Compression Ratio/ Timing / Reliability

[sleestak]

I have been debating between and reading about two Big Bore Kits and it has brought some questions to mind.

The two kits I am trying to decide on are the NCY 72cc kit and the Airsal 72cc kit. Both are fairly different and I am hoping to gather some input and opinions here.

I know one is cast iron while the other is aluminum. One is supposed to cool easier than the other and there is the plating on the Airsal, the pistons - one domed the other isn't, etc.

My real debate is in the raising of the compression ratio and weather it will remain reliable with my intended set up.

I plan on using the stock head with .003 intake .004 exhaust. I would bump those up another thousandth but don't want to have to worry about too much clattering. I also plan on using a CDI (blue) with advanced timing and an intake w/a re-jet on the stock carb - guessing I will start with 90 or 95 and see how it works out.

While running 93oct, do you think the higher compression ratio and spark advance from the CDI will be a bit too much? Should I opt for the NCY instead?

Please lay some knowledge on me, thanks!

[felliott]

I say yes on both questions. I have had these engine sparkknock using the first qustion. High compression is very hard on the starter. It will not turn the engine over at all sometimes.

[reveeen]

The general consensus in these parts is:

Without increasing the combustion chamber size the installation of a big bore kit will land you in difficulty.... especially with a domed piston.

The "usual" compression ratio for a 50cc (39mm) GY6 is around 10:1, increase the bore size (without opening up the head) and the compression will go up.

The guy offering the GY6 jet kits (for $6) has a deal going on his so called 100cc kit:
www.scooterpartsmania.com/100cc-big-bore-kits-139qmb-gy6-50cc-engine64mm-valve70002-p-1070.html

I *think* we all got our jet kits........ and I ordered a $7 intake (which I got) so I would not be afraid of ordering. Just verify your valve length (see the tech section under "heads" to see what to check for).

[vvtr]

Have a look on ebay for a seller called robotechlegacy.... Hes doing some nice 80 kits for the 4 strokes that come complete with new head, full gasket set and uprated oil pump.
Good quality too ..... And probably cheaper than both the kits you mention.
I think theyre listed as 100cc but i worked it out at 88.
If i cba i will add a link later.....

[sleestak]

Thanks everyone for the input. I will check out the places/kits mentioned above however I am pretty set on getting something that is not Chinese as I have read a fair amount of horror stories about crappy pistons and the like - not to mention my past experience with Chinese casting. I guess this is why I have been leaning toward those two kits. Another reason is the reading about increased wear on the crank with the 80+cc kits. If anyone has input on that, I would appreciate hearing it as well.

Higher combustion doesn't bother me TOO much, but there is a point where it gets to be a too high and I think that domed piston pushes it to the edge for sure, even though it states on numerous sites that it was made to be used with the stock head. We get 93oct out here and I plan to use it. I just don't want to get to the point where I am mixing in Toluene to get 95+, lol.

[reveeen]

getting something that is not Chinese

Old fella scratches head and wonders: you are starting with a Chinese motor, and most manufacturers are "batch manufacturing" in China now (be it Harley Davidson, or GM), so he is having trouble connecting the (those) dots.

In all situations I have found you get's what you paid for. Sure that $88 kit may have a dodgy piston in it, but chances are just as good it doesn't. The majority of piston troubles documented here were caused by the piston hitting the head..... this being caused by using the stock head on a larger bore cylinder (easily fixed by using an extra cylinder base gasket, or two).

The only part of the crank that is going to give difficulty is the rod bearing. The main bearings are the same as used in 2 stroke engines, so if they are reasonable quality to start with, I would not look there. The rod bearing will self destruct under 2 conditions: you over-rev the engine, and you neglect to supply a good supply of clean, cool, oil.

If you limit yourself to garbage gasoline, then compression is going to bother you...... resulting in piston/connecting rod bearing failures.

[sleestak]

Jan 12, 2012 3:13:27 GMT -5 reveeen said:

getting something that is not Chinese

Old fella scratches head and wonders: you are starting with a Chinese motor, and most manufacturers are "batch manufacturing" in China now (be it Harley Davidson, or GM), so he is having trouble connecting the (those) dots.

LOL, yes I realize that the scoot is Chinese. It fell into my lap for a whopping $100 and I figured I would tinker with it. There is difference between a NCY kit and some random no-name kit from eBay. Say what you will, but I know there are many who would beg to differ with you. Regardless, I don't think there is any problem with wanting to replace a part with something of a little better quality.

I am trying to only do this once and not every few months because I have a Cafe bike to build and a car that is regularly upgraded. Read as "enough projects".

As far as the piston hitting the head, I will check that during assembly and already plan on getting handy with a dremmel. Since it's winter, I can take the time to port things and such.

[vvtr]

i know if i had the time & money i would get a load of performance parts manufactured in china and market them as a uk product.... why not, everyone else does lol.
mark my words... in a few years chinese products will be at the top of the tree.... just the same as japanese cars & bikes were garbage in the early 70s - look at them now.

it wont be long before youre seeing jinlun & hyosung etc on the podium of wsb

[Deleted]

www.scooterpartsmania.com/100cc-big-bore-kits-139qmb-gy6-50cc-engine64mm-valve70002-p-1070.html

DONT get that, the rings are never right, always too big and not just the compression rings but the oil rings too, i have had problems with those kits...get a good kit like Airsal or NCY or HOCA or Naraku

larger gauge starter cables and/or a higher cca battery is sometimes needed too.