Chain saw stutters at full throttle - diagnosis please?

[dswensen]

I have a chain saw that will start easily, idle well, and accelerate smoothly - until I get to almost full throttle. When I pass that magic mark close to full throttle, the saw will cut out and back in, almost like the ignition switch is being turned on and off. Occasionally, it won't cut back in and it will die - when it does this it is easy to restart.

I first thought it was ignition, but I have tried new two coils, with all new associated wiring including kill switch wiring, and three new spark plugs. No change.

I am now thinking perhaps it is flooding as it approaches full throttle, and then is clearing as the RPMS come down when it cuts out.

Does that sound close to reasonable?

 

[s sidewall]

Was thinking ignition system but, mmm, hard to say, maybe tank vent?

Steve Sidwell, Samsung On5 using Tapatalk

 

[Snowchaser]

Carb might have junk in it. Something in the carb floating around and partially blocking a port, starving it of just enough fuel to prevent full throttle, but not enough to drain the carb dry.

 

[cornfused]

Could be a fuel line collapsing at high draw rate then relaxing at lower draw rate...Had a Poulan PP5020 that did the same thing & that was the cause.

 

[kevin j]

what saw?

-carb could be set too lean and the RPM is bouncing off of the rev limiter coil.
-carb could be sent too rich and seriously for stroking.
-several fuel related possibilities: dirt for water in fuel etc.
-plug misfiring is long shot.

need more info

 

[dswensen]

Saw is a new Chinese aftermarket build. Copy of Stihl MS380. All parts are new. Admittedly, all parts are Chinese copies. However, it's a good mystery that I'd like to solve. I've built both an 044 and 066 clone and have had excellent results.

Have had the carb apart - clean as a whistle. It's a new replacement carb - I couldn't get the carb that came with the kit to run consistently at all. AND, the saw with that first carb installed had the same high-speed issues this newer carb has. Adjusting "L" screw has the expected effects on idle and throttle response. Adjusting the "H" screw really rich creates a 4-stroke just before the saw starts to cut out (holding the throttle carefully just this side of full-throttle). Adjusting the "H" really lean doesn't change the problem. Carb is a "face-feeder" for impulse, and impulse air seems to be pumping just fine. There is no kink in the impulse line that goes from cylinder to handle assembly where carb is mounted.

Really, when the throttle is fully open, the saw acts like the ignition switch is being turned off, and then turned back on just before it dies. Sometimes it does die.

Ignition coil that came with the kit (along with all the wiring for the kill switch) was replaced when I thought the problem was coil-related. Replaced the kit-supplied spark plug with a new NGK, and then two more new NGK plugs - no change.

I tried pump-fresh fuel mix yesterday thinking it could be bad fuel - no change.

I am wondering now about the fuel line and filter. It's new, and not kinked in any way - and doesn't seem mushy. No holes in it I could find. Collapsing fuel line seems logical for the symptoms assuming no ignition issues. But I am wondering if a collapsing fuel line would react as quickly as this saw "stutters" and then recovers. I haven't replaced the new Chinese fuel filter yet - just thought of that. I'll do that.

One of the bellows-folds in the intake boot extends a bit far (less than a third of the way) into the intake throat. Although it is visible, it doesn't seem to be blocking the throat too much.

All good ideas guys. Any more?

 

[Westboastfaller]

Obviously it appeared to be a high rev ignition failure. Sounds Like it is from what you have described. Water miss will happen at any rpm's as will most shorts.
I have had a saw that had a generated flywheel with the wire clipped off short on remove and the conection separated and caused a hi end short that I couldn't see.
A few questions:
The model?
Any changes to it other than stated above? Flywheel, chokes ...
Choke is not getting pressed at hi rpm?

Has it always done this since you got it?
Good exhaust flow?
Any gas blow back into the filter?
It's usually shorting or choking from fuel, exhaust, fire or water in my case.

Anyway it doesn't sound like tank or carb vent hole or impulse. Check impulse line anyway make sure it has a good seal. Clip the line a bit and stuff it back on if it seems loose.

Sometimes it can be hard to tell between a carb and ignition/spark issue.

Next...

I agree with the other guy about buildup or something behind the needle?


May need a good soak in a heavy duty brake cleaner in a coffee can or replaced. Remove plastic and everything if you do that with the right stuff.

 

[HarleyT]

What would happen if you put an OEM coil on one of these things? Are these saws supposed to be copies, or just saws that look like this Stihl model? I mean, should real parts interchange?

 

[Ozhoo]

Try it without an air filter to eliminate the choke flap closing.

 

[dswensen]

Obviously it appeared to be a high rev ignition failure. Sounds Like it is from what you have described. Water miss will happen at any rpm's as will most shorts.
I have had a saw that had a generated flywheel with the wire clipped off short on remove and the conection separated and caused a hi end short that I couldn't see.
A few questions:
The model?
Any changes to it other than stated above? Flywheel, chokes ...
Choke is not getting pressed at hi rpm?

Has it always done this since you got it?
Good exhaust flow?
Any gas blow back into the filter?
It's usually shorting or choking from fuel, exhaust, fire or water in my case.

Anyway it doesn't sound like tank or carb vent hole or impulse. Check impulse line anyway make sure it has a good seal. Clip the line a bit and stuff it back on if it seems loose.

Sometimes it can be hard to tell between a carb and ignition/spark issue.

Next...

I agree with the other guy about buildup or something behind the needle?


May need a good soak in a heavy duty brake cleaner in a coffee can or replaced. Remove plastic and everything if you do that with the right stuff.

