Stihl FS 85 loses power after a few minutes.

[John from Cle Elum]

I am clearing a lot of area with an FS85 brush cutter running a tap-to-feed string canister with .090 gaterline. Pretty heavy load for the FS85. After a few minutes of running it slows down and eventually quits. If I leave it sit it starts right up after 10minutes. I think the mixture is ok. Basically I lean it out as fast as it will go and then richen it until the speed starts to fall. It is hard to tell with that large canister on it. The head has quite a few hours on it. Running 40 to 1 mix using Husquvarna synthetic. Plug is the right shade of tan. Compression feels a little light but ok. I probably should measure it with a guage. I am thinking it may be the tank is not venting but that is my last and only idea.

Any other ideas??
John

 

[ZeroJunk]

If it is the tank pulling a vacuum you should be able to loosen the cap and the power come back instantly or close to it.

 

[John from Cle Elum]

Not yet looked at it. That is an easy thing to check. I saw a similar thread and it pointed to vapor lock. I am using ethanol fuel at 3000ft alt. Probably should find some real gas.