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PCM 454 Backfires above 2800 RPM

  • Thread starter Capt. Mark Block
  • Start date
C

Capt. Mark Block

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I have a PCM 454 which has st

I have a PCM 454 which has started backfiring when the throttle is advanced above about 2800 RPM. I tried retarding/advancing timing and no change. Engine runs up above 2800 RPM with no load (not in gear).

Any ideas??

Thanks
 
"Assuming the igniton system a

"Assuming the igniton system and advance curve is okay and you have uncontaminated fuel, I'd look for a fuel flow restriction like dirty filters, etc., creating a too-lean condition. If you see black smoke during the backfiring, the opposite would be true, a too-rich condition from an intake restriction or stuck choke. The too-lean condition is more likely.

"
 
"Assuming the igniton system a

"Assuming the igniton system and advance curve is okay and you have uncontaminated fuel, I'd look for a fuel flow restriction like dirty filters or a vacuum leak, creating a too-lean condition. If you see black smoke during the backfiring, the opposite would be true, a too-rich condition from an intake restriction or stuck choke. The too-lean condition is more likely.

"
 
Thank you - yes it was a fuel

Thank you - yes it was a fuel restriction.
 
" I have a 1979 Chris with twi

" I have a 1979 Chris with twin 305 in good shape with only 640hrs. Max rpm of 4500 was available but this year in spring rpm would not go above 3700 on one engine and 4000 on the other. Both engines are perfectly tuned. What are normal causes of engines not reaching specified rpm level or at least close to it.
Thanks, Steve (Toronto Canada) "
 
"I've got a 305K in my &#3

"I've got a 305K in my '80 Catalina, too. I'm reading your post as two engines that ran fine to red line at 4500, then sat over the winter and both have dropped down in max rpm after sitting.

I'll assume gas, oil and ATF in the tranny's are fresh and clean. And the props have not been changed or damaged. And the flame arrestors are clean.

No other symptoms (like smoking or rough running) than low rpm's would seem to point to the carbs getting gummed up