Troubleshooting: Stihl saw won't idle (dies)

[jdhupp]

Hi, all.

I've been troubleshooting an idle problem with a Stihl saw (TS-400 cutoff saw) and I'm nearly out of ideas. Hoping someone here can help. I feel invested now and don't want to simply hand it over to the service department.

By the way, I bought this saw used, so I don't know anything about its history.

The saw runs fine at higher speeds, but if I try to let it throttle down it dies immediately below a certain rpm. It also hesitates when I throttle up (does not accelerate responsively -- may actually drop a little for a half-second).

Making sure the saw is well warmed up does not help.

It was interesting that earlier, I could adjust the saw so that it would idle for a little while (less than a minute). Trying to solve that, I disassembled the carburetor, gave it a dip treatment, and reassembled with a full carb rebuild kit. I also adjusted (bent down) an inlet control lever ever so slightly to sit flush with the carb chamber floor, just as it is supposed to. But afterwards the idle performance was actually worse, and no carb adjustment corrected that.

I disassembled the carb and bent the inlet control lever the other way a bit, but that had little if any effect.

The saw passes all 3 vacuum and pressure leakage tests for the carb and crankcase.

The spark plug was replaced not long before I got serious about solving this.

Also replaced:
- a fuel hose
- in-tank fuel filter
- auxiliary air filter
- spark arrest screen
- exhaust gasket
- decompression valve (lets the Stihl start easier with less kick-back)

 

[D'Animal]

What is your lbs of compression?

 

[jdhupp]

What is your lbs of compression?

Suppose I should check that to cover the bases, but I would have thought that unless it was really low, it would idle -- at least with idle set high -- but bog down under load.

 

[D'Animal]

It will run at 100lbs compression but only 1/2 and full throttle. 100lbs is not enough compression for it to idle.


90% of the Concrete saws suffer debris ingestion in the form of concrete dust due to lack of maintenance on the air filter. The piston will be worn and the intake side of the cylinder will be worn as well.

 

[jdhupp]

It will run at 100lbs compression but only 1/2 and full throttle. 100lbs is not enough compression for it to idle.


90% of the Concrete saws suffer debris ingestion in the form of concrete dust due to lack of maintenance on the air filter. The piston will be worn and the intake side of the cylinder will be worn as well.

Compression (just checked) is 120 psi. I think nominal might be 137-139. One guy was telling me I needed about 130 for decent idle. What do you think?

I attach photos which I hope someone can judge. There is a light half-circle under the intake port, and on the top of the cylinder on the intake side, a light scoured half-ring (not carbon like the rest of the top).

 

[D'Animal]

The intake side of the cylinder is worn out. The white scuffing you see is the chrome worn off.

Look at the bottom edge of the piston skirt. It should be the same thickness on the intake and and exhaust.

 

[jdhupp]

The intake side of the cylinder is worn out. The white scuffing you see is the chrome worn off.

Look at the bottom edge of the piston skirt. It should be the same thickness on the intake and and exhaust.

The skirt is the same thickness on both sides, at least to within 0.004".

Since the intake side typically gets more wear (airborne abrasives), how far would you typically let the cylinder wear out before replacing cylinder and piston?

Apart from that, with compression at 120psi, is the cylinder wear likely to be the cause of the won't-idle problem?

 

[D'Animal]

The cylinder is no longer round and the "BlowBy" going by the rings is what is making it hard for the fuel mix intake charge to stay in the crankcase instead of being blown back through out the carburetor. That is why it will not idle.

 

[jdhupp]

On account of obvious wear on the piston skirt (worn semi-circular area on cylinder below intake port), I installed a new cylinder-and-piston assembly, but that did not solve the problem.

Though the carburetor low-speed ports did not seem to be plugged (squirted carb cleaner through with no obstructions) and I had already done a carb soak and installed a carb rebuild kit, I had other advice from a mechanic that works on Stihl saws who said that despite all that, this was probably a carburetor problem. He sees it often with the Stihl cutoff saws. He advised that I simply replace the carb at this point.

I did, and now it idles beautifully.

Thanks to all!

 

[fj40charles]

On account of obvious wear on the piston skirt (worn semi-circular area on cylinder below intake port), I installed a new cylinder-and-piston assembly, but that did not solve the problem.

Though the carburetor low-speed ports did not seem to be plugged (squirted carb cleaner through with no obstructions) and I had already done a carb soak and installed a carb rebuild kit, I had other advice from a mechanic that works on Stihl saws who said that despite all that, this was probably a carburetor problem. He sees it often with the Stihl cutoff saws. He advised that I simply replace the carb at this point.

I did, and now it idles beautifully.

Thanks to all!

So it sounds like you had 2 problems. Bad carb and worn out piston and cylinder.

Good job getting it fixed.