Looking For Impulse Pressure For A Stih 026....Cool! The forum is in Dutch today.

[buddie]

Perhaps I am not using the correct search terms but I cannot find the proper pressure value for the impulse system on my 026. My MityVac says I've got about 6 (+ -) lbs pressure when I crank the saw while drawing off of the impulse tube, can anyone please offer me some acceptable pressure readings? Thank you!!

 

[mtngun]

That's a new one to me. I've never heard of anyone testing for the strength of the impulse line. Normally we test to see if the saw has an air leak.

 

[buddie]

Thanks for your response mtngun. As you can probably tell, I am a rank novice at this sort of thing but trying my best to determine why my saw won't start normally. Runs great after I prime the carb and it seems as if the carb is not getting fuel at cold start-up so I'm trying to figure out why. Here is what I have done so far:

Rebuilt the carb to spec's
New Stihl air filter
New Stihl fuel line
New Stihl tank vent
New Stihl master control switch
New spark plug (gapped)
Crankcase pressure 158 lbs
Impulse rubber line holds good pressure

I can get prime and start saw if I slightly pressurize the impulse nipple located on the carb, but that's my best shot so far. That seems to energize the carb with fuel. Also, I have had luck getting this saw to kick if I hold my finger over the carb throat while pulling magnito- I feel lots of suction on my thumb before it kicks. That's why I think it may be an impulse issue but I have yet to test for an air leak. I will try to do that tomorrow. I am thankful for all help offered.

 

[mtngun]

I doubt if an air leak would cause hard starting but it never hurts to leak test if you are set up to do it.

Not getting fuel could be many things. Do you have a way to leak test between the fuel pick up and where the fuel line connects to the carb ? Usually I fish the fuel line from the tank, take out the fuel filter, plug in the mighty vac, and block off the carb end of the fuel line while I leak test. Takes longer to describe than to do.

If the fuel line checks out, it could be something in the carb. Make sure the diaphragms and gaskets are in the right order -- it's shown on the IPL.

Sometimes carb problems can be tough to track down. :msp_mad:

 

[buddie]

Thanks again mtngun, I'll read up on pressure testing and attempt to do it tomorrow. Fuel pickup tube is new and pressure-tests fine. New carb kit installed correctly I'm certain. I've re-built about 5 of these little Walbro critters on various yard equipment and they are all good runners. I rebuit it because the no-cold-start is the reason why I tore into this saw in the first place. This is the first impulse carb I have touched so that is why I am questioning that detail. I bought the saw new years ago and it is a low-hour saw, roughly 20 hours on it altogether. Anyway, I'm kind of glad I face this little problem because thanks to the kind folks on this forum, I am learning a lot about this saw. I'll stay at it until it works. Thanks!

 

[ttyR2]

I hate to say it, but even though you said it was assembled right, I'd be looking into that again. Get yourself an IPL and compare the carb parts. If you're seeing any vacuum pulses through the impulse tube, that's not the problem. It's got to be the card. Did you double-check the fuel line? (I generally replace all the hoses as a habit on any saw I work on, they're cheap)

 

[Steveo_supremo]

Sounds to me like the choke flapper on the air filter isn't closing all the way or isn't sealing against the carb when You set the master control to the "choke" position.

 

[pioneerguy600]

Sounds to me like the choke flapper on the air filter isn't closing all the way or isn't sealing against the carb when You set the master control to the "choke" position.

+1 on that.

 

[buddie]

I hate to say it, but even though you said it was assembled right, I'd be looking into that again. Get yourself an IPL and compare the carb parts.

Have IPL and checked and rechecked assembly several times.

If you're seeing any vacuum pulses through the impulse tube, that's not the problem. It's got to be the card. Did you double-check the fuel line?

Okay, I thought a little impulse pressure was a good thing because it drove the diaphragm.
Fuel line is new. Hopefully I will be doing a pressure test today.

Sounds to me like the choke flapper on the air filter isn't closing all the way or isn't sealing against the carb when You set the master control to the "choke" position.

I know, it does sound like that is the culprit and it was the first thing I replaced but it is engaging properly. I put a Stihl brand filter on it, still no-go. But even the new filter flap isn't 100% air-tight. Is is supposed to be? I thought it needs a tiny bit of by-pass air so it won't just detonate and stall during start up.

This no-cold-start issue was the reason why I began repairing/replacing parts. In other words, it's still doing the same thing it was doing before any repair/replacement. Please keep the suggestions coming!!
Thanks!

 

[pioneerguy600]

The next logical step is to get that vac test done, there is very little crankcase pressure in a good running tight saw, 5-6 lbs is what I have observed, but a saw with an air leak will have even less. When a engine has an air leak it will have less vaccum available to draw in fresh fuel/air mixture through the carb, it will get too much air from the leaking area. The 026 is known for its leaking radial seals, especially the flywheel side seal. The rubber bellows type manifold also is prone to cracking.

 

[buddie]

...... The rubber bellows type manifold also is prone to cracking.....

YOU GOT IT! That ended up being the problem - starts and runs great now. The intake bellows was not clamped down on the head. Noticed it while I was taking saw apart but would not have thought to check it otherwise. Thanks!!