1982 YZ490 trail machine

Plus one.

I had that same bike back in the 80's.

I think that long silencer may be causing you some tuning issues.

Also make sure the packing is not saturated with oil.

I ran a DG pipe on mine and put some air box filters in it to let it breath better those 490's need a lot of air. Once you get the jetting figured out it will run great.

Yes I'll try some bigger main jets. The silencer could now be soaked with oil. I'll check and maybe repack it. It's pretty easy to do, just a snap ring holds it all together. The pipe is a DG, I could get a DG silencer for it. It would be better if I could find a silencer with a spark arrestor. I though about opening the air box. With the porting done, it really wants to rev out more. Maybe it needs more air too.

I removed the 420 main jet and replaced it with a 440 main jet. It runs a lot better. I tried holding it at different throttle settings and it ran steady and clean at all, except 40 percent was not as smooth as the rest. I might be able to tune that out with the air screw.

40 percent should be needle territory