Thread: how to increase compression on a 2 stroke?

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I'll deffinetly send an update with a pic.. probably change the brake pads out with a bigger unit from a mountain bike.. I'v got the wheels off right now, and I'm putting brand new tubes and tires on...

factory oil spec is 16:1 for break in and 25:1 after.. if I find it needs more go power, I'll do some milling... maby run an aftermarket exhaust pipe for the low end... parts for these bikes are cheap enough... exhaust pipes: $20 carbs: $20 new engines: $90.. list goes on and on, and theres not much over $200...


here is a completed moped conversion bike, from a pic on Ebay
http://img.auctiva.com/imgdata/9/7/2...4482846_tp.jpg

while doing some searching on a forum for these bikes, and found that high compression slant heads are available for them.. sparkplug is slanted 45* and the chamber is smaller... $13... hotrodding this thing will be easier than I thought

Originally Posted by Matt167

while doing some searching on a forum for these bikes, and found that high compression slant heads are available for them.. sparkplug is slanted 45* and the chamber is smaller... $13... hotrodding this thing will be easier than I thought

I suspect you could gain more from a properly tuned pipe than from any other modification.

probably could.. there about $25-$30 for a tuned pipe.. supprising on how, somthing I did not know existed 2 days ago, has a fairly large following and aftermarket support..


I got 1 bike tire on today. havn't done 1 since I was a little kid and forgot how much of a pain it was took me a good 1/2 hr to figure it out and do it

Yeah, then you go to air it up and find that you've pinched the tube with one of the tools.

nope, that didn't happen.. I was afraid the tube might have gotten punched but it wasn't, and holds air fine... I didn't need tools ither, managed to do everything with my hands.. I never put a new tire on b4, I have changed and patched tubes.. so when the tire came out of the package and it was flat instead of curled around like a tire, I was kinda clueless. but all I needed to do was inflate the tube a bit, fit it over the rim and squeeze it togther. got hard at the end, but I got it

Two stroke engines make compresion on the up and the down stroke. Shaving the head (or swapping the head for a 'high compression' head) is only half the solution. You need to add what used to be called a 'stuffer box' to the bottom end to complete the solution.

A properly done 'porting job' will work wonders to get the powerband in the RPM range you need. On the older Mac engines you could also add multiple carbs by adding one or more carbs to the bottom end. Three lifetimes ago (back in the late '60's) I had a three carbed Mac 75 that came off a racing go-cart that was clocked at 90+ MPH. I wanted to add a 'pull-start' and made a 1/8" spacer that I installed between the head and the cylinder to lower the compression. I wasn't aware (at the time) about the lower end compression and the 'pull start' would barely work because of the high compression. I also removed two carbs from the bottom end, as I was installing this engine on a mini-bike I had made for my son. Ended up being much too fast for him.

Contact someone that is familiar with building racing two strokes for additional info.