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Chainsaw that dies when throttle applied - impulse line?

Daniel Duffy

Daniel Duffy

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Hi all.

I'm so glad I've found this forum. Been fighting a problem with my saw for months and have been too stubborn to pay a shop to fix it when I know the answer's out there and I enjoy hands-on work. Here's the situation.

Have an Echo CS-400 saw that I've used only a couple of hours a year for last 6 years or so. Never had a problem. Then when I fired it up recently, it would only stay running on full throttle - it would not idle. Researched and it seemed a carb rebuild was in order.

Disassembled carb and installed rebuild kit. Carb was pretty clean to start. None of the gaskets were brittle, and the screens were not clogged but went ahead and replaced all anyway. Aired out all lines/chambers and followed up with carb cleaner but nothing was clogged to begin with - but wanted to be sure. Carb was now spotless and air/fuel would pass through every chamber freely.

Made sure all fuel lines were good and not cracked/soft, and fuel filter was clean. When I'd depress the prime bulb, fuel would flow freely up from the tank, and through the carb, so didn't appear I needed to replace fuel filter. Additionally, because it would run fine on full throttle (but ONLY full throttle), that also seemed to indicate fuel was flowing fine.

Made sure the air filter was clean. Checked spark plug for good spark with spark line tester, and checked spark plug gap. Checked compression, which also seemed fine.

Put it back together and followed Echo instructions for setting carb high/low and idle.

Now it starts up and idles better, but as soon as I apply throttle, it dies. I have however noticed that once it's warmed up, if I slowly apply the throttle, it CAN increase up to full throttle in a controlled manner. But if I want to just brap the throttle, the way you'd normally use a chainsaw, applying throttle as needed, it dies.

So did some more research, tore it apart again, triple checked carb was clean. Removed and cleaned spark arrestor, which didn't have much carbon build up anyway.

*Thought* I'd read that using straight, unmixed fuel might help. Tried that and the damned thing started up but wouldn't shut off - even when I turned the ignition switch to OFF. Had to dump the fuel to get it to finally die. Don't think it did too much damage - only ran like this for <30 seconds but it was crazy it didn't stop. No idea what that was about but it didn't fix the problem anyway. I was also mistaken - I needed to get pure fuel without the ethanol in it.

Replaced with regular 50:1 mix, cleaned out the tank, lines and carb (again), and it started fine - but again, would only idle. But again, once I applied throttle in a normal manner, it dies. It will only accelerate if I very slowly increase throttle.

Took it to a saw shop and they said they'd probably just replace the carb. But I don't want to pay someone to do what I can do myself, or pay to replace a carb that doesn't need replacement. And after having rebuilt the carb, I don't think it's the problem. I'd really like to figure this one out and have the satisfaction of fixing it myself.

Read through this thread but couldn't find anything new I don't think I've already tried:
https://www.arboristsite.com/community/threads/chainsaw-dies-under-load-or-full-throttle.229941/

The only thing I don't think I've checked is the impulse line. If I understand it correctly, it basically provides pressure to the carb to pump the fuel through, regardless of position of saw. Any time I've been testing, the saw has been level. Still, this seems to be the only part of the equation I've not checked. But if fuel flows enough to idle then could this even be a consideration? Could it be that it's partially clogged and under full throttle not providing *enough* impulse to pump enough fuel into the carb fast enough?

Any ideas out there for this? Have I missed something?

Thank you in advance for any ideas.
 
dougand3

dougand3

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I'd read that using straight, unmixed fuel might help. Tried that and the damned thing started up but wouldn't shut off - even when I turned the ignition switch to OFF. Had to dump the fuel to get it to finally die. Don't think it did too much damage - only ran like this for <30 seconds but it was crazy it didn't stop. No idea what that was about but it didn't fix the problem anyway. I was also mistaken - I needed to get pure fuel without the ethanol in it.
So, you straight gassed saw? That REALLY won't help. Maybe you survived it but I'd pull muffler and look at piston/ring for scoring.

Original problem could be cracked impulse line or just carb not tuned properly. Open the H&L screws 1/2 turn CCW.
 
Huskybill

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Sounds like the spark arrestor is plugged up. Before the straight gas. With straight gas the carbon build up glows like a glow plug on a model air plane. Not good.
 
Ketchup

Ketchup

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So, you straight gassed saw? That REALLY won't help. Maybe you survived it but I'd pull muffler and look at piston/ring for scoring.

Original problem could be cracked impulse line or just carb not tuned properly. Open the H&L screws 1/2 turn CCW.
Yeah. If the saw ran until it starved for fuel, you very likely damaged your saw. Pull the muffler and have a look.

If you’re okay there, then you can get back to your problem.

Have you done a pressure and vacuum test? Probably not the problem, but since you’re doing work anyway, rule out air leaks.

What did your spark plug look like when you pulled it? How much carbon is on the top of the piston? Is the muffler outlet wet from fuel after you run the saw?

I agree with above that your carb setting may be your problem. Your H needle may be supplying too little fuel or too much.

Your impulse line is definitely a possible problem. Look for kinks, clogs or leaks.

Ultimately, it possibly your carb is just junk. I think it’s a WT. WT’s are known to spontaneously flood. They often don’t come back after a rebuild even if they look right.

Just some thoughts. I haven’t worked on a cs400 specifically.
 

rd35

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If you really only ran it for 30 seconds on straight gas, you likely did not do any damage to the piston. (Lets hope) Never Ever run a 2-stroke engine with straight gas!!!!!! From your description it sounds like you simply had the "H" needle setting too lean. Those factory settings on the needles are only a starting point for tuning. If it idles and you crack the throttle open and it dies, then you need to open (counter clockwise) the H needle 1/4 turn and try it again. You should find that it will come to life and run better once you get the H and L needles tuned properly!
Check out this link: https://web.archive.org/web/20051018212959/www.madsens1.com/sawtune.htm
It should help you get your carb set properly! Practrically all 2-stroke small engine carburetors get tuned the same way...so this should help you out!
 
Sam R

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I'd like to add to the above that it's a good idea to use non-flammable brake cleaner for small diaphragm carburetors. The reason being is that there are o-rings, and check valves that can get dried out and brittle from regular carb cleaner. I kind of doubt that's your problem, I agree that it sounds like your H is too lean.
 

SEAM

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L adjusts low speed and acceleration, H adjusts high speed... I guess a 1/8 to 1/4 turn counter-clockwise of the L screw will improve things.
After setting the carb to factory specs proper tuning is a must!
 
huskihl

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L adjusts low speed and acceleration, H adjusts high speed... I guess a 1/8 to 1/4 turn counter-clockwise of the L screw will improve things.
After setting the carb to factory specs proper tuning is a must!
Richening the L screw should get rid of a slight hesitation once the H screw is set, but not sure its going to rid of a bog that has to be feathered to get to WOT. I bet his high speed circuit has an obstruction or the H is lean
 
SteveSr

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This could also be a carburetor main nozzle check valve that is stuck closed. Saw will idle all day but will never rev up because the check valve to the high -speed circuit is stuck closed. The Walbro service manual tells you how to test it. It is not easily replaceable so for a low value carburetor you just replace the carb.
 

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SEAM

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Situation after carb work:
Now it starts up and idles better, but as soon as I apply throttle, it dies. I have however noticed that once it's warmed up, if I slowly apply the throttle, it CAN increase up to full throttle in a controlled manner. But if I want to just brap the throttle, the way you'd normally use a chainsaw, applying throttle as needed, it dies.
Sounds like simple hesitation ("L" too lean) to me for starters, if the saw can be manipulated to achieve WOT when warmed up some... The impulse line on Echoes usually lasts virtually forever (but we have better fuel over here).
Another possible culprit might be the primer check valve.
A proper carburetor tuning ist Step 1 - if that does not help a systematic troubleshooting is in order.
 
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