2-Stoke Fails After 15 Minutes

[1990C4S]

I have a 2-stroke Shindaiwa backpack blower that has been trouble free for ten plus years. This summer it has been harder and harder to start which turned out to be loose carburetor bolts and some pretty worn out gaskets.

I sealed the carb up and it starts and runs great for 15 minutes or so. After that it won't hold high rpm's. It drops down close to idle speed, then comes back briefly and dies off again. It never stalls, but it won't hold full throttle.

The plug looks good, the fuel lines are good, and when it runs, it runs like a top.

I'm sort of at a loss. It can't be a dirty carb, it runs perfectly at first. I took the fuel vent off, so it's not creating a vacuum in the fuel tank. The spark arrestor is also off right now, there's no back pressure problem.

The only thing I can think of is that the carb is leaking air in when it gets hot and then engine leans out? But I've tried adding some choke as it stalls, and that makes it worse, as does giving giving the primer bulb a squirt while it's dying. I've also sprayed ether onto the carb seals while it's running and there is no surging, so the seals seem to be good. I don't think I have a vacuum leak...

I pulled the cylinder off, the internals are clean and like new, no scoring, almost no wear, the rings look fine.

Maybe I'm losing spark? Maybe it is the plug? I would go get a plug today but they aren't available locally.

I'm stumped and the leaves are piling up. I'm stumped. What have I missed?

[unclebilly]

Fuel filter plugged? Sounds like it’s fuel starvation to me.

[p911dad]

Gas tank vent plugged up?

[1990C4S]

Originally Posted by unclebilly

Fuel filter plugged? Sounds like it’s fuel starvation to me.

I took it off and blew through it, seemed okay. But I do think it's fuel. Maybe I will try it without the filter today. Worth a try. I will order one from Amazon.

Originally Posted by p911dad

Gas tank vent plugged up?

Stalls with and without the vent.

[dad911]

Replace fuel line, ethanol screws them up.

[wdfifteen]

Does it still have a spark arrester screen?

[LEAKYSEALS951]

Get a new plug. You've ruled out a bunch of hard stuff already.

[1990C4S]

Originally Posted by dad911

Replace fuel line, ethanol screws them up.

I replaced one line, the other 'appears' to be okay. I will order some. Ethanol free fuel was available here until this summer, so it has only had ethanol in it for a few months. Also, the primer bulb works fine.

Originally Posted by wdfifteen

Does it still have a spark arrester screen?

Removed.

Originally Posted by LEAKYSEALS951

Get a new plug. You've ruled out a bunch of hard stuff already.

On order...

[flatbutt]

Is the fuel filter one of those sintered metal things? I've been able to blow air thru them but they still didn't pass fuel. But the 15 minute thing is odd.

[rsrfan]

Originally Posted by wdfifteen

Does it still have a spark arrester screen?

Check the muffler. Used to own a landscape company and the stihl backpack blower spark arrester screens plug up and they stall out when hot…

[1990C4S]

Originally Posted by rsrfan

Check the muffler. Used to own a landscape company and the stihl backpack blower spark arrester screens plug up and they stall out when hot…

Muffler is 100% clear.

[rsrfan]

Sorry, did not see that. It “might” be the coil. When they get older they can fail when hot then work again when cooled off…

[greglepore]

I have a stihl chainsaw (like 30 yrs old) that bogs like that when it gets hot. I need to swap the plug-my suspicion is that it either opens the gap up when hot beyond marginal or something in the insulator/electrode.

[john70t]

Gas would happen at all times (or randomly) so I second heat-related coil or plug.

[1990C4S]

I cleaned my plug and re-gapped it, but I also pulled the front half of the in-tank fuel filter off.

It 'appears' to be fixed, but now I regret making two changes at once; since I posed the problem here I would like to post a definitive solution.

New plug and filter on the way from Amazon.

Thank you for your suggestions, I think it was fuel (as BB suggested above), but I have a bit more R&D work to confirm, I will reassemble the fuel filter and see if it fails me tomorrow.

[Bob Kontak]

Originally Posted by 1990C4S

....but I also pulled the front half of the in-tank fuel filter off.

Was there dirt in the tank? I have seen filters pack up with crud after 15 minutes and bog way down including stalling. Let it rest a half hour and the dirt falls out and it runs normal again for a little increment.

This was on a riding mower though but same concept should apply.

[1990C4S]

Originally Posted by Bob Kontak

Was there dirt in the tank? I have seen filters pack up with crud after 15 minutes and bog way down including stalling. Let it rest a half hour and the dirt falls out and it runs normal again for a little increment.

This was on a riding mower though but same concept should apply.

I will check, that's not a bad supposition. Very old blower, large tank, never been cleaned....yea, I am guessing it's dirty.

[1990C4S]

I'm 99% sure the problem was the in tank fuel filter. I installed the new one today and it runs great.

Inspecting the old one, I would say that ethanol was dissolving the white metal body of the filter. I had the same issue with a moped carb a few months ago. I generally find that two strokes handle the ethanol better than four strokes, but I guess it's a problem in all my small engines now. I will empty the crab and soak it in trans oil of Marvel Mystery Oil before it goes away for the winter.

Thank you to everyone who suggested some of the things I missed.

[javadog]

Separate the water out of the ethanol gas and quit running ethanol gas through your blower.

[greglepore]

Yeah, we're lucky to have ethanol free available here at the pump. Stopped using regular gas in power equip this year, after replacing several carbs. My small engine guru swears by seafoam (an ounce per tankful) as a fuel stabilizer in anything that sits.