Thread: how to increase compression on a 2 stroke?

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Ok, I like to ride bicycle a lot, and my current bike is very neat.. not in the best cosmetic condition, but it's solid.. it's a late 60's GrantSport Easy Tour 26", sold from W.T Grant when they were still in business.. being a low end bike, it might be worth $5 or so, not collectible like an old schwinn.. it was a 3spd but the 3 gear axle was replaced with a single gear, and the old pull type dérailleur does nothing.. easy bike to pedal going on the level but due to no gears, hills arn't easy.... searching on craigslist for a new truck I found that bike-moped kits are made ( was in the auto section ).. 48-66CC 2 strokes with manual clutch and pedal start... searched Ebay and found the kits for $129 for a '66CC kit.. specs list 6.0:1 compression and 6hp ( seems logical for a 2 stroke ), which will deffinetly haul me up the hills, or at least assist... but how do I get more compression and power?.... there listed at 40 MPH top speed on a 26", so I'd like to hotrod it to go 60 if I can...


yes, I know I'm asking how to hotrod a moped

60 Mph on a bicycle with skinny ass tires and terrible flimsy little brakes?
eek:

You got a death wish or something??

Take the 40 mph and live to enjoy it. Actually 40 might even be a stretch.

I know, really don't want 60 MPH.. the main reason for wanting a little more power out of it, is I live on a very large hill, probably 60* incline and it's about 1 mile up.. with the ignition and fuel off and some decent compression, I should be able to descend down the hill using the brakes very little, and then the engine should be able to take me up the hill...


I'll have to see how it does first, because really.. I'v pedeled up to 30 MPH down hill, and that's pretty fast on a bike for me.. 15 MPH would be just right I think

Wowzer Zowzer!!

I'm with Bob!

That said, I used to race the daylights out of go-karts when I was a kid. Homelite and McCullough engines that we would squeeze 20-25 horse out of. The compression ratio is fairly easy to raise by shaving the head as you have no valves to worry about. Thinner copper gasket does the same trick.

A word of caution - the spark plug clearance is critical and you need to check before you mill the head - Also bear in mind that these little buggers run hotter than the hinges on the gates of hades and will burn a hole in the piston in a heart beat. If you're going to raise the compression, back down one heat range on the plug and go a notch richer on the mixture and use a good two-stroke oil. I like RedLine or Honda HP. We raced Suzuki and Kawasaki motocross at 50:1, but rebuilt the top end every other race. The sponsor had a fresh motor in the truck (it was a nice truck - complete machine shop inside) ready to go and rebuilt between motos at the track You probably need to stay at 32:1 for what you're looking at.

Have Fun

P.S. Update the brakes on the bike and send a picture.

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.

Matt, since you have already started the hot rod modifications and are gungho about getting this little speedster on the hill, I have only one other suggestion. Perhaps to top off your little jet a full set of leathers may be a wise investment. Road rash happens.....and it hurts. Been there, done that and it weren't fun.

I wish you much success in your endeavor. It would be nice to be young enough again to attempt such an adventure.

I'v had road rash b4 also.. I laid a dirt bike down on the pavement 1 time, and rolled my go kart sideways down the road ( 3 times ) another... it does hurt... I want enough power to ascend my hill.. right now, I just have the basic kit on order, along with a spring loaded automatic chain tensioner

Well--besides top fuel and prostock type engines we have been into karting -----your package as sold is probably pretty well optimized for what its purpose is---biking as on flat ground---higher compression in itself won't be a lot of help altho some porting changes would---porting is like camshaft changes not about the size of the port but how the vertical diminsions effect inlet and exhaust---just like cam changes tho, it really effects the rpm level where hp/torque occur

if you want to climb a hill you should be looking at adding to the low rpm range that will be happening closer to the top of the hill, not max rpm----

I found 50 tooth sprokets for $20, probably order 1... factory has a 41 tooth sprocket... because it is the low end grunt I need... if this 1 goes good, and I deem them pretty fun to ride. Maby I'll get a Schwinn Stingray chopper and really gear that 1 for speed