026 Revs Up, Bogs, Revs Back Up Again-Only With Bar On?

[IHDiesel73L]

I just tuned up my 026 which had been sitting for a while-new air filter, plug and fuel filter, gave the carb a good spray, etc... It fires up on the first pull but when I WOT it runs full out for a few seconds, bogs down like its going to stall, and then revs back up again and does it over and over. I also just put a new 18" bar on and I thought maybe something was binding so I took it off. It revs and holds WOT just fine with no bar. Put the bar back on, adjusted the chain, nothing binding, etc...surging problem is back. What could be causing this? I'm running fresh 50:1 100LL fuel which my 025 and PM650 have had a few tanks of so far and are loving it.

 

[Wood Doctor]

Sounds like it's starved for fuel under a load, even a small load. Try opening up the carb's hi-speed setscrew about a quarter turn. If that fails, it could be the fuel line or fuel filter, but check the carb setting first.

 

[IHDiesel73L]

Sounds like it's starved for fuel under a load, even a small load. Try opening up the carb's hi-speed setscrew about a quarter turn. If that fails, it could be the fuel line or fuel filter, but check the carb setting first.

Will do-the fuel filter is brand new-should be ok. Is it possible to kink the fuel line when you're pulling it out to pop in the new filter?

 

[Bob Hedgecutter]

Have a serious look at your clutch and chain brake area- you would not be the first to discover today that a heap of crap in the clutch cover around the brake band can bog a saw down.
This is assuming you replaced the chain, sprocket, bar with the correct sizes.

 

[Wood Doctor]

Will do-the fuel filter is brand new-should be ok. Is it possible to kink the fuel line when you're pulling it out to pop in the new filter?

That's possible but especially if you replaced it with the more common fuel line sometimes supplied for these saws that has a longer neck at the top portion that extends above the tank. Be sure that's not pinched. Usually the kink does not occur during removal. That will sometimes cause a crack in the line, in which case the line is old and ready for replacement.

 

[r1stgei]

I just tuned up my 026 which had been sitting for a while-new air filter, plug and fuel filter, gave the carb a good spray, etc... It fires up on the first pull but when I WOT it runs full out for a few seconds, bogs down like its going to stall, and then revs back up again and does it over and over. I also just put a new 18" bar on and I thought maybe something was binding so I took it off. It revs and holds WOT just fine with no bar. Put the bar back on, adjusted the chain, nothing binding, etc...surging problem is back. What could be causing this? I'm running fresh 50:1 100LL fuel which my 025 and PM650 have had a few tanks of so far and are loving it.

Bar on and off. Will it do it if the chain break is on ? Sounds like it’s starving for fuel. When the bar and chain are in there is additional load added to the scenario.

 

[SteveSr]

I just tuned up my 026 which had been sitting for a while-new air filter, plug and fuel filter, gave the carb a good spray, etc... It fires up on the first pull but when I WOT it runs full out for a few seconds, bogs down like its going to stall, and then revs back up again and does it over and over. I also just put a new 18" bar on and I thought maybe something was binding so I took it off. It revs and holds WOT just fine with no bar. Put the bar back on, adjusted the chain, nothing binding, etc...surging problem is back. What could be causing this? I'm running fresh 50:1 100LL fuel which my 025 and PM650 have had a few tanks of so far and are loving it.

Pull the air filter cover off so that you can watch the choke lever on the filter while you run it. Stihl launched a batch (or 3) of these with wrong or weak springs. It will rev and pull the choke flap in and then die until the speed drops and the choke opens again. Rinse & repeat!

 

[IHDiesel73L]

Pull the air filter cover off so that you can watch the choke lever on the filter while you run it. Stihl launched a batch (or 3) of these with wrong or weak springs. It will rev and pull the choke flap in and then die until the speed drops and the choke opens again. Rinse & repeat!

Had some time to screw around with it tonight. For the hell of it I took the air filter off entirely to see if it would do it. Not only did it still do it, when the speed drops it spits fuel out of the throttle body-not sure what that means. I decided to just take the whole damn thing apart to check the fuel line for cracks/leaks. Pulled the carb and the fuel line and pumped some air into it-doesn't seem to have any leaks. At that point I sprayed the crap out of every orifice in the carb, sprayed out the fuel line, etc... Going to put it all back together tomorrow and if this doesn't do it I'm saying uncle and taking it to a local shop that specializes in Stihls.

 

[SteveSr]

Had some time to screw around with it tonight. For the hell of it I took the air filter off entirely to see if it would do it. Not only did it still do it, when the speed drops it spits fuel out of the throttle body-not sure what that means. I decided to just take the whole damn thing apart to check the fuel line for cracks/leaks. Pulled the carb and the fuel line and pumped some air into it-doesn't seem to have any leaks. At that point I sprayed the crap out of every orifice in the carb, sprayed out the fuel line, etc... Going to put it all back together tomorrow and if this doesn't do it I'm saying uncle and taking it to a local shop that specializes in Stihls.

Does it still do it with the air cleaner off? You'll have to warm up the saw first so that it will start without the choke.

If it still does it without the air filter it might be some sort of weird ignition failure.

A video of the symptoms in action would be VERY helpful at this point.

 

[rupedoggy]

STOP spraying carb cleaner etc all around and take the carb off the saw. Replace the stretched diaphragm and set the needle fulcrum with a new diaphragm and good to go. As an extra measure check the pulse line/passage to assure it is clear and ,if it has a hose, it is not cracked.