Stihl 050av chainsaw with points(breakers),.

There is a sheer key on the fly wheel, it is probably bent and out of timing. Take it off and replace the key.

http://www.mediafire.com/view/79dalxfo44648wx

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as you didn't mention any details of this saw , im gonna suspect its at least 10+ years old , and I can say if its still got its original points and condenser , even its orig coil that's where id start looking for a problem ! , I do assume you have already tried a new plug as they do short out sometime and wont fire even in air !! , these are great saws they have been making this design since 1968 (I have a 72 that still runs !!!) and they still make them ,although they renamed it a farm boss nowadays , its still the same design , although they don't come with points any more no there all electronic now

Have checked your spark arrestor in the muffler?

041 timing pdf

Stihl 041 Timing

I had something similar a long time ago with my Stihl chainsaw. I removed the small carbeurator and found that the gas/ carb float was at times sticking in the open position allowing too much gas to flow into the piston cylinder. The spark and compression can theoretically burn so much fuel on each compression stroke of the piston and the residual ie; overflow of gas/oil mixture is forced out the muffler on the exhaust stroke. Keep in mind the engine is a two cycle stroke engine meaning it fires each time the piston compresses and therefore if it is the float sticking open it will seem like a great deal of raw fuel being forced out the muffler. I hope this will give you a starting point to look at. Good luck and hope you don't get discouraged because the Stihl product, especially the chainsaw is one of the finest produced and on the market today.

Hello there, the correct 'air gap' between the magnet and the ignition unit is around 0.25mm (or 0.010") Use a feeler gauge to achieve this setting and use the power of the magnet to sandwich the feeler gauge between the magnet and both poles of the ignition unit. There should be sufficient clearance around the mounting screws to achieve this. If your still having problems check for sideways and up / down movement of the flywheel / crankshaft. If you have movement here and it isn't a loose flywheel on the crankshaft, the main bearings are probably shot.

If you have changed the plug and have it firmly earthed against the cylinder and there is no spark pull the earth cut out wire off the ignition unit to rule out the switch and wire, now try again, if still no spark replace the ignition unit.

0.3mm should be the air gap.

Check the filter inside the fuel tank
Check the fuel line for cracks or holes
Clean the carb

If still not working,replace the pump diaphragm on the carb