Stihl HL94 engine racing

[vrex]

Have a set of long handled sheers at the tree service i work for. We've tried debugging everything we could think of, new spark plug, air filter, even changed the clutch. The sheers will be running along as usual, then rev up for 1-2 seconds then back to normal. It'll do this at intervals, maybe 30 seconds maybe more, I've not timed it. I figured if it was a fuel filter it would bog and have no power, if it was the air filter itd run rich (it does run a bit rich, I will say that, little oil around the muffler exhaust). But the racing throws me. Bad coil? Like, it gets hot and changes resistance and runs off?

 

[cus_deluxe]

crack the fuel cap when it starts to act up and see if that does anything.

 

[Duce]

Have you done a vacuum check?

 

[stretch5881]

Racing is usually a sign of an air leak. Needs a vacuum test.

 

[vrex]

its not racing consistently, a zing here and there. Does that line up with an air leak?

 

[stretch5881]

Yup, an air leak leans the mixture out. By the time it's racing constantly, the piston is close to becoming toast.

 

[vrex]

makes sense, intermittent air leak somewhere. I get that it would cause a lean condition, i've just never heard of an air leak developing slowly, more of an it does or doesnt leak air and roach the cylinder. I'll advise it get sent off for a pressure/vac test.