Vibration out of control

G

George Tirebiter

Member
The vibration has gotten progressively worse over the last few weeks on my 66cc china girl. I now have about 275 miles on it. I raise the needle clip up one notch but it still misses a lot especially from the 10-20 range. It feels like I'm riding on a jackhammer. I took off the front motor mount and re-tightened it and everything else. Even the gas line from the gas tank shakes like crazy. I greased the drive sprocket, the right side gears and the chain too. Not sure what else I should do. Any ideas? Thanks,
George
 
First question. What engine do you have?

Second: Are you using rubber or any resilient material in the engine mounts?

Third: Is the engine mounted securely, solidly to the bike frame? Do you have good contact betwen the engine mount and the frame, no large air gaps?

Forth: Are all the engine mounting fasteners in place and tight? No broken or missing fasteners?

Tom
 
if one can grab the top of motor and the bike frame near the tank and not be able to push or pull the motor at all, it should be tight enough

that leaves a cracked frame as the cause or a pre-2012 motor that had the standard 'bone shaker' crankshaft
 
It's a 66cc, with a 40t sprocket. No rubber used, and everything is super tight. The attached pic is of the rear mount and as you can see there is a gap between the spacer and the seat tube. A few days ago I took off the front mount to inspect it and I adjusted the angle to be more level. Well, that just made the vibe even worse, but the engine is on tight as a drum.
George
 

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George, did you take that photo with the back part of the mount off? If not then you are missing the part that wraps around the other half of the mount. Also, That gap is likely a part of your vibration issue. You will have to sand that mount to match the angle of the seat tube so that it fits flush.

The vibration has probably become more noticeable because your engine is broken in and has gained power.
 
crassius said:
if one can grab the top of motor and the bike frame near the tank and not be able to push or pull the motor at all, it should be tight enough
Click to expand...
Exactly crassius, that is THE test.
If your your engine moves at all pushing as hard as you can that is fail.

that leaves a cracked frame as the cause or a pre-2012 motor that had the standard 'bone shaker' crankshaft
Click to expand...
Yep.
An unbalanced engine will tear itself apart.

The thing is, the $100 kit engines are that cheap for a reason, they use the cheapest parts possible inside and typically about the time you get them broken in fuel mix wise, those crucial bearings are already worn in past their useful life and have way too much play in them, which is exasperated with an unbalanced crank to begin with.

Hopefully that is not what you have, and you just need to get the engine to be 1 with the frame.

I'll cross my fingers for you.

One side note...
Ignition timing could be a factor, just not a likely one.
 
:) That explains a lot.

The "missing" part is a metal strap that straddles the frame tube and has a semi-circular area that should fit the frame and two holes to allow those studs to be used. It essentially 'squeezes' the frame between it and the front part of the mount which you have.

Good luck and let us know how things go once the engine is secured.

Tom
 
Well, George, I'm glad that it's a simple fix. When you speak about getting it level, are you meaning to get the carb level? Having the carb level is the ideal. Can you post a photo of your front mount just so we can check that over to see if that mount is flush with the frame.
 
that motor won't stretch across that gap no matter how hard you tighten it once you get the strap on - cut a "C" shaped section off of a PVC pipe and stuff it between the mounting block & the frame
 
Good eye Crassius, I only glanced at the photo and it seemed off kilter. Closer look shows a pretty parallel space. I would suggest abs pipe which is harder but, George, if you can find a metal pipe to cut the spacer out of that would be the best. Or flat metal with appropriate spaced holes that you can put between the mount you have and the engine case.

keep em flying Tirebiter
 
The pipe idea will work great! I had a lot of space between the rear mount and frame, and I have one of the GT2-A frames with the welded-on front mount. What I ended up doing was spacing the mount farther back with longer studs and some washers between the engine and mount. Still a very solid mount, and I have hundreds of miles on it with very little vibration at all. I feel pretty lucky that the little 48cc engine I bought was so well balanced.
 
I was trying to get the carb level when all of the vibration problems got so much worse.(Duh!) I tore it all down today to inspect everything. When I get all the right parts together I'll post some pics. Thanks,
George
 
Take a section of PVC pipe (maybe 1" diameter?) And cut it so that you have a 1 or 2" piece. Now take that and cut it lengthwise. Now you have a spacer to fit between the frame and motor. That way (when the back piece shows up) you can bolt it up and there won't be any gaps.

You could also make your own strap for it. Remove one of the studs and take it to a hardware store and get the right size nuts. The back strap is only a piece of bent metal with some holes drilled in it. Making one would probably be faster than ordering one.
 
Agreen said:
Now you have a spacer to fit between the frame and motor. That way (when the back piece shows up) you can bolt it up and there won't be any gaps.
Click to expand...
You don't need nor want anything between your engine and the frame.
Those frames are designed so any 48 or 66 engine will drop right on and fit.

Bummer you were shorted your back bride, double bummer I didn't catch that as I have a plethora of them in a box.

They also make for dandy light stand-offs for separating hot components like a DVR and Blu-ray disc player and Amp.

2SbackBrace.jpg


When it comes to carb angle you don't adjust that with engine mount.
That brace needs to mate perfectly square at a 90 to the seat tube.
That frame will, or least should.

All your torque to your back wheel is carried against that mount block, if it isn't flush you just have an edge against the seat tube that won't want to play friendly with the seat post.

You'll see when you do the 'Shove Test' how done properly, your engine is one with the frame.http://motorbicycling.com//www.pinterest.com/pin/create/extension/
 
Thanks for the info. Very helpful. BTW, is it Ok for the right side gear housing to rest on the chain guard? I've attached some new pics. Hopefully everything is right. I did the grab and shake test. It's solid. Let me know what y'all think. Thanks,
George
 

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