Danger of working on chainsaws...

[mrcaptainbob]

It's the ongoing issue with my Stihl MS390. Was a 310. CaNOT get it to run! I thought I solved the fuel tank pressure issue, but.... The bar is off, clutch was removed in an effort to see where the oil film is coming from. Took it outside and pulled several times, it started, but ran pretty poorly. A few more pulls and nothing. Took it back to the bench and started removing the carb. Was coming off okay, except the fuel line kept a grip on the inlet tube. Holding the carb in the right hand, used left thumb nail to slide the fuel hose off when...there was a SOLID STREAM of fuel/mix that SHOT up my nose, beneath the eyeglasses and into the right eye!! I immediately redirected the sasw and just let it lose pressure. Was a quite a bit of fuel spurting out. (Rushed back to the house, yadda-yadda-yadda...all is well. Doc gave me a clean bill of health.)
My question is: What in the word is causing such an extreme amount of positive fuel tank pressure??
I'm at my wit's end and about ready to chuck this thing!

[lxskllr]

Heat? The tanks have a vent I believe. If yours is defective, I could see it building up pressure.

[mrcaptainbob]

It 'ran' for all of maybe ten seconds. Not near enough to build that kind of heat.

[lxskllr]

I was thinking ambient conditions. Same way plastic gas cans puff like baloons in the summer.

[mrcaptainbob]

Well, it was a hot day. But I was working on the bench in the shade of the barn. Hard to imagine it would build that much pressure with temps at 90*'s.

[Old saw fixer]

I could have missed something, but all the vents in hand held two stroke equipment I have worked on were one way, to let air in the tank as the fuel is used.