A bike issue. Loud noise.

[Rambunctous]

My dirt bike started making a loud screeching noise while in 1st and 2nd gears and when I push it when its not running and in neutral it also makes the noise. Also when its running and in neutral it does want to roll forward slightly. It seems to shift fine and never had to be forced or jammed up. The bike is a 1988 Yamaha YZ 250. The transmission oil is new and full. Any hints / ideas?

[odduc]

Sounds like the clutch plates might be sticking. You may want to open it up and check.


OR, adjust the clutch cable.

[Rambunctous]

Do you know if motorcycle gears have synchros?

[pcakes55]

Pretty sure no motorcycles have synchromesh gears

[Kurlon]

By definition a constant mesh setup doesn't have, no do I believe it could even utilize synchros.

[Rambunctous]

The noise disappears as soon as I shift into third. What is the common denominator between 1st neutral and 2 gear?

[TheIglu]

Gears. You may of lost a bearing in your tranny.

[Rambunctous]

I think your right. I have split cases before but never touched the gear box. I guess I will be learning something I don't want to learn.

[stoinkythepig]

By definition a constant mesh setup doesn't have, no do I believe it could even utilize synchros.

Car transmissions are constant mesh too.


The synchros are part of the dog clutch assemblies (purple). All they do is ensure that the two mating shift dogs are at the same speed before they can attempt to engage. The 1-2 synchros take the biggest beating because the speed differential is greatest between those two gears.

Bike don't need them because the gear ratios are much closer together, so the speed differentials between gears are are small; and the parts are much smaller, so they have less inertia to overcome.

[aldend123]

I thought it also was that motorcycle gears are straight cut (hence the supercharger-like whine) while car gears are helical.

But for the record, I'm not seeing how that that matters right now, more of a question.

[stoinkythepig]

I thought it also was that motorcycle gears are straight cut (hence the supercharger-like whine) while car gears are helical.

But for the record, I'm not seeing how that that matters right now, more of a question.

You are right, regarding synchros, the gear style does not matter. Helical gears are only used to reduce noise (and maybe to add strength, though the difference is negligible). They are not desirable, otherwise, because they generate side loads (thrust) that have to be dealt with and that means shims (much more complicated assembly since shims have to be selected, installed, verified, and swapped if they are incorrect) and thrust bearings. Bikes can have straight gears because they make so much other noise while moving that the whine of the gears meshing is drowned out.

[SVRACER01]

usually 1st gear is straight cut. that why it whines when you downshift into 1st