Good ideas all. But all the parts are brand new including the carb, and the saw hasn't seen wood yet. It's a copy of an MS380/038. The muffler is a hollow can. I have had the carb apart, and it's clean as a whistle.

What would happen if you put an OEM coil on one of these things? Are these saws supposed to be copies, or just saws that look like this Stihl model? I mean, should real parts interchange?

I have a OEM Stihl 290 that should have the same coil - I will look into trying that coil on this saw. Yes, parts are interchangeable. Thanks Harley

Try it without an air filter to eliminate the choke flap closing.

I will do this - it makes some sense that this might be happening, the way the "on/off" pulses. Thank you.

 

[pioneerguy600]

Try it without an air filter to eliminate the choke flap closing.

That is a well known issue with this series of saw, especially with these cheap Chinese airfilters with their goofy return springs.

 

[dswensen]

That is a well known issue with this series of saw, especially with these cheap Chinese airfilters with their goofy return springs.

Ohzoo and Pioneerguy600 win the prize. It runs great without the air filter. All the way up the throttle range and back down again.

This is great! I would have sworn on a stack of bibles that it was the ignition system.

Now the question is; can I just remove the choke shutter, use the resulting air filter, and still get the saw started from cold, or do I need a replacement filter/choke assembly. A question that I will answer tomorrow.

Pioneerguy, you mention this is a known problem with the Chinese copies. Is there a solution that doesn't involve OEM?

 

[HarleyT]

He mentioned that it was a problem with that series of saw.

 

[pioneerguy600]

Ohzoo and Pioneerguy600 win the prize. It runs great without the air filter. All the way up the throttle range and back down again.

This is great! I would have sworn on a stack of bibles that it was the ignition system.

Now the question is; can I just remove the choke shutter, use the resulting air filter, and still get the saw started from cold, or do I need a replacement filter/choke assembly. A question that I will answer tomorrow.

Pioneerguy, you mention this is a known problem with the Chinese copies. Is there a solution that doesn't involve OEM?

The spring is not strong enough to hold the choke open in the position it rests in the way it is but if you can grind away some of the plastic that stops the choke from opening a little more the choke will stay open. I have ground away .025 - .035 with a carbide bit in a Dremel, this allows more air to pass the choke flap and it does not suck it shut. Other than that I have found no resolve using these new filters, allowing the flap to open further is also a great power boost for the saw.

 

[dswensen]

The spring is not strong enough to hold the choke open in the position it rests in the way it is but if you can grind away some of the plastic that stops the choke from opening a little more the choke will stay open. I have ground away .025 - .035 with a carbide bit in a Dremel, this allows more air to pass the choke flap and it does not suck it shut. Other than that I have found no resolve using these new filters, allowing the flap to open further is also a great power boost for the saw.

This is very helpful. I have the choke shutter out of the air filter assembly and I see how it operates. This part of your post is not clear to me. -"... some of the plastic that stops the choke from opening a little more..." . Do you mean making the shutter lay FURTHER open when it is at rest, out of the way? I understand that the intent is to stop the shutter from being sucked closed when the larger volume of air is passing it.

Thank you for you time and experience.

 

[pioneerguy600]

Yes, make the choke shutter lay open further, it does as you said, it gets sucked closed as the opening is not large/wide enough and the flow of air is too great for the spring to hold the shutter open under full throttle. Take a good look at the area behind the choke shutter when it is in the normal open position to see if there is any plastic/nylon that can be ground away to let the shutter sit further back away from its closed seat position. On the ones I have modified there was a stop tab that was easy to grind off and even went a bit more by grinding on the shoulders of the bosses around the mounting screws.

 

[dswensen]

Yes, make the choke shutter lay open further, it does as you said, it gets sucked closed as the opening is not large/wide enough and the flow of air is too great for the spring to hold the shutter open under full throttle. Take a good look at the area behind the choke shutter when it is in the normal open position to see if there is any plastic/nylon that can be ground away to let the shutter sit further back away from its closed seat position. On the ones I have modified there was a stop tab that was easy to grind off and even went a bit more by grinding on the shoulders of the bosses around the mounting screws.

Understood. I will try it! Thank you.

 

[rd35]

Those air filter choke flaps can be a problem on any Stihl saw that has one including OEM versions. Buddy called me the other day telling me his 024 would not start. I told him to press down on the choke lever (ignition/choke thumb lever) while pulling the rope. Fired up first pull! The choke flap on many of these units does not fully close when the thumb lever is set to "choke" and released. But on these clone 038's I never would have thought about a flap getting sucked shut by air flow at WOT. Great diagnosis and fix guys. I love reading this posts. Learn something new every time! Thanks all for a great read!

 

[pioneerguy600]

The 026 China filters are even worse but can be fixed up,the seal between the two halves are poor and fine dust can get through easily, the choke shutter on most does not close tightly, some heat and tweeking can make them fit quite well. Even seen some OEM ones that were not up to snuff, just one of the not great designs these saws inherited.

 

[dswensen]

Yes, make the choke shutter lay open further, it does as you said, it gets sucked closed as the opening is not large/wide enough and the flow of air is too great for the spring to hold the shutter open under full throttle. Take a good look at the area behind the choke shutter when it is in the normal open position to see if there is any plastic/nylon that can be ground away to let the shutter sit further back away from its closed seat position. On the ones I have modified there was a stop tab that was easy to grind off and even went a bit more by grinding on the shoulders of the bosses around the mounting screws.

Your fix worked beautifully - thank you! She's a runner now!