031 AV Help needed

[Rotten1]

I need some advice on a Stihl 031 AV. Saw is 26 yrs. old, was not getting fuel. System was all clean, Carb diaphragm & needle were hard. I Put a kit in carb, now I can start it, but can't keep it running to tune carb.
There is no exhaust restriction, strong compression, & no crankcase leaks. Pulse hose is good & working, Intake boot is good. L & H screws are 1 turn from seated. Strong fire , points are at .015". Oh, and fuel line and filter are good.
Any help would be Greatly appreciated.
Thanks, Rotten

 

[ShoerFast]

Rotten1
Not much more to check,,,, something I would try, maybe you had allready, but just taking the fuel line off at the carb and putting a hose in the filler hole and block the rest with a rag and just blow intell a good streem of mix comes throught,,,,,, I have had air bubbles traped in the feed line that wouldent purge.

 

[stihltech]

leak

You need to put a little pressure to the fuel inlet barb and see if it holds air. If not, you have found your problem.

 

[Justin Garrison]

Those 031s are real buggers to work on, that is why I gave up on mine, It had a simmilar problem, I did all the things you mentioned, but it still would not really run. The muffler would fall of if it did run for awhile.

 

[ShoerFast]

Giving it some thought, I had worked on an 031 av once that had a coil that would crap out,,,, the resastance checked well, but as it ran, it just wouldent keep the spark comming,,,, at that time I had an automotive osilliscope that it showed up on. but it was very consistant on when it wanted to stop working. As soon as it would stop, give it a tug and it would try to run again.

 

[Lakeside53]

You may have a bad condensor...

 

[ShoerFast]

Lakeside
Your right, but in this case the scope should have shown the high voltage spike (coil secondary desapating and plug fireing) and the osalations crossing both sides of the line (condensor disapating the coil secondary that cant bridge the spark plug gap),,,,, But when the fun stoped, the scope would just flat line,,,, about the only thing that would take that much voltage would be a winding in the coil shorting to ground. It was a thrower cause it would happen right at the point of full transfer slot in the carb - main or "H" jet. What lead me to hook a scope up to it, was just putting in a new plug (with out checking the gap, bad shoer) it gave up at a lower speed, resetting the gap from .025 to .020 was about the same as the old plug,,,,, but dropping the gap to about a frog hair, (lets try anything once) you could get a little speed, but no power.
This was almost 25 years ago, funny how we rember when we got a but kicking!

 

[Rotten1]

Got back out this morning and tried air pressure at fuel barb, holding pressure good. Blew through filler cap, no air bubbles in fuel line.
It doesn't seem to be losing fire at all. I can make it run for a minute at idle, but cuts out some. At partial choke I can rev it up (about 1/2 throttle) and it smooths out for about 30 seconds then dies. Plug is dry after it dies (no flooding).
I Can't make it run yet, but it now has a new pull rope, & I could use a new arm.
Thanks, Rotten

 

[Fish]

Take your carb apart again, remove the needle, blow out the passage, and visually
ensure it is clear.