Hot start problem

[John from Cle Elum]

I have a Stihl ht75 pole saw which has become very difficult to start when hot. New plug, old plug looked ok, dark tan. The spark tester shows a great spark when cold. Fuel filter looks good. One clue:. The clear primer bulb does not seem to get filled regardless if how many times I pump it when hot. When it's cold it will fill 1/2 to 3/4 full. When hot, it tends to pop a couple of times, run for a second and then quit. Then nothing but a random pop. When hot and running correctly, it would take three pulls with no choke. Compression is good cold (130psi), have not measured it hot. It takes about three pulls to get a pop when cold with the choke and another 3 to get it run with no choke. Runs great once started. I'm working it pretty hard brushing out a huge thicket of 2-3 inch vine maple for a couple of hours at time. On thing, I did just mix a couple gallons of gas (40/1) with probably about 10% ethenol. Last mix was ethenol free. Seems to have made the problem a lot worse but it may just be a coincidence.

I am thinking vapor lock within the carb or a crankcase leak. Is there a way to verify? Any other ideas? I can't take a half hour break every time it needs fuel.

Any help or new ideas would be appreciated.

Thanks
John

 

[SEAM]

Might be a clogged fuel tank breather or a leaky check valve in the primer assy... Try opening the fuel filler cap once next time it gets hard to re-start.

 

[lone wolf]

I have a Stihl ht75 pole saw which has become very difficult to start when hot. New plug, old plug looked ok, dark tan. The spark tester shows a great spark when cold. Fuel filter looks good. One clue:. The clear primer bulb does not seem to get filled regardless if how many times I pump it when hot. When it's cold it will fill 1/2 to 3/4 full. When hot, it tends to pop a couple of times, run for a second and then quit. Then nothing but a random pop. When hot and running correctly, it would take three pulls with no choke. Compression is good cold (130psi), have not measured it hot. It takes about three pulls to get a pop when cold with the choke and another 3 to get it run with no choke. Runs great once started. I'm working it pretty hard brushing out a huge thicket of 2-3 inch vine maple for a couple of hours at time. On thing, I did just mix a couple gallons of gas (40/1) with probably about 10% ethenol. Last mix was ethenol free. Seems to have made the problem a lot worse but it may just be a coincidence.

I am thinking vapor lock within the carb or a crankcase leak. Is there a way to verify? Any other ideas? I can't take a half hour break every time it needs fuel.

Any help or new ideas would be appreciated.

Thanks
John

Winter gas what temp was it when this occurred?

 

[rupedoggy]

I may be wrong but I think the Av gas has a high tendency not to vapor lock. Try 5 gallons of that and if it solves the problem then you will be able to use the tool without waiting.

 

[PV Hiker]

Mike yes vapor lock. He is running the saw hard and long witch will heatsaok the saw. If he stops to refill the fuel after it runs out of fuel then the motor heats up more and cooks what is left in the carb. End result is very hard to start.

Be better if the saw was allowed to run for a minute or two, then shut it down and fuel it up. If you run it hard and long fuel before it runs out of fuel. This makes it easier to restart.

Give the saw a good cleaning especially around the head fins and flywheel.