Rev-limiter on Chinese "Happy Time" 80cc?

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11:41 PM
Jul 25, 2008
I installed the 80cc motor kit on a nearly new mountain bike. Everything is straight, well lubed and runs true. After about 100 miles I decided to test the top speed. Up to 25mph everythings great. Plenty of throttle left and when fully open engine sounds like it is going to gain speed rapidly. After about 27mph the engine acts like it is being controlled or held back by some kind of restrictive device. My son in law has the same engine kit and it responds the same way. I have installed a new carbon plug wire, plug and carb. o-ring. I modified the muffler by removing baffles and cutting the small exhaust pipe to about 3 inches. I would appreciate any ideas or suggestions to engine can continue reving. Thanks, Mike
I think alot of it has to do with the size of the intake manifold. You can buy larger intakes, but they are hard to find.
Dude. Don't. 27 mph,44 tooth sprocket I would say that's redline,dawg.
You don't want to go past that "rev point" or you'll start breaking parts.
Dude. Don't. 27 mph,44 tooth sprocket I would say that's redline,dawg.
You don't want to go past that "rev point" or you'll start breaking parts.

True. after that speed the engine starts to vibrate like crazy. It can't be a good thing...
you should not be able to over rev a two stroke and start breaking your engine cause it cant get over 30 sounds like its either not flowing enough fuel/air or is running rich
Just an idea, If you need more speed, buy a kit from these guys. You can shift through your normal derailer gears ive heard.
Otherwise you will blow it up. If you need proof, go out for a ride with it wide open for 2 miles and come back to the post with your results. LOL

Or If your that set on the rrrr-ee-vvvvv, get one of these engines that can handle the RPM's :cool:
The rev limiter is the carb's inability to provide enough fuel to the engine at those engine speeds. 27mph is not too fast on a 44 tooth, I've done that on a 48 tooth sprocket with no problems.

Some adjust their float to allow more fuel, some adjust or bore out their main jet to a slightly larger size, and some do both.
Like I've said before, MY HT is literally screaming at 26mph with 44 tooth. Most of that is the gearbox howling. I'd say 26-27 is definitely the red line with a 44 tooth and my particular type of engine.

Now I've held it at 26mph for quite a long time (over 5 minutes). On a long stretch I usually get up to max speed, listen to it scream for a minute then release the clutch and throttle and coast down to 15mph and do it all over again. Over, and over for about 90 minutes strait was my longest continuous run so far.

I don't think Max speed will kill the HT in a few minutes but It's definitely not good for the long term. While 26MPH may be my top seed, it feels like there is next to no load on this motor. I know this motor can handle way more torque as I've reached top speed even going up a slight incline. The motor even sounds better going up hill. That's why I'm installing a 36 tooth. I should be able to do 26 mph no problem without making the motor scream and increase my MPG. Any tweaks that just increase RPM will only shorten the lifespan of your motor. For speed, smaller sprocket is the only way to go.
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