Chain saw stutters at full throttle - diagnosis please?

[pioneerguy600]

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

Good to hear, hope you have many enjoyable hours running it.

 

[DBeard]

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?

I was just going to say that I found this thread after having the same problem with my hisquvara. I had it serviced recently and had only used it once…. Then let it sit for 3 months. I emptied all the fuel and ran it until it quit , left open the fuel cap for a couple hours and put in new fuel.

It ran perfectly

So I think this problem COULD be bad fuel. So, for future readers of this thread… I’d check the fuel first as that’s the easiest fix

 

[a. palmer jr.]

Slightly lean mixture, bad spark plug, etc.

 

[pioneerguy600]

The problem was solved back in 2017, no need to dredge it back up with off base guesses of what was causing the problem.

 

[GrizG]

The problem was solved back in 2017, no need to dredge it back up with off base guesses of what was causing the problem.

That's no fun....