NormK,
It still can be spark related as I have seen plugs to fail under compression and still fire outside the engine when not under compression. I even had one compressor that would run and idle when started but quit as soon as the compressor was to again build up the tank pressure. Simply would not run if the engine was needed at 90psi tank pressure load. I could drain the tank pressure and engine would again build the tank to the cutoff point and return to idle. It would idle all day unless it the a pressure build up needed. Any time an engine is under load the compression load is even higher.
When comes plugs misfiring under compression some strange things can happen. I had one Husqvarna backpack blower last Spring where the operator swore the connecting rod was the problem as knocked so bad above idle speed. It turn out to be a failing plug.
Even Ignition coils can fail the same way just not as often. Most either works or not but I have some very coil problems too. With one that would set fire to air filter on a Homelite Chainsaw. I also had Husqvarna 570 just a few month ago to run great when first started to only go bonkers later doing all kinds things including backfiring out the carb and shooting flames out the muffler.
Now if this is a L-head (I think you guys called side valves) then a valve not closing completely can cause the described problem as there can be an overactive ACR.
Last edited by AVB; 28/01/17 03:00 PM.
