Stihl 026 revving problem - newbie question

[spudulike]

Sometimes you can tell a saw has an air leak by turning in the L screw. Normally the idle speed rises until the engine dies when the screw is nearly in.

If the saw has a significant air leak, the idle speed increases as the screw is turned in and stays high even with the screw in with the idle screw not lowering the idle as it should.

If you know carbs and adjusting them, this can help diagnose air leaks.

A typical pressure test is a good idea.

If the saw was set at 1 1/4 turns out on both screws and making around 13000rpm and stable with the throttle held open. It shouldn't have failed if the fuel/oil were good.

 

[LumberDutch]

The saw was set at just under 1 1/4 turns out on both L and H, so I thought that should be ok. I did not check the revs and maybe it was a bit high (by ear). I have heard of 4-stroking, but am not confident I would always correctly identify it. I mixed fresh fuel and oil and it should have been the right ratio - maybe even a bit more oil (40:1).

When hot, I did turn out the L a bit as it was idling too fast. I did not try to turn it in, so an air leak is still possible.

So yes, I have been doing some reading on pressure/vac testing and will give that a go. One question: will the pressure at the top of the cylinder (spark plug hole) be the same as in the crank case? I assume with my cylinder in this state that would be the case, maybe it is always the case. It may help me with a diy pressure test setup.....

[spudulike]

The pressure or vacuum in the crankcase will be the same as above the cylinder but if you use a converted spark plug to inject pressure or vacuum to the engine, move the piston to below the upper transfers so the pressure or vacuum above the piston is transferred to the crankcase.

Remember we are not talking about secondary compression which is the pressure between the piston and combustion chamber once the piston covers the exhaust port during pulling over the engine 

You can't guarantee carb settings at the factory standard of 1&1. You should always tach a saw or live with recognising the four stroking note when flat out. 

If the fuel was good then you are left with carb settings/issues or air leak. There are other weird reasons for machines to seize but are not common.