pioneer p26 fuel tank pressurizing

[Navara]

Hi guys. I have a problem with a pioneer p26 chainsaw that is pressurizing the fuel tank. It works fine for about 5 minutes then pressure starts to build in the tank. The pressure is coming from the fuel pick up pipe in the tank. As this is fizzing bubbles out of the filter. Have checked all the usual things blocked fins, pipes, filters etc. Stripped carb and everything seems ok there to. Would be real grateful for any help as saw is still in good condition.

 

[ehp]

check the vent for the tank, it is plugged,

 

[Navara]

tank vent has one way non return valve in vent pipe. this only lets air into tank and will not release the pressure build up. have reversed the valve in the pipe but get fuel flooding out of pipe as pressure builds.

 

[tony marks]

well thanks navarro.. i got one saw that does that .. looks like im taking a leak whem im usin it.. not to mention being dangerous.. dont recall if it the p26 tho ..
im thinking its my farmsaw.. 1st cup an all..

 

[ehp]

yes you will get pressure
when the saw warms up the heat from the engine heats the gas because the fuel tank is together with the crankcase not like a husky or stihl
i have seen those saws boil the gas in the tank when ran hard and long, the 922 homlite is another saw that does the same thing

 

[Navara]

I don't think it is the heat from the engine pressurizing the tank. It seems to be getting back pressure from the carb as the filter on the end of the fuel pick up pipe is bubbling. It keeps bubbling even when the saw is stopped and gradually the bubbles stop coming out of the filter after a short while.The saw runs fine for 5 minutes then as pressure in tank builds the saw dies out and stops even when idling. Thanks for your help ehp any other ideas?

 

[NevadaWalrus]

Some newer Stihl's only vent to let air into the tank as well. I went in circles for a while till talking with the rep from Stihl. They're doing it for environmental reasons, fumes escaping into the atmosphere.

The inlet needle has to hold a bit more pressure then in the past although I don't have any specs just yet. Instead of settleing on 5psi as in the past I now look for 10psi when pressure checking the carb inlet needle.

Using alky fuel will increase the potential. Running vibration seems to be as much a cause for vaporization pressure as is heat. Check the inlet needle holding pressure and if not getting at least 10psi, replace the needle, dress the seat with a Q-Tip and fine valve lapping compound. If that doesn't correct, stretch the spring a bit to create more spring pressure.

 

[Navara]

Thanks Nevadawalrus. Checked in let needle and adjusted to increase pressure on needle and saw works much better I may still have to do a little more adjustment. Will wait and see. Thanks again another problem solved.