025 problem adjusted carb, changed filter still runs bad

Diesel JD · Oct 4, 2005

Hey guys,
I'm having trouble getting my 025 to work right. It will start right up and then die as soon as I begin to accelerate. Funny thing is it works great some of the time. Plenty of power. the compression is in the 90-100 PSI range even cold, so I don't think that would be an issue. Where would you all begin to look...here's what already has been done...
1) Fuel filter in tank
2) carb adjustment
3) cleaned carburetor
4) checked compression
Any help is appreciated,
J.D.
PS if it makes any difference, if I lean it over on the side where the fuel tank is, and bury the trigger it will often snap out of it and start running great again. Thanks

12guns · Oct 4, 2005

Well, I'm no pro, but if you can tilt the saw and let the gas run over the filter causing it to run fine, sounds like a filter problem. Does it act up w/ a completely full tank? or just when it's low? Also I've found that after checking/cleaning the carb more than once sometimes it starts working fine. Don't put too much stock in what i have to say, wait for the pro's to reply...

Diesel JD · Oct 4, 2005

I thought that too, so I tried filling up the tank...no change, changing the fuel filter ...noo change...in answer to your question. It happens both on a full and < half full tank.
J.D.

12guns · Oct 4, 2005

Did you adjust the carb before you changed the filter? Did you have the carb completely apart? jets and all?

Diesel JD · Oct 4, 2005

Not sure I had teh carb completely apart, just had the top and bottom and gaskets off,a nd cleaned it with some fresh gas or something and reassembled. The carb was adjusted again after reassmebly and after the air filter was cleaned. I just don't get it...

12guns · Oct 4, 2005

I know, it can be frustrating. I fought an old 045 a " buddy " sold me for 50 bucks...said it ran great. After a couple months playing w/ it I found out it had a cracked case. Go figure. I would start by taking the carb completely apart, every little spring and screw you can comfortably take out. make sure the ends of the needles are clean and smooth, spray some cleaner on the parts, rinse w/ gas, and put humpty back together again. If that does not fix it, you have to move to a higher power than this 24 year old.

Simonizer · Oct 4, 2005

Sounds like an air leak, crank seals, or manifold. Do a pressure/vacuum test on crankcase. Cheers.

bugfart · Oct 4, 2005

Does the diaphragm have a little hat on it that slides into a hinged fork? If so, make sure he's engaged during assembly.

Is that compression of 90 - 100 a good reading for a Stihl of any variety? It sounds low to be but I'm absolutely no authority on anything.

Lakeside53 · Oct 4, 2005

If it really has 90-100lb compresson then that a big problem and probably wouldn't even start. The saw should be 140-155lb to run really well, and will "ok" (maybe) down to about 125-130lb. Check that your compression tester has a schrader valve (like a bicycle tire valve) in the end. If it doen't then you can't use it on a small engine as the volume of the tube to the meter is a significant ptoportion of the the saw displacement.

Lakeside53 · Oct 4, 2005

Check the fuel hose... they can get a small hole where they bend into the tank. Just pressure test the tank (REMOVE ALL GAS FIRST) though the fuel hose. This will show the problem.

The outlet valve on the carb can get gummy or blocked. Does your saw have both high and low adjust screw, or just a Low? The single screw carbs are a problem for blockage. If you don't have access to an ultrasonic cleaner (like for jewelry), get a new carb kit, knock out the old galley covers and soak the carb in napha over night. Then replace the new galley cover(s) carefully, etc etc. If you have a cleaner, just open the carb and put it in the naptha in the ultrasonic cleaner for about 3 minutes.

Diesel JD · Oct 4, 2005

Ok, I'll check compression and yes the tester does have a schrader valve for pressure relief at the top. Should the compression be tested engine hot or cold. How do you test for pressure and vaccum, or should I let a shop do it? Is it worth new rings and piston for an 025 or is it junk?

Lakeside53 · Oct 4, 2005

Doesn't really matter hot or cold. I only see a small difference. Most important is to make sure the bore has some lubrication.

If it is low on compression, Rings won't normally do it : the cylinder/piston is probably shot. It's a pain on an 025 to economically replace the rings or piston unless your time is worth little. A shop really can't do it for you at any price that makes sense as the entire motor has to come out and be disassembled - you pretty much have to strip the entire saw. ANd you'll be shocked by how much Stihl pistons cost.

Unless you have the carb adapters, you probably can't pressure/vacuum test the saw. You can pressure test the fuel line/tank though by just pumping it with bicycle pump.

However : you say it sometime runs great. That doens't sound like low compression. Sounds much more like fuel starvation, either by delivery to the carb or by the carb itself. So get your compression tester checked...

Lakeside53 · Oct 4, 2005

Diesel JD said:
Ok, I'll check compression and yes the tester does have a schrader valve for pressure relief at the top. Should the compression be tested engine hot or cold. How do you test for pressure and vaccum, or should I let a shop do it? Is it worth new rings and piston for an 025 or is it junk?

you say "at the top". The value NEEDS to be at the VERY botton of the hose right where it screws into the plug hole... Did I misread you, or did you me?

Lakeside53 · Oct 4, 2005

Diesel JD said:
Ok, I'll check compression and yes the tester does have a schrader valve for pressure relief at the top. Should the compression be tested engine hot or cold. How do you test for pressure and vaccum, or should I let a shop do it? Is it worth new rings and piston for an 025 or is it junk?

Sorry, I didn't answer your question "how to pressure/vacuum test". In short, you block off the muffler port by slipping a rubber sheet between the muffler and the cylinder, then tighten down the muffler. Remove the carb, put in an adapter which covers the carb inlet and impulse line, and then you can pump up to .6 bar (say 9-10psi). It shouldn't leak. If it does, start spraying with soapy water. If the pressure test passes, do the same with a hand held vacuum pump. If it looses vacuum in say less then 20 seconds to .3 bar, your crank seal(s) are shot.

Your problem is likely NOT crank seals. Crank seals generally give problems at idling or with starting, not high end or power. Be careful with seals on these 021/23/25 saws - there are two types depending on whether you take the engine apart or insert the from the outside.

Diesel JD · Oct 5, 2005

Okay good answers, maybe if the compression reads low my gauge isn't right for teh job. It's teh same kind you get at a typical auto parts store, goes up to about 300PSI and is designed for cars and boats and whatnot. It has great power when it has power....just sometimes can't get it to stay running.

bugfart · Oct 5, 2005

Crank seals doesnt sound like an easy job - just a guess.

georgodjungle · Oct 5, 2005

filter but

the fule line?

Fish · Oct 5, 2005

Sounds like a common fuel line full of cracks, the 025s and 029s have crap for fuel hoses. You will likely have to blow the crap out of the carb as well, dust enters through the cracks.
Remember, check the simple, obvious things first.

Fish · Oct 5, 2005

When you go through the carb, you need to remove the lever, spring, and needle,
and blow out that passage especially, so you can visually observe the hole is
clear of gunk.

jowen · Oct 5, 2005

maybe a plug going south

might want to change the plug and gap it, and check the spark timming. just a thought

025 problem adjusted carb, changed filter still runs bad

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