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.
China parts or Stihl?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?
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.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.