250cc valve lash not holding
 

I have been working on an RPS Tornado 250. Motor would not start because the valves were so tight. No biggy, lashed the valves. Started right up. The next morning it won't start. Valves are locked right again. Ok... Set the lash again. Starts up. Sits overnight, very hard to start in the morning. Rode for a bit and now it won't start at all.

What would make the valve lash continually go out after one ride?

You should be setting the valves when the engine is cold for the best results.

10-4, engine was cold. It starts fine until it gets cold. Then I have to set the valves again.

The only other thing that could be happening but very rare is the the valve seat might be getting hammered into the head. It has happened but I doubt that's happening though. Are you tightening the locknut tight enough?

I keep wondering that, but ya I'm putting some ass on it for sure. I work on a lot of 150cc engines and never had an issue with this. Next step is to yank the head and see what's going on. I was just hoping to know what I might be looking for before I do. Thanks for the replies!

I agree with Jerry that it must be the lock nuts. I don't think that valve seat damage can happen that fast. :hmm:

It is possible that the threads on the threaded tappet in the rocker arm and/or locking nuts are worn out. When you loosen a nut, does it feel like it wobbles a little on the threads? They should not, at all. Maybe you can just replace the nuts?

If those tappets are gone, that just means new rocker arm assembly-about $15-$20, depending. Not expensive. On the ZS 172FMM, there are feet on the tappets with a universal (ball joint) to allow flex. You might have rollers on the rocker arms. It might be just impossible to find the tappets and nut assemblies separately. I see them on AliExpress for like $1, but I shy away from AliExpress for anything I really need now, and correct! Worth a gamble at $1, unless they take 6 weeks and aren't the correct ones when they arrive!

How many hours/miles on the engine?

You could work on the head this Winter, maybe install a new cam while you are at it, and maybe do a little porting work.

Just wondering, can you use loctite or nylock nuts on those lock nuts?

No. Those parts are oil bathed. They rely on machining of threads, good parts, and especially on not getting abused. But they do just wear out.