OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Chainsaws and Concrete Cutters Petrol Chainsaws and Concrete Cutters Oily air cleaner element

HI,
This is a general question on two strokes a mate called me awhile ago and asked why the air cleaner would be oily on his echo wiper snipper, I made up a couple of suggestions but nothing to convincing. Generally he felt it was not running as well or not as powerful as it used to be.

Then I discovered a similar problem with my Stihl chainsaw and I have the same symptoms ( possibly imaginary but it does not seem to be running like it used to) but the air cleaner is definitely attracting oil.

I am guessing that fuel is blowing back onto the air cleaner element some how either rings or valves or both. Is there an easy explanation?

Hi,
No valves in a two stroke, I would tend the think worn rings . a bit off oil shouldn't be a big problem
if there was no oil then there is a problem hence no lubrication

Only thing I can think of is a partly worn metering diaphragm. If it has lost some of its pliability, it can prop the needle valve open, flooding the engine.
It starts off minor, and gets worse. They will generally run rich with this issues, hence slower at full throttle and less powerful

If this is happening, and there is only minor flooding, fuel mix could be running out the carby throat whilst its sitting in the shed, and if the air filter is tilted slightly down, the mix will run onto it.
The fuel evaporates, but the oil doesn't.

That said, a blocked fuel cap vent could exhibit similar thing - if the pressure cant be released, overcomes the needle valve, and 'floods' the carby. Less likely though.

I also guess that you mean paper filters, not oiled foam filters. Some people have a habit of dunking the foam filter in a pot of oil, and installing it saturated.

Two notes here:

1) Although KenW is correct there no physical valves in most 2 cycles like 4 cycles, there are some that do have reed valves. These reeds do sometimes fail to close when they should.

2) Tyler you are basically incorrect about the tank venting outward. They only vent inwards on most 2 cycle except for very old equipment where they use slivered bronze vent. This is an EPA requirement here that gas vapors must not be expelled un-burned. This why you find that when you loosen the fuel cap you will find it pressurized most times. The inward venting is so the tank does pull a vacuum causing lost of fuel flow. Most 2 cycle cube carbs are design to hold back a pressure 5-10 psi depending on the design. Now needles and seats do wear; therefore, they can get to where they will not this pressure level and need replacing. As note a stiffen diaphragm will also cause this leakage as well as carburetor with either primer as part of the carburetor or as an external primer for their check valves can go bad and leak backwards.

Now some engine do have a slight blow back during operation due how a 2 cycle moves the fuel mix. As the crankcase might be slightly pressurize by ring blow-by just the piston passes the intake port. Some of the old saws that I have actually have quite a bit of spit back from the carburetors.

There is other note here but it related to the handheld 4 cycles that uses a separate oil tank. Many are design in such a way that PNC fails from lack of proper oiling and oil is blown into the air cleaner.

Thanks for that guys,
It has been that long since I took a 2 stroke apart that I had forgotten about valves
My Stihl is pretty old (AV31) so normal could be possible it only has a mesh filter. I will use it for awhile and see if anything develops.