Vintage Dirt Bike Q & A

Hey guys,
I fiddled around with different main and pilot jets on the 34mm VM Mikuni. I am now trying a 310 main and 35 pilot jet with a Q-6 needle jet. I have the needle at the lowest setting (leanest) and it runs pretty good. I think it would be better if I went with a leaner needle jet, maybe a Q-2 and raised the needle up a clip or two. Anybody out there know what an appropriate needle jet size for this application would be?

I also noticed a strange rattling sound coming from the engine at all RPM's. There is no noticable vibration - just the rattle. I changed the piston, wrist pin and wrist pin bearing so that is not the problem. I guess it could be the needle bearing on the rod at the crank. I don't know. I'm not familiar with the old 2-strokes (or new ones for that matter) and it may be a normal noise for all I know. I do know that it is coming from inside the engine and is not some attachment rattling. Anybody have any input on this? I could post a sound clip if that helps. I am thinking it may be best if I tear down the engine and check everything before I break something! I am just having trouble believing it is a bearing issue since the bike has such low hours but who knows.


thanks,
Charles in New Orleans

Does this rattle appear when reving while static or when your riding ??

If its static then that sounds like piston slap (you should have some noise anyway with the motor being aircooled)

Is the bore still using the origonal chrome ?? I'd be inclined to have the cylinder Nikasil plated with a new piston. If you've been fitting a new piston to an excessively worn bore you will get piston slap.
While you've got the cylinder off check the big end bearing,you should have no lift at the rod ,get the rod vertical and just try lifting it up and down.

If its whilst your riding it could be pinking (pre ignition), check you using the correct octane fuel, check the timing isn't to advanced,you maybe running to lean,check you using the correct plug.