Stihl 076 trouble milling saw

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Jerome

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I recently rebuilt a 70s era 076 for a milling saw. I replaced all gaskets, crank bearings, seals, piston, rings, cylinder, worm gear and the drive for the pump. I am running it on 40:1 mix. I have had the normal vibration issues and air leaks that are so common with these saws and think/thought/hope that I have them sorted out. When the saw is cold it starts 1st or 2nd pull, I let it idle for a few minutes to warm up a little before I start to cut. It runs and cuts perfectly for around 5-10 minutes, revs build and fall nicely, it idles well and does not stall out. However, after about 10 minutes it starts to behave like there is an air leak, the revs will rise and fall with no change in pressure on the trigger (I have the trigger lock on as I am using it on a mill and don't always have my hand on the saw handle). I have let the saw stay running when it does this to it see if it sorts itself out and it does not, it always dies within a minute. After such an episode I cannot get the saw started again until it has cooled down completely and unless I let it sit overnight it will not run correctly if I can manage to get it started. Also if I can manage to start it it runs like the choke is on and pours white smoke for around 20 seconds and then will die if I let off the throttle.

I have loctite on the decomp valve, the little M4 bolt perpendicular to the decomp and check them to make sure they don't move. They don't. The intake manifold bolts don't come loose and neither do the nuts holding the air filter base onto the top of the carb. However, I can tell that something is screwy because if I take the air filter and its base off right after the saw stalls the first time the gasket between the carb and the filter base is SOAKED and the filter is very wet if not soaking too. All gaskets from the cylinder up to the carb were replaced again today just to be sure and the same thing happened.

I have not done a vacuum test as I do not have a tester but think that I can rule out an air leak as the saw runs nearly perfectly for the first 10 minutes. I have not replaced the carb or done a rebuild. I have thoroughly cleaned it but reused the diaphragm and other parts. Again not sure that this is the issue as it runs well to begin with. I am running the L set screw 2.25 turns out and the H set screw out about 0.75-1 turn out.

Any help would be greatly appreciated, thanks!!
 
I don't have a lot of experience on these saws, only worked on a couple, but the basic carb settings should be 1 L and 1 H.
 
I started at the 1 turn setting and worked to where I am at now. The H isn't very far off but maybe my L is out too much because my idle screw is off?
Would the leak not show itself until the saw is really hot?
What kind of electrical issue would cause these symptoms?
I have a new gas line from tank to carb and the tank seems to vent well as there is no pressure equalization when I unscrew the gas cap.

Not a NEW plug but a nearly new, few hours on it, that I took off strong running saw.
 
Typically the ignition coil starts to fail when hot. Electrical resistance goes up with heat.

I'd replace the plug first and check the plug wire for shorting.

Personally went nuts on a power washer with a honda 4 stroke. Clean looking plug. After a new carb, gas tank and coil-I finally replaced the $2 spark plug and it ran fine. Face palm.
 
Gotcha, I have a new style ignition module off a TS510 that I can swap in, no idea what condition it is in however... Is there a way to check with a volt meter or something?

Checked the plug wire casing, it is totally intact and not even crushed anywhere. The boot looks good too. I will try a new plug tomorrow, bit late here now.
 
You can check the ignition for heat related failures easy, run it again, when it stalls out by itself, pull the plug, ground against the case and yank it, check for spark.
 
After such an episode I cannot get the saw started again until it has cooled down completely and unless I let it sit overnight it will not run correctly if I can manage to get it started. Also if I can manage to start it it runs like the choke is on and pours white smoke for around 20 seconds and then will die if I let off the throttle.

However, I can tell that something is screwy because if I take the air filter and its base off right after the saw stalls the first time the gasket between the carb and the filter base is SOAKED and the filter is very wet if not soaking too. All gaskets from the cylinder up to the carb were replaced again today just to be sure and the same thing happened.

I am running the L set screw 2.25 turns out and the H set screw out about 0.75-1 turn out.

How did you tune it? ( You can't use a tach or your ear to set H without a load on the engine, see below). Try one turn out on the H and one and a quarter on the L.

Do you have a spare carb?
The 1111 series saws are rev limited via the carb. At at a threshold RPM the vibrations cause a control valve to open and the air/fuel mixture is made richer to prevent the RPMs climbing any further. It sounds like your control valve might be opening and not shutting again when it gets hot causing your saw to flood.
It is possible to bypass the control valve with an aluminium shim.
 
I am running the L set screw 2.25 turns out and the H set screw out about 0.75-1 turn out.

The final adjustment on the H has to be set "in the cut". You will be lean if you just set it up at WOT.
 
How did you tune it? ( You can't use a tach or your ear to set H without a load on the engine, see below). Try one turn out on the H and one and a quarter on the L.

Do you have a spare carb?
The 1111 series saws are rev limited via the carb. At at a threshold RPM the vibrations cause a control valve to open and the air/fuel mixture is made richer to prevent the RPMs climbing any further. It sounds like your control valve might be opening and not shutting again when it gets hot causing your saw to flood.
It is possible to bypass the control valve with an aluminium shim.
I've had 1111 series saws that weren't limited, some of the carbs I know we're limited but I'm not sure about all of them.
 
The carb is a Walbro 21. How would I tell if there is a limiter? I had done my best to set the H screw "in the cut". I would adjust put the saw in the wood take it out, adjust, repeat until it had the most power while cutting.

After putting a new plug in and running the saw this morning and with the above info I think that the ignition module may be the issue.
1) I grounded the plug after the saw cut out and there was no spark. I waited for the saw to cool for about 30 minutes, its only 35˙ out so that was plenty of time, and then grounded the plug again and there WAS spark this time.
2) I saw the white smoke again and managed to keep the saw running long enough to see where it was coming from and it comes from the flywheel/pull cord area. The smoke also smells slightly like burning metal/plastic not exhaust.
3) Once the first or second stall-out has happened it doesn't matter where I set the limit screws the saw will not stay running, IF I can start it, until it has cooled off.

I am also going to do a compression test the evening when I know for a fact that the saw is cold because when I first rebuild it I had to use the compression release to start now I don't. I wouldn't think that slightly low compression would be such an issue however. If the compression where so low that the saw cut out how would it even start and stay running for 5 minutes??
 
A picture for those who are interested:
 

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The carb is a Walbro 21. How would I tell if there is a limiter? I had done my best to set the H screw "in the cut". I would adjust put the saw in the wood take it out, adjust, repeat until it had the most power while cutting.

After putting a new plug in and running the saw this morning and with the above info I think that the ignition module may be the issue.
1) I grounded the plug after the saw cut out and there was no spark. I waited for the saw to cool for about 30 minutes, its only 35˙ out so that was plenty of time, and then grounded the plug again and there WAS spark this time.
2) I saw the white smoke again and managed to keep the saw running long enough to see where it was coming from and it comes from the flywheel/pull cord area. The smoke also smells slightly like burning metal/plastic not exhaust.
3) Once the first or second stall-out has happened it doesn't matter where I set the limit screws the saw will not stay running, IF I can start it, until it has cooled off.

I am also going to do a compression test the evening when I know for a fact that the saw is cold because when I first rebuild it I had to use the compression release to start now I don't. I wouldn't think that slightly low compression would be such an issue however. If the compression where so low that the saw cut out how would it even start and stay running for 5 minutes??

On my 075 I had a heck of a time keeping it running after it was hot. I ended up finding the intake cracked. It was fine cold but hot I guess it was expanding and causing a leak. Once I found it I expoyed it and it seemed to correct the issue...it now has a new one. Other issues I found during my trek with this beast: poor connection to the spark plug connector (spring looking thing) and most recently oil pump o ring leaking causing air leak. You really need to pressure/vac test it before doing anymore trouble shooting.
 
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