Stihl 4 mix engine troubles

[Lawn man]

I'm having an issue with over revving of engine when warm. Any advice out there?

 

[davhul]

How do you know it's over revving? The rev limiter is about 10,500

 

[Lawn man]

How do you know it's over revving? The rev limiter is about 10,500

Sorry I should have been more specific, the throttle works fine until engine gets warm and then just starts to rev up even when I am not trying to and the cable is not sticking in full throttle position

 

[davhul]

Can't think of anything specific. Could be a air leak. But weird only when hot. If you could post a vid more people might can help.

 

[Lawn man]

Can't think of anything specific. Could be a air leak. But weird only when hot. If you could post a vid more people might can help.

Thanks I will if I can't figure it out tomorrow, already had enough of it for today lol, possible ethanol in gas ruined my carb but styhl supposedly took care of that, I have adjusted valves once already for a hard starting condition mabey they need it again but I will have to get the tool from stihl as I lost the first one

 

[davhul]

Ok. Did you put a valve cover gasket on too? It needs replacing when you adjust. You know you need a valve adjustment when the rope gets harder to pull.

 

[Lawn man]

Ok. Did you put a valve cover gasket on too? It needs replacing when you adjust. You know you need a valve adjustment when the rope gets harder to pull.

No I did not it seemed ok but later read that you should every time, mabey its suckling air I will try that
Thanks

 

[davhul]

That's a good cheap start. It compresses when its old and doesn't seal. There is negative and positive crank pressure there so it's a good chance it leaking.

 

[HuskStihl]

@lone wolf seems smart about these. Maybe he'll wander thru

 

[lone wolf]

Sorry I should have been more specific, the throttle works fine until engine gets warm and then just starts to rev up even when I am not trying to and the cable is not sticking in full throttle position

Sounds like it could be you dont have enough slack in the throttle cable or an airleak. Does it do it at idle or high rpm?

 

[earlthegoat2]

I've had a similar problem like this that turned out to be a bad throttle return spring on the carburetor. I e also had simlar when the throttle cable gets worn and sticky. Also when the cable is not adjusted properly AND the carb is not adjusted properly. A bad clutch that is slipping when hot but not when cold has caused this problem as well.

I'm guessing an air leak somewhere as well. I've not changed valve cover gaskets after a valve adjustment hundreds of times without a known problem. I know Stihl recommends it and it is just prudent but I will just say it is unlikely to be causing this problem.

 

[Ted Jenkins]

Air leak. Double double check intake gaskets. Thanks

 

[Lawn man]

Sounds like it could be you dont have enough slack in the throttle cable or an airleak. Does it do it at idle or high rpm?

Thanks everyone for the insight, I will try to get a new gasket today, I don't think it's the throttle as it seems to snap back just fine even when hot
Once I use it for a couple minutes it will go to almost full throttle all by itself and hang there for a minute or so and idle back down and then go back up and stay there all by itself

 

[Streblerm]

The impulse hose was the source of a massive vacuum leak on my km110. Sounds like a vacuum leak is your problem. I thought the hose was formed and bought the Stihl part but it was just a short straight piece of hose.

 

[lone wolf]

Thanks everyone for the insight, I will try to get a new gasket today, I don't think it's the throttle as it seems to snap back just fine even when hot
Once I use it for a couple minutes it will go to almost full throttle all by itself and hang there for a minute or so and idle back down and then go back up and stay there all by itself

I still suspect the cable.

 

[Lawn man]

I still suspect the cable.

Thanks again for everyone's help, turns out I couldn't get a valve cover gasket over the weekend so I put some gasket sealer on it witch didn't help but in the process of taking the plastic cover off I loosened a screw that seams to adjust cable tension and that solved the problem , I never would have guess there was a throttle tension adjustment
Thanks again

 

[lone wolf]

Thanks again for everyone's help, turns out I couldn't get a valve cover gasket over the weekend so I put some gasket sealer on it witch didn't help but in the process of taking the plastic cover off I loosened a screw that seams to adjust cable tension and that solved the problem , I never would have guess there was a throttle tension adjustment
Thanks again

 

[Vintage Engine Repairs]

Sorry to bring up an old post, but I’ll take any opertunity to learn. Lone Wolf, how did you figure that was the issue? I’d have thought a tight / improperly adjusted throttle cable would maintain a set rpm rather than fluctuating as the op mentioned? Thanks in advance!

 

[lone wolf]

Sorry to bring up an old post, but I’ll take any opertunity to learn. Lone Wolf, how did you figure that was the issue? I’d have thought a tight / improperly adjusted throttle cable would maintain a set rpm rather than fluctuating as the op mentioned? Thanks in advance!

Because I had an HT 131 here that did the same thing and I could feel there was zero slack in the cable. If you work on the same things you get a feel for everything that is right or wrong.