Stihl 075 fuel tank building pressure

[Kfd518]

First time out milling with this saw… cut great for first and second cut, sputtered a bit and died. Hissing from fuel cap vent hmmmm, attempt to remove cap and sprays fuel at me. Release slowly… fuel inside tank is bubbling! Saw cranks promptly dies. Rinse repeat, x2 or 3 times had enough quit for the day. Ethanol free fuel 91 octane 35:1 stihl hp ultra. both air and fuel filters fresh out of packages. Got home cranked back up and runs fine again. Any ideas?

 

[john taliaferro]

I run 93 with 50 to 1 ultra . Sounds like the coil is weak if you had a tach i bet its dropping spark , sometimes they will act out warm and run perfect cool . Post it on the chainsaw forum they can help better dignose .

 

[BobL]

sounds like a gummed up carby that self cleared.

 

[Kfd518]

New filters fuel and air, flywheel in tact, fins as clean as I can get them did that after posting. Weren't to dirty to begin with surprisingly. Retuned from scratch, 1 turn out from seated high needle 1.25 out from low side needle. High side has been fattened up 1/8-3/16 turn slight burble at full throttle haven't had chance to get back in wood yet. Cap allows pressure in and out for some reason. Carb is freshly rebuilt and did well cutting a dead oak with plenty of solid wood left still after rebuild new impulse and tank to carb line waiting on new pickup line to come in and may go on ahead and order new fuel cap as well .


Would the old heat the coil in the oven kind of like testing automotive coil help in diagnosing it and check primary secondary resistance tests while warm?
Probably a no no but when I didn't get a response on this one I hit the chainsaw forum up already…

 

[BobL]

Humm, have you done a crank case pressure test? It could have a heat sensitive leak so you should do the test both before and after warming up. If there is a leak from the crankcase into the fuel tank it could be pressurising the tank.

 

[Kfd518]

Humm, have you done a crank case pressure test? It could have a heat sensitive leak so you should do the test both before and after warming up. If there is a leak from the crankcase into the fuel tank it could be pressurising the tank.

Correct me if I'm wrong but isn't the Tank completely isolated from the crank case except through impulse line to carb via tank to carb fuel line? Tank/ starter housing are integral pieces with gasket between which in turn is held on with 4 bolts. And if this was the prob wouldn't it likely have blown out the diaphragm in the carb due to pressure from crank case leakage making pressure instead of vaccum?
Just trying to straighten things out in my mind and correct my theory of operation if I am wrong on how things work on these older saws.

 

[Yellowbeard]

I had this same thing happen to me last summer. Saw was running so hot that the fuel was boiling. Mine turned out to be leaking around the crank seals so that it was running way too lean which was making it really hot. In the summer it would just keep dieseling after I switched the power off.

New seals seem to have fixed it, but I am having similar issues with starting it when it's hot. Cool it works fine but hot it seems to give me trouble.

 

[Kfd518]

Since I have finally gotten a vac/ pressure tester this the current diagnosed problem. Figure while she's down I'm gonna repaint and clean everything up while she's stripped down that far.

 

[Kfd518]

Still waiting for oil pump worm oil seal to come in…stihl 9642-003-1440 backordered from stihl as well as the anti vibe mounts under the handle assy. 2-3 week availability 1.5 weeks down can't re-test till it get is hopefully this is the problem. Went ahead And replaced fly wheel side and clutch side main oil seals while in there.