No power, bad vibration

[Tim_B_172]

So I wanted to run this by you all as I am at the end of my wit. My Dads bike engine is having a weird problem. It's a Grubee Starfire 48cc BTW. Anyway a little history first, he had this engine on a bike with a shift kit from Staton Inc. and it seemed to work fine there, but after experimenting with the shift kit he decided that he didn't like it and it was too much trouble. So he set the bike up the standard way, directly driving the back wheel with the rag joint etc. Anyway, at that point we noticed that the engine develops very little power and vibrates insanely at high RPM. I mention the shift kit because while using it the engine usually stayed in the lower end of the RPM range and maybe the extra gear reduction was making up for the lack of power the whole time. We tryed a few different things and ended up checking the muffler for clogs, no clogs found. But we decided to just try running the engine with end cap off of the muffler and voila! it sounded like a string of fire crackers going down the street but all of the power and speed was back where it should be. It still vibrated a lot though. So we tried a different muffler that I had lying around and the power was gone again, ran it with no muffler and the power was back. So what's going on? I have a few theorys, but I'm not an expert on 2 stroke engines. I'm thinking that maybe the piston rings are bad and the extra back pressure from the muffler is causing ring blow-by and thus reduced compression? Maybe?

If we can't figure it out it's not the end of the world. Dad already has a new engine coming for a chopper bike that he is building so he will be up and riding soon anyway, we just want to know what happened.

 

[MikeJ]

I will toss out some comments. Maybe the muffler has a bit too much restriction? I'm not sure how to clean out a muffler.

Vibration and shaking.... I learned to tighten the mount bolts, especially bolts that are hard to get to. The harder the bolts are to reach, the greater the chance they are the ones needing tightening. If you hit the resonance range of the bicycle frame at higher RPM, try tie-wrapping a weight (like a two-pound cloth bag of buckshot) to the downtube. That will move the frame's resonance range into the much lower RPM range. (This is only a thought; I have not tried this. I keep my RPMs under that where severe vibration occurs.)

MikeJ