Stihl FS-38 trimmer mystery!!!!!!!!!

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Yeah I believe I'm going to try and get a leak down test done on it. What gets me about the whole thing is; I've always been able to get one to hit using a small shot of starting fluid. Not this one. Nothing.
 
If you got no results with ether then you have a compression issue. While you had the engine disassembled I hope you cleaned the ring groove and checked the ring gap. Also, be certain the purge circuit on the carburetor is functioning to ensure fuel is in the carburetor. Please let us know what you determine. I recently brought an FS-38 back from the dead for a neighbor but their only issue was with old fuel and dirty carburetor.
 
VaBchBum; About how much starting fluid in the spark plug is good? I don't want to blow that new cylinder piston kit up. It's got 120# compression now since I put that new cylinder piston kit on it. Is that a good compression? What about the previous 90# compression I had; is that low?
China parts or Stihl?
 
1) A leakdown test and a Vac/Pressure test are two completely different tests.
IF the dealer will/can do a Vac/pressure test, the intake manifold must be on.
2) Most modern 2 cycles have a variable ign timing circuity. It adds in reduced starting effort. If that portion of the module is defective, and they do fail, you'll never get it stated. It might be sparking at the wrong time. That would explain the backfiring. The timing is separate from the magneto portion, meaning it will show good spark.

I will get you credit for you vigilance in trying to get it running.
You already have, in labor and parts well over what the trimmer is/was worth.
On its best day.
 
1) A leakdown test and a Vac/Pressure test are two completely different tests.
IF the dealer will/can do a Vac/pressure test, the intake manifold must be on.
2) Most modern 2 cycles have a variable ign timing circuity. It adds in reduced starting effort. If that portion of the module is defective, and they do fail, you'll never get it stated. It might be sparking at the wrong time. That would explain the backfiring. The timing is separate from the magneto portion, meaning it will show good spark.

I will get you credit for you vigilance in trying to get it running.
You already have, in labor and parts well over what the trimmer is/was worth.
On its best day.
Did you gap the coil when you put it back on? I usually use two sheets of paper or an envelope if I can find one.
 
Maybe your new coil and flywheel aren't compatible and spark is occurring at the wrong part of the cycle. Two things would happen if this is the case , big time flooding or the pull start would get jerked out your hand or be very hard to pull over . Why did you buy a new flywheel???

If your machine isn't even try to fire starter fluid isn't going to help it will only make matters worse because it takes longer to fire an engine using starter fluid to clear the cylinder out before it does any good . Two stroke mix is the best for obvious reasons.
 
For anyone reading this old thread, as Toyboy stated, one of the first things you can check is the spark arrestor. It costs nothing and can be done with the supplied wrench. If your muffler shows any signs of wet oil, or even if it doesn't, remove it and burn it off with a torch. Get it red hot and give it a quick scrub with a small wire brush, the put it back. This is a common problem for trimmers and saws when not run at high RPM because the muffler doesn't get hot enough. The original poster could have saved a bunch of time and money had he checked this first, as the symptoms point to a plugged exhaust. The unit tried to fire, but the exhaust could only come back out the intake.
 
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