Vibration after 24 mph WHY!!!!!

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showcaller

Member
Mar 9, 2011
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6
Los Angeles
I have a 49 cc china girl on a beach cruiser. I have done several upgrades, mostly from SBP's. (exhaust, U Mounting bolt for engine, spark plug)

Up to 24 MPH everything is fine. She just purrs along, minimal vibration. Once I hit 24 MPH it's like the world is becoming unglued... the vibration is so intense I can't even stay on the seat. I know she will do 28 MPH and probably more but I don't dare give it more gas after 24 mph.

I have checked and rechecked and rechecked everything and from what I can tell everything that is supposed to be tight is.

Any ideas here?

Thanks

Sho
 
ocho ninja

ocho ninja

Member
Jan 14, 2012
564
2
16
San Jose, CA
The manufacturing process and the crudely balanced flywheels that are partly at fault. Also make sure the the mounts are sitting flush with the frame with no pinch points, that could also be factor.

Mine behaved much the same way, i would be fine up to a certain speed and revs... then it would start to vibrate so bad that i was certain the bike was gonna come apart.

But after break in and switching over to a smaller sprocket, all is better now.
Smaller sprocket gives you more top end speed, but also helps you cruise along at lower rpm = less wear tear and longer life for your china girl.
 
2door

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
If you have any rubber or other resilient material in the engine mounts, remove it. The engines are designed to be firmly mounted in the frame. This means the engine mounts must have good contact with the frame, no gaps or air space between the two.
Make sure your sprockets are precisely aligned, the chain is tensioned correctly and the exhaust/muffler is secured in a way that keeps it from vibrating also.
As was said above you'll notice an improvement in the way the engine runs after break-in and decreasing the rear sprocket size will help unless you live in a hilly area and need the lower gearing for climbing.

Nevertheless keep in mind that these little 2 strokers vibrate. Some worse than others but they all will produce some vibration. It's the nature of the beast.
Use the Google Custom Search feature, keyword 'vibration' and you'll have plenty to read on the subject.
Good luck. Keep us posted.
Tom
 
glennbo

glennbo

Member
Aug 24, 2010
347
13
18
HAMMOND
on my two strokes i use a 36 tooth sprocket if you have done everything right with your install and it still vibrates like **** gear it down less rpm less vibration smaller sprocket will do this and increase speed. only bad is hill climbing will suffer with less torque.if your running with little vibration at 24 on a 41tooth you should run 29 with a 36tooth with less vibes hope this helps ya

glenn
 
Mannhouse51

Mannhouse51

New Member
Jun 2, 2011
277
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West Michigan
My bike got some serious vibes goin at around 24 too . I had a 44 tooth sprocket . I went to a 41 tooth and now I cruise at 25 - 28 nice and smooth . I think I'll take some of this great advice and go to a 36 tooth . Yeah man , a nice 30 mph + sounds great.
 
rustycase

rustycase

Gutter Rider
May 26, 2011
2,746
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Left coast
I think you should try that option, if the marginal power you have has been acceptable , so far.... try dropping to a 41T sprocket and see if your acceleration and hill climbing remains good enough.

We all have different circumstances and needs.

Good luck
rc
 
Al.Fisherman

Al.Fisherman

New Member
Sep 9, 2009
1,966
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0
Calera, Alabama
I had one frame that vibrated so bad that I would have tingle in my fingers for 15 minutes after I got off of it. Changed frames (mounted the engine the same), and the vibration was gone.
 
rustycase

rustycase

Gutter Rider
May 26, 2011
2,746
5
0
Left coast
...the best of questions! Are you sure it's the engine vibrating? ...

On mine, the poor quality chain, mangled by repeatedly being thrown off and bound up by a poorly mounted rag joint and tensioner, would definitely produce a lot of vibration at higher speed.

There are good reasons to purchase a hub adapter and quality chain.

If you DO make the determination it is the engine vibrating, MSRFAN wrote a thread on how to balance a little ht crank.

Static balancing is not the best, but it can be done without removing the crank from the cases, and would at least show if your engine is badly out of balance so you could make some improvement.

Best
rc
 
F

flmotorbikes

New Member
Mar 23, 2012
174
1
0
Florida
I would make sure all your mounts are tight. Once you reach a high speed the engine RPM is increased causing more vibration then at idle
 
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