72cc BBK

[mono]

Compression ratio only determines the maximum gas pressure that results when the piston reaches top dead center. (disclaimer : for a certain volumetric efficiency that is, which is rpm and engine dependent, probably close to 100% at low rpms for our engines, and also excluding the effect of rising gas temperature by compression itself, which will be less at extremely low rpms, and comparable in effect between a 50 and an 85cc)...

The area of the top of the piston, together with this pressure, determines the force needed to turn over the crankshaft towards and over that point...

Since an 85cc's piston has an area that is larger by a factor 1.7 than that of a 50cc (if it's really an 85cc), it's easy to see why an 85cc with a compression ratio of 9:1 will be harder to crank than a 50cc with a compression ratio of 11:1...

[LoveMyKymco]

I run the 50mm ncy kit and I get 8:1 compression just as you should if you have it as well, I dont know why anyone calls it a 85cc its simple math.
Theres quite a few other things that come into play but I decided not to delve into them such as the dcr, compression pressure, cam duration, the list goes on and on, but in the end the answer to the question is if you get the 50mm kit your stock starter will do just fine.
Ive got quite a few, probably near 30, 50mm kits installed and everyone of them starts with the stock starter fine. The 72cc kits actually do have an issue with starting due to higher compression ratio due to small combustion chamber heads used, and I found that using a higher cca battery will help alleviate the problem.

[jaws]

Hey guys, its been a while since I've posted on here, but scooter season is almost here and its time to fire it up. I installed a 72cc kit last fall, and only got to ride it a couple of times. I was wondering what main jets are you guys using, and with what other mods? I've got a 90 jet in it now with a polini variator/6.5gr sliders, 2000k springs, open foam filter and stock head, cam, and exhaust (muffler has extra holes drilled in the end from the previous owner).