New motor randomly revs a letting off throttle

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Rusty_Nail

Active Member
Been starting to do my break in riding on a new 100cc. Seems to have plenty of power even still in break-in. The problem I'm having right now is randomly when I let off the throttle and pull in the clutch, the engine will start to rev up a bit but it doesn't do it every time, far as I can feel, all bolts are tight shy of stripping them so can't still be an air leak far as I know. Need some ideas to try to attempt to resolve this small problem. I've checked everything I can think of and it still happens at random. I bought a new carb for it. Could it be a jetting issue? it has an M65 clone and a high compression head since the original head came warped and wouldn't seal. Sometimes it drops to idle perfectly. Any help is appreciated.
 
Rusty_Nail said:
Been starting to do my break in riding on a new 100cc. Seems to have plenty of power even still in break-in. The problem I'm having right now is randomly when I let off the throttle and pull in the clutch, the engine will start to rev up a bit but it doesn't do it every time, far as I can feel, all bolts are tight shy of stripping them so can't still be an air leak far as I know. Need some ideas to try to attempt to resolve this small problem. I've checked everything I can think of and it still happens at random. I bought a new carb for it. Could it be a jetting issue? it has an M65 clone and a high compression head since the original head came warped and wouldn't seal. Sometimes it drops to idle perfectly. Any help is appreciated.
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This exact issue happened one time on my buddies pitbike. The problem turned out to be the spring that was supposed to hold the throttle slide down under the cap wasn’t fully seating in the barrel on top of the V-shaped bent piece which pushes down on the E-clip on the needle, which allowed the needle to move up and down slightly, enough to cause different idle speeds randomly. What I did was put a few small washers on top of that V shaped clip to take up the slack.
 
I tried riding it again today and the same thing again... bike runs fine when the clutch is engaged, but soon as I pull in the clutch, it will randomly either go back down to idle normally, or it will rev hang a little then go down to idle or the other thing I've seen it does is just rev up. I've also checked the needle clip and its on the center notch. But it never does the same thing every time so it's making it difficult to diagnose. Everything is tight so don't see how there could still be an air leak. Unless this is a known problem with new motors? Only upgrades on it is a high compression head and expansion chamber with silencer.
 
I may be wrong, but I swear I saw this same post somewhere else. The op was not pulling the clutch before braking. Then pulling the clutch while still on the throttle. Let off the throttle. Then pull clutch. Then brake. Almost simultaneously
 
OK. So I tried an idea and it seems to be working so far. Doesn't rev out constantly like before. I mounted up my old carburetor which was still working fine. It's an NT Speed carb. This is the third engine it's been on and it still hasn't let me down. The new carb I bought did nothing but leak fuel like no tomorrow. So the only thing is I had to take the intake off the Triple 40 I had that seized and put it on the 100cc so the carb would fit on the intake. It seems to work a lot better. I've also been noticing that when I'm riding and let off the throttle the bike feels like it's bucking until I pull in the clutch is how it would describe it. What causes that if anyone knows? Other than those two things, I'm starting on my second tank for break in so if all goes well I can't wait to see what this thing can do!
 
Bucking is usually seen on high compression engines, I’ve got a 66cc spare engine that does it pretty bad, it runs great till you let off the throttle! I balanced the crankshaft and it didn’t change the bucking.
 
Is it bad for the engine to do that or will it not hurt it? That was my main concern. I thought it was because I have a spring chain tensioner on my bike that actually turned out to work out well for me.
 
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