Engine Trouble Terrible vibration

[Maxwell]

I bought a engine kit about two weeks ago and assembled it without any problems but recently I have been having terrible vibration in the engine at around 10 mph but it runs fine at anything under that. I have checked the mount and it is very secure. It is a 49 cc with all stock parts. what do I do now?

 

[crassius]

if it is the old unbalanced crank, that vib usually starts at about 25mph with a 41T or 44T sprocket - otherwise if motor is tight in frame and frame is not cracked, it might just be so rich that it is 4-stroking really bad

 

[Maxwell]

I adjusted the carburetor to make it run more lean and the same problem still occurs. I can post a video if that would help.

 

[crassius]

that leaves only looseness somewhere to cause shaking - I'd be really surprised if it had enough power to shake the whole bike rather than just the motor

you didn't use rubber for any mounts did you?

 

[Maxwell]

I'm not using rubber mounts for anything and the frame is super solid.
Here is a video of it running hope it helps: https://photos.google.com/share/AF1...?key=cWR4Um12MmtGTXI2TDZnYkR6SlVNNVFyUlF4OGVR

 

[crassius]

all I get is a picture of what appears to be a 66cc

 

[FurryOnTheInside]

Idk if the 49cc needs extra balance holes drilling in the crank? I have been advised that the no-name 66cc really needs them. Is your engine a Grubee or a no-name generic non-Grubee?

When the subject of drilling extra crank balancing holes was being discussed, I was advised to not attempt to split the case and remove the crank in case I knocked it out of true, and being ill equipped to retrue the crank.
A crank can be splayed, pinched or twisted at the big end pin where the two flywheels are joined. I think the flywheels can be off centre too (?) Some posts here have referred to these problems being present on brand new engines anyway.
I dont think there's a lot you can do about it if this is the case, but I would be checking by a very basic method, just touching something against the flywheels to see if the move it up and down when the crank is rotated by hand. Idk but this might show up if it's grossly out of true (?).


Also, (this is probably not the reason for the vibration, but) the lower end of the exhaust muffler should be supported to the frame so it doesn't snap the studs in the cylinder when the vibration shakes it.

 

[Maxwell]

all I get is a picture of what appears to be a 66cc

Here is a new link:

 

[Maxwell]

Idk if the 49cc needs extra balance holes drilling in the crank? I have been advised that the no-name 66cc really needs them. Is your engine a Grubee or a no-name generic non-Grubee?

When the subject of drilling extra crank balancing holes was being discussed, I was advised to not attempt to split the case and remove the crank in case I knocked it out of true, and being ill equipped to retrue the crank.
A crank can be splayed, pinched or twisted at the big end pin where the two flywheels are joined. Some posts here have referred to these problems being present on brand new engines anyway.
I dont think there's a lot you can do about it if this is the case, but I would be checking by a very basic method, just touching something against the flywheels to see if the move it up and down when the crank is rotated by hand. Idk but this might show up if it's grossly out of true (?).


Also, (this is probably not the reason for the vibration, but) the lower end of the exhaust muffler should be supported to the frame so it doesn't snap the studs in the cylinder when the vibration shakes it.

It is a generic engine with no markings on the box it came in except saying that it was 49cc and that it was made in china, I can get the link to the exact one I purchased on amazon but I don't think it make a difference since they all look like they come from the same factory. Also do you think the crank could have been splayed, pinched or twisted during the break in period because it didn't run like this at the start so the crank wasn't messed up out of the box.

 

[FurryOnTheInside]

It is a generic engine with no markings on the box it came in except saying that it was 49cc and that it was made in china, I can get the link to the exact one I purchased on amazon but I don't think it make a difference since they all look like they come from the same factory. Also do you think the crank could have been splayed, pinched or twisted during the break in period because it didn't run like this at the start so the crank wasn't messed up out of the box.

No I don't think it can go out of true by itself, unless a bad bearing can do it (?). I'm really no expert, I am just reading the forum while building my bike and it is taking a lot longer than two weeks.
Support the muffler anyway before that thing snaps, though.