Engine won't go past 30MPH

it may if you do the run in for a while longer, but it could be that 32T is too small for the total weight of you & the bike (assuming you have no bearings dragging or rubbing of the drive train anywhere) - 50 is real hard to hit for anyone over about 150lbs

I am 210 lbs stripped naked and dried off. My mountain bike Grubee Skyhawk 66cc does 30-37mph on the flat with a 44 tooth sprocket before any mods. It did noticeably improve during break-in over the course of 3 tanks of fuel. I had detonation problems that were solved by correcting the squish band shape and gap.

When I ported the intake manifold to match the cylinder, I lost speed. This was verified by switching back to a stock intake. The mis-match has an anti-reversion effect I'd suggest.

Matching the exhaust flange to the cylinder increased the torque in a slightly lower range making the bike have stronger power peaks at 35kph and 50kph. This is with the stock Grubee chrome exhaust. The black exhaust was noisier and made less power so I did not use it any more. The chrome exhaust with the end cap (baffle) removed is much noisier and has less power and speed.

Jetting is important. Plug should be a tan colour, not black and oily. I'd recommend an oil mixture in the 32-40:1 range with quality oil as there is too much carbon fouling with heavier cheap oil mixes.

I'm no expert but the sprocket choice sounds like the problem, a 36 or even a 40 may put the rpm's in the pipes range to increase speed. What it will do is increase your hill climbing, some engines are better on top end than others.

IMHO, the best way is to let your local terrain will determine your gear ratio.

if you live in a flat area it doesn't matter, go with which ever you like best. the stock 44t is a nice "middle of the road" choice. (no pun intended)

i tried a 28, 32, 44, 50 and finally settled on a 56.
the smaller ones dramatically increase top end speed on level ground, but they DO NOT climb hills. even a moderate grade will eventually defeat them over a long incline.

gear for the terrain that you will actually be riding on.

my top speed on level ground is around 25-30 mph, with 29er wheels.

Leo, I have 3 motors. The first 66cc 44t 26"wheel I got would not go over 30mph, and would not climb moderate hills at all. The new Grubee Skyhawk 66cc 44t will do 35-37mph and will climb moderate hills with ease. With a few mods I am getting the Skyhawk to climb steep hills at over 30kph (18mph). Under that speed I still lose torque. I agree with your post, but would add that torque adding mods are something to consider as well as low gears.
These motors are not all the same. It is obvious the new Grubee Skyhawk is better balanced than the older 66cc motor and will run to higher rpm and makes slightly more power. I have the older motor apart right now and see no counterbalancing on the crank. The cylinder porting looks similar but has some huge casting flaws. I've cleaned up the porting flaws (no mods or further porting) and while the case is apart will try stuffing it to see the effect. I'll take a guess at what to drill for counterbalancing the crank. Crank balance and piston ring thickness are big limiting factors for these engine's rpm.

Steve

I do 30mph (gps verified) and it's plenty fast enough for me. My bro goes a bit faster on the top end...I think about 35mph. These are on new engines, we haven't even fully broken them in yet.

My bike is completely stock with a Sporstman Flyer hub 40T and 29" wheels. I did change the plug wire, but that was because the other was a dud right out of the box. Also an NGK plug.

My bro's bike is stock with a 36T hub, banana pipe, rejetted, new CDI, SBP intake filter, and a bike berry tensioner.

Keef B, are you running a rag joint? One of the things we noticed the most is that going from a rag joint to a hub adapter and sprocket, we have less vibration, smoother acceleration.

Rejetting helps as well as getting the correct fuel mixture. The stock exhaust pipe is better for low/mid range, but I find my bike is lacking up top. My bro's banana pipe helps him pull more top end out of his bike. A proper expansion chamber would be even better.

Greg58 said:

I'm no expert but the sprocket choice sounds like the problem, a 36 or even a 40 may put the rpm's in the pipes range to increase speed. What it will do is increase your hill climbing, some engines are better on top end than others.

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I agree. When I slapped a 32T on my 36T performance build before I did more major mods the top speed actually decreased by 2 mph. It wasn't getting into the power band enough to maintain acceleration.

I think anything more than a 36T on a 26" wheel is just asking for trouble. You can attain 50mph with a 36T it's just why would anyone want to go that fast. My 34T with 24" wheels is about the same as a 36T with 26." Given it's high compression and running 110 octane It's too fast for normal cruising. Reason that I like where I am at now is it makes 30 mph a joy to cruise at.

A 50 mph china girl is going to have a very short lifespan. You need about 5 1/2 to 6 hp to do 50 mph.

After careful tuning and break in, most of my china girls would top out about 35 to 40 mph with a 36T on 26" wheels. They cruise at 30 mph pretty dependably, but riding at 35 vibration takes a toll in short time. 40 mph is a lot worse.

It's not a good idea to turn a bicycle into a motorcycle.

Way to fast, but good luck.

Dan

Yep, I concur the the others.... switch that sprocket to a 40t or 36t & you'll see the speed increase due to the engine RPMs getting into the proper power band.



One more thing I'd like to add... If you're serious about going real fast?, INSTALL GOOD BRAKES FIRST!!!

Keef B said:

Thanks Guys much needed info. I was racing my friend over the weekend and his bike did 45mph. He was giving me head starts and was still whooping me on the top end. Any Ideas on how I could increase acceleration, I have a basic arrow motors sort of modified exhaust but it has leaks.

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Mine is stock with a stock carb and stock exhaust on regular fuel. I can reach 35mph to 37mph on the flat with the engine "tuned up" lightly.
1) Carb jetting set up by looking at the soot-up of a new plug insulator.
2) Cylinder head squish area reshaped with sandpaper to match the piston dome.
3) Piston to head gap set to around 1mm by sanding the cylinder and/or head, and/or shimming the basegasket height.
4) Aligning the drive chain to run straight
5) matching the exhaust pipe flange and gasket to match the exhaust port.

This is where I am right now. I am using this engine as a testbed to test whether "known" 2 stroke tricks really work. I have been doing them one at a time so that I am sure what works and what doesn't.

So far one that didn't work was port matching the intake.
The stock mismatched intake (too small compared to port) works better!
The reason is likely due to reversion pulses. Also runs leaner this way.

Next steps, which I hope will push me over 40mph, are going to be:
6) File the casting flaws off the port edges of the intake and exhaust.
7) file the gross ridges off the transfer ports
8) trying a longer smoother intake and a trumpet like tapered intake to the carb.
9) Drastic step, pull the engine apart and stuff the crankcase with epoxy.
I will be doing this first (am doing it now actually) to an older 66cc engine that will not rev as high as my new engine due to crank balance issues. So I will drill a couple balance holes, plug them with foam and epoxy, then fill the case with several more tubes of epoxy to reduce case volume and primary compression. I am doing this all before a tuned exhaust on purpose. When the engine is maximized first, it is easier to tune in the pipe, so:
10) Tuned pipe.

I see many on here do 4 or 5 mods at the same time, never learning the true effect of any of them. I see failures from ill conceived "improvements". Try things one at a time on these engines. They are so simple I can have the crankshaft out of the motor in 1 hour.

Steve

Mr Happy Piston...

You should try your stock 44 tooth sprocket. Less stress on the motor and it would give you more torque.

A 40T is a pretty good compromise. You can get decent acceleration, and 35 mph top speed