDI, bigger head and valves, Kiehin carb with the bigger jet before Thanksgiving and been running ever since and have put about 2000 miles on the kit and no problems I run WOT at about 40 to 45 and average 47 (I live in the mountains not much flat areas for testing) my max was 55 but it was on the down hill side and once hit 58 off a steep road these was by phone gps tracking speeds
As for humanshield I think he might have a restricted cdi killing power at higher rpm or a wrong jet in the carb these would be my first suspects in your issue
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Post by isurffl on Feb 11, 2015 14:50:34 GMT -5
Most commonly what size jet should accommodate and 80cc and which for a 100cc? I'm buying an 80cc bore kit with an assortment of jets and I understand this question is subject to atmospheric conditions. But against, this thread is not just for me.
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Post by niz76 on Feb 11, 2015 18:37:14 GMT -5
Most commonly what size jet should accommodate and 80cc and which for a 100cc? I'm buying an 80cc bore kit with an assortment of jets and I understand this question is subject to atmospheric conditions. But against, this thread is not just for me. Brent (90GTVert) has hammered it into our heads over and over- rightly so- that it's difficult to give a jet size requirement because everyones setup is different and no two scoots seem to be the same even though they might have the exact same parts... That being said, the 100cc BBK I bought came with a 91 MJ and it just happened to be perfect... Always good to get a selection like you mentioned and tune the right way: start pig rich and work your way down. Be sure to study plug reading in the tech section and it will eventually become pretty easy! (of course just when you think u have the hang of it, some carb gremlin will have u pulling your hair out for two days haha) Here's some info- might not all be relevant but I like what you're trying to accomplish here... Many 2t stock 49cc's seem to like 70-75 main jet. The snorkel can be a great tool to pop in and out to richen and lean and see which way you need to go on the jets... The 4t's seem to be using pretty similar sizes. Whenever I install an "open filter" or UNI of some sort, the jet sizes needed jumps way up into the 140-150 range. Again, not very detailed info, just hoping to illustrate the huge difference the open filters make and what kind of tuning to expect when installing one. I would guess an 80cc to be using a MJ around 80-85, but it's very dangerous to just pick a size and go with it. Learn to "feel" the tune and use the snorkel or your hand to restrict air/let more air flow as a tool to help you figure out if you're too rich or too lean, and learn to read your plugs! Don't be afraid to buy 5 spark plugs at a time so you can see how each jet change is affecting the color.... I know these thoughts are a bit scrambled but hope it helps someone! |
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Post by runningbuck on Feb 11, 2015 22:58:43 GMT -5
to go along with niz76 it also depends on carb size 18 mm 19 mm 20mm ect. ect that changes the jets also so now you might see why people say it's a guessing game and is it a true 18mm carb or a just close aka 17.5 to 18.5mm carb too many little things to pin it down to what size a carb needs to preform on your bike just fyi
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Post by isurffl on Feb 12, 2015 11:20:11 GMT -5
Now I don't see much mention on idle jets, is it important we upsize those too?
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Post by niz76 on Feb 12, 2015 12:32:31 GMT -5
I honestly haven't ever needed to change pilot jet but that's just me I guess. I read many people do- I just have never found the need yet... Totally depends on your setup...
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Post by isurffl on Feb 14, 2015 17:09:11 GMT -5
Ok I see a bunch of stuff about gapping the compression rings, what about the oil retaining rings?
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