Another Stihl 025 Mystery

[Fish]

No, it is not that hard to get at.
I agree, it is not the impulse line.

 

[Fruecrue]

You could try a blast from an air gun. That could however have some negative effects, rupturing the impulse line if it's plugged or putting debris into the motor.

 

[Wood Doctor]

No, it is not that hard to get at. I agree, it is not the impulse line.

Wait a minute, Fish, I thought we concluded that the problem is the impulse line (blocked, cracked, etc.). There appears to be no air movement through it at all. Doesn't the carb wait for a signal from the impulse line before it squirts fuel into the combustion chamber? If it gets no signal, then the fuel does not flow.

 

[TJ the Chainsaw Mechanic]

Just to double check, put a dab of grease over the impulse hole. If it sucks in or spits out the grease the hose is probably fine. You were getting Some fuel at one time....so that more than likely means the hose is cracked not broken. You don't need the plug in the cylinder to check impulse. These small saws don't produce a whole lot so give it a few fast pulls.
But like said, if the hose has a small crack it more than likely will show some small signs of impulse.

 

[Fruecrue]

I may need to be corrected here, but, I believe the impulse drives the diaphragm, which pumps the fuel into place to be then pulled in by a Venturi when air flows through the carb.

I fix boilers for $, saws are a hobby.

 

[TJ the Chainsaw Mechanic]

I may need to be corrected here, but, I believe the impulse drives the diaphragm, which pumps the fuel into place to be then pulled in by a Venturi when air flows through the carb.

I fix boilers for $, saws are a hobby.

it drives the fuel pump gasket at the bottom of the carb which pumps the fuel and also drives the diaphragm. You are correct.

 

[Wood Doctor]

Just to double check, put a dab of grease over the impulse hole. If it sucks in or spits out the grease the hose is probably fine. You were getting Some fuel at one time....so that more than likely means the hose is cracked not broken. You don't need the plug in the cylinder to check impulse. These small saws don't produce a whole lot so give it a few fast pulls.
But like said, if the hose has a small crack it more than likely will show some small signs of impulse.

With spark plug removed, I covered the impulse hole with a dab of Moly grease and blocked the manifold with my thumb. Several fast pulls produced nothing. I repeated with the spark plug inserted. Again nothing. The grease blob stares at me like a dead mouse. There is no feedback signal getting to the carb diaphragm. That would explain why no fuel mixture is making it to the combustion chamber.

 

[big t double]

Vac/pressure test performed?

So......

 

[fearofpavement]

Well, if you pull the handle, you can get at the impulse line. It may be off the cylinder. Sounds like you're bracketing the target.

 

[Wood Doctor]

Well, if you pull the handle, you can get at the impulse line. It may be off the cylinder. Sounds like you're bracketing the target.

I did that, Fear. The impulse line looks good. No cracks anywhere. I haven't blown out the hose yet with high pressure, but that seems like a fruitless exercise. Replacing the impulse hose on an 025 is impossible unless the cylinder is also removed. So all of my fears of completely disassembling the engine to replace the impulse hose are justified.

So, I think we are back to a blown engine with air leaks that prevent any operation of the impulse hose. Fish may have been right. The crankshaft seals are likely gone and those leaks allow air in so that nothing makes it to the impulse hose. Seem reasonable?

 

[fearofpavement]

A seal would have to be completely blown out for the saw to not make any impulse at all.
Also, I don't think you have to have the cylinder off to change the hose. Should be able to fish it around with a forceps. I have an 025 in the shop that's partially apart, I will try to take a look if I have time tomorrow.
Did you look at the intake side of the piston through the intake port while rotating the engine? I had a piston with a broken skirt once and I spent many hours trying to figure that one out.

 

[blueberrymuzik]

Sounds like you've pulled the handle, this is a good time to inspect the intake boot for a tear. It seems unlikely that the crank seals would give up completely as you are running the saw. It is easy to tear an intake boot if you get the saw hung up and yank on it to get it loose.

 

[kyle.kipple]

So, I think we are back to a blown engine with air leaks that prevent any operation of the impulse hose. Fish may have been right. The crankshaft seals are likely gone and those leaks allow air in so that nothing makes it to the impulse hose. Seem reasonable?

So I'm confused...you said the saw was running and just stopped. Said to determined it was getting fuel because of a wet plug. I don't see a leaking crank seal stopping a saw dead in its tracks during a cut. Unless it just completely shredded.

 

[Fruecrue]

I'd be figuring out a way (possibly a straw or small diameter hose) to check for blockage/ flow through the impulse hose to the crankcase.
I agree with fearofpavement, even with no seals, there's enough displacement to get a pressure change at the impulse port to the carb.
Not to mention with air leaks that big, the top end would likely be torched.

 

[Chainganger]

Arborist Site has a thread on replacing 025 impulse line
Google
Stihl 025 impulse line

 

[Wood Doctor]

Well, this is definitely a team effort and I appreciate all the feedback. The intake manifold boot looks OK. Remember this saw has never been taken apart to any degree until today. There is no indication that the impulse hose is defective, nor is it's connection to the carb housing loose. I hate to pull it apart if it's OK. I'm sure FearofPavement understands this.

The problem is that there is no fuel being pumped to the jug and that's triggered by the engine sending signals to the carb through the impulse hose. If I said that the plug was wet earlier, I believe I blew it. Please forgive me. The only way I can wet the plug now is by squirting fuel into either the plug's threaded hole or the carb intake.

The impulse line could be still be blocked, but only at the exit point from the cylinder. A compressed air blow out might work for awhile, but whatever grime is blocking it now would still be inside the chamber and ready to collect and coagulate there again. I think a bunch of crud got into the crankshaft area and eventually blocked up the entrance to the impulse hose. Any comments on this conclusion or suggestions?

I believe this is the thread Chainganger was talking about:
http://www.arboristsite.com/community/threads/stihl-025-wheres-my-impulse-hose.76881/

WindThrown's post is the best in this thread. He figured out a way to replace the impulse line without removing the cylinder.

 

[Jimmy in NC]

The025/MS250 had 2 different ignitions, you have to make sure that you have the correct flywheel.

Exactly my thoughts. Flywheel wonky.


Ahh read ahead and see its a fueling issue. I would just tear it apart, clean it up fully, and go from there. After you get the rear handle off the rest comes apart quickly.

 

[Milkman31]

A 20min pressure/vac test would answer a lot of questions.

 

[big t double]

A 20min pressure/vac test would answer a lot of questions.

Best. Answer.

 

[kyle.kipple]

It's a pain in the rear to tear apart for sure but it only takes time. It's always worth it if you're able to fix a problem child.