Chainsaw fuel boiling?

[GBertolet]

I just finished refurbishing my Poulan 3.7 chainsaw. It runs great, but, I have an issue. I have been ripping and noodling some tree sections, that were too big to handle and transport. They are about 20 inches diameter and 24 inches long. The saw is running great for maybe half the tank, and the saw quits suddenly. I can hear boiling sounds coming from what I believe is the carb.

The saw will not restart, and when I pull the plug, it is dry. Possibly vapor lock. If I let the saw sit for several hours, it starts right up again. The temp is around 75 degrees, and I had been running the saw very hard with an 18 inch bar. I originally had the HS screw about 1 1/4 turns out, and increased it to 2 full turns. The plug is starting to look sooty now, and it didn't help the issue any. The cylinder fins are clean, and I do have a plastic fuel tank.

Could it be heat transfer to the carb? If so, would thicker, or of a different material carb gasket help, giving more insulation? Or is this just something I will have to deal with when cutting hard? How common is this?

 

[nbbt]

Try opening the fuel cap. to release the pressure/vacuum. If it starts and runs good for a while , find where the fuel tank vent is and try cleaning it. A friends Poulan had this same issue, the tank vent was full of super fine wood dust causing the issue.

 

[GBertolet]

Will do, thank you.

 

[Nutball]

The fuel can easily boil in the crank case if excess fuel pools in it especially if the saw overheats. I have heard fuel boil in the crank case before, and the sound comes from the carb. You can take the gas cap off to see if the gas is boiling in the tank which is possible too. That can vapor lock the fuel line.

 

[Al Smith]

I've only seen this happen once .Circa 1974 west of Vail Colorado ,elevation around 8500-9000 feet .Two small saws,a Pioneer and my Poulan s-25DA cutting camp wood on a deer hunt .Both boiled the fuel but they never stopped running .I had assumed because they were both horizontal cylinder engines with metal fuel tanks the heat from the cylinder at higher altitude was causing this .However because of the altitude both saws needed to be leaned up a tad .Oxygon level you know . The Poulan which I still own and still runs great never once before or since has done that .Keep in mine the elevation in Northern Ohio is a tad less than 900 feet above sea level .

 

[rogue60]

What fuel are you running is it pump gas?
Sounds like classic vaper lock to me. boiling fuel and vaper lock is becoming more common when it's warm weather as they no longer make pump fuel at servos for anything that runs a carburetor and low fuel pressure.
If you can get a hold of some real fuel something like true fuel or even avgas for when it's warm and vaper lock strikes just dump the boiling crap out fill with good fuel and continue cutting.

 

[huskyboy]

What fuel are you running is it pump gas?
Sounds like classic vaper lock to me. boiling fuel and vaper lock is becoming more common when it's warm weather as they no longer make pump fuel at servos for anything that runs a carburetor and low fuel pressure.
If you can get a hold of some real fuel something like true fuel or even avgas for when it's warm and vaper lock strikes just dump the boiling crap out fill with good fuel and continue cutting.

+1.... plus some saws are more prone to it like the 562 and others are more resistant to it.

 

[CR888]

Either wait 20mins and try to possibly start saw or if your quick refueling and hold throttle W/O you have a slight chance or do what I do in those situation and not for the life of God hit the kill switch until job is complete, refuel while saw is running...hope a spark don't set you, your saw & 2T fuel can ablaze all at once. Seriously don't try that one if your accident prone type.

 

[GBertolet]

I am running 87 octane pump gas, with StaBil ethanol treatment, with 32-1 fuel mix, B&S 2 stroke oil.

 

[Nutball]

I've only seen this happen once .Circa 1974 west of Vail Colorado ,elevation around 8500-9000 feet .Two small saws,a Pioneer and my Poulan s-25DA cutting camp wood on a deer hunt .Both boiled the fuel but they never stopped running .I had assumed because they were both horizontal cylinder engines with metal fuel tanks the heat from the cylinder at higher altitude was causing this .However because of the altitude both saws needed to be leaned up a tad .Oxygon level you know . The Poulan which I still own and still runs great never once before or since has done that .Keep in mine the elevation in Northern Ohio is a tad less than 900 feet above sea level .

I had it happen to a 015. It has a metal tank on the engine, but kept running.

 

[Al Smith]

The only one I've had die in the heat was a Mac 700 .It was 95 degree heat .On a refuel it would cause the coil to fail .Take about half hour 45 minutes before it would spark again .It didn't boil the gas though and the gas tank is right above the cylinder .Come to think of it on a PM 610 the bottom of the cylinder can get clogged with chips cutting off the air flow .The coils on those will fail with heat but again recover .

 

[GBertolet]

I had this happen again. During heavy cutting, I released the throttle, to turn the log, and the saw died again. I heard boiling sounds, coming from the carb/fuel tank area. I removed the fuel cap, and saw that the fuel was not visibly boiling. I had about 1/3 of a tank left. The boiling sound stopped within two or three seconds. I replaced the fuel cap, and the boiling sound resumed. I could not restart the saw. Let it cool and it starts and runs fine. It never did this before until recently. What is going on here, and how do I stop it?

 

[Nutball]

I have what may be a similar problem with one of my saws, but I haven't gotten around to working on it. In my case I can't tune it rich enough, but after a few minutes of running (lean) it gets the crank case hot. Then for some reason gas pumps out of the carb in surges and starts filling the crank case with gas, which then boils. I never tried restarting it, but it would have taken a week or so for that to clear itself before restarting.

 

[Al Smith]

I think a lot of the problem could be air flow .All that shrouding etc was put on there for a reason .If I'm not mistaken some saws like a Stihl Ms 200T have an insulating thing over the muffler which is bottom exit to fend off the heat .I could be wrong about that though .What I do know is you have less problems if you keep the built up gunk and crud cleaned off the fins .I'm as guilty as most people are about that .

 

[Al Smith]

What you don't want to do is flood a large displacement saw .Choking a hot saw more times than not will flood the crankcase full of fuel mixture .You will jerk on that thing until cows come home .Then they might come back on you and try to pull your arm off .Then you have two choices .Let it sit a long time or pull the plug and blow the fuel out .