What would cause a chainsaw to surge?

[trooney]

Got this bastird of an 034 together and started it up. Whoever owned it certainly did not know how to tune a chainsaw. Anyway, it idles fine and throttles up fine but every once in a while it surges like its leaning out. L is about 1 1/2 out. H is 1 but could take it in a little. Any ideas? Stiff diaphram? New fuel line.

 

[Squareground3691]

Got this bastird of an 034 together and started it up. Whoever owned it certainly did not know how to tune a chainsaw. Anyway, it idles fine and throttles up fine but every once in a while it surges like its leaning out. L is about 1 1/2 out. H is 1 but could take it in a little. Any ideas? Stiff diaphram? New fuel line.

Pressure/vac test for possible leak maybe? New carb kit ,

 

[trooney]

I think a leak would be constant, not sure. First time this has happened and I've dealt with alot. Just want to get some ideas before I dig into it and hopefully somebodys had it happen to them before...

 

[jacob j.]

Could be the fuel tank vent - does it have the older style black vent with the grub screw?

If so, look in the tank and see if the red umbrella check valve is there or not. It could be doing it in
concert with a fouled metering barrel in the carb.

 

[trooney]

Could be the fuel tank vent - does it have the older style black vent with the grub screw?

If so, look in the tank and see if the red umbrella check valve is there or not. It could be doing it in
concert with a fouled metering barrel in the carb.

It does have the tower vent. And the umbrella valve is not there. What kind of valve does it take? The kind with the tit?

 

[trooney]

Number 13 correct?

 

[jacob j.]

Yep. #13. You can buy those on eBay fairly cheaply on Ebay as Holley 26-41 carburetor check valves.

Holley 26-41 Carburetor Accelerator Pump Check Valves Umbrella Style 10 Pack | eBay

Fits Holley 2300, 4150,4165 2 bbl and 4 bbl Carburetors.

www.ebay.com

 

[trooney]

Yep. #13. You can buy those on eBay fairly cheaply on Ebay as Holley 26-41 carburetor check valves.

Holley 26-41 Carburetor Accelerator Pump Check Valves Umbrella Style 10 Pack | eBay

Fits Holley 2300, 4150,4165 2 bbl and 4 bbl Carburetors.

www.ebay.com

Thanks

 

[Outback]

When you say surging do you mean that its losing power and then picking it back up again in the cut? I can't see how having no tit would cause that. have you checked for a hole in the fuel line. Your missing tits make me wonder if the fuel tank had the pleasant aroma of varnish when you started. I had a hell of a time with small flakes of varnish cloggin the fuel screen in a little husky 440. Took me forever to figure it out. The screen looked clean until you held it up to the light.

 

[cus_deluxe]

im guessing he means at idle, not in the cut , that it gains rpm and then settles back down. sort ur tank vent and do a carb kit and that “should” cover it. i had an ms250 that did a similar thing and a small adjustment of the metering lever was all it took.

 

[trooney]

im guessing he means at idle, not in the cut , that it gains rpm and then settles back down. sort ur tank vent and do a carb kit and that “should” cover it. i had an ms250 that did a similar thing and a small adjustment of the metering lever was all it took.

Yup, thats what I mean. I was going to tear apart the carb anyway, but forgot about the valve in the tank vent. Good thing JJ said something about it.

 

[Outback]

Ah, so erratic idle then. carb kit country.

 

[Wilhelm]

It's a female, she will have her surges!
Just wait for her to calm down again, have a beer on the porch or tinker on another saw in the shop.

If You try to calm her down Yourself she'll just take off!

 

[Wood Doctor]

This thread really intrigues me. I just rebuilt the carb on a vintage 1978 Echo CS-500 EVL that now runs almost perfectly at top end, acceleration, and idle with just one exception. It idles fine at just the right speed when resting on a flat surface or the ground. But, when you pick it up by the outer handle and prepare to cut, it surges up about 200 RPM, just enough to start the chain creeping. Set it back down and it resumes at the correct idling speed and no chain creep.

I know that's not supposed to happen and I can probably live with it. The saw might repair itself after using it a few times. My other two 500 EVL's with identical engines do not do this. Any ideas? TIA.

 

[066 redeye]

This thread really intrigues me. I just rebuilt the carb on a vintage 1978 Echo CS-500 EVL that now runs almost perfectly at top end, acceleration, and idle with just one exception. It idles fine at just the right speed when resting on a flat surface or the ground. But, when you pick it up by the outer handle and prepare to cut, it surges up about 200 RPM, just enough to start the chain creeping. Set it back down and it resumes at the correct idling speed and no chain creep.

I know that's not supposed to happen and I can probably live with it. The saw might repair itself after using it a few times. My other two 500 EVL's with identical engines do not do this. Any ideas? TIA.

I have the same problem with a 12mm 044.
Nothing I have done has changed it.
Holds vac/press all day.
Only thing I haven't done is change the carb,
like something carb internal is loose.
I can bend/twist handles when running and no change.
Sit it back on the ground and revs change.

 

[drf256]

Tom, I would definitely do a vac and pressure check.

The other thing is the carb main nozzle check valve. It stops the metering area from sucking air back in through the main nozzle at idle. It causes wacky issues.

At least for me, tank vents caused saws to lean out under load, and it got worse and worse after a refill. Lack of the flapper will cause saw to leak fuel, but if you can see through the flapper hole and you can clean the black vent, it shouldn’t be happening. You can pull the vent off and run 1/2 tank of fuel and see if it happens as a cheap diagnostic experiment.

 

[trooney]

Tom, I would definitely do a vac and pressure check.

The other thing is the carb main nozzle check valve. It stops the metering area from sucking air back in through the main nozzle at idle. It causes wacky issues.

At least for me, tank vents caused saws to lean out under load, and it got worse and worse after a refill. Lack of the flapper will cause saw to leak fuel, but if you can see through the flapper hole and you can clean the black vent, it shouldn’t be happening. You can pull the vent off and run 1/2 tank of fuel and see if it happens as a cheap diagnostic experiment.

Yeah, I meant to chage the flapper valve anyway. Just forgot about it til JJ mentioned it. I think I'll rebuild the carb, also got another carb I can put on it. If that doesnt do it then I will p&v test. I can't believe it is an air leak though with saws anything is possible I guess...I've certainly seen some weird shat before...

 

[pbillyi69]

one of my 357s surge when running wide open sometimes. it passes all the tests. it has done since it was brand new. i havent ever figured out why. i just dont worry about it anymore.

 

[Moparmyway]

I’ve learned, even if it’s new, I replace the filter first, then kit it, and while I’m in there, check metering lever height and blowoff pressure as well as re-seat pressure.

I’ve been running a saw when the diaphragm had enough and was very stiff when I pulled it out of swap with a new one. They thin out in the cut and don’t want to idle cleanly.

 

[drf256]

I had an Issue with the harder new blue pump diaphragms that came in a new oem carb as well. Did all sorts of crazy stuff. So a stiff older one could be the issue.

Tom, see if you can pop out the main nozzle and see if the check valve is leaking. A piece of tubing over in should make that easy. Should be able to suck air through it but not push any back. Some rebuild kits come with the small flapper valve replacement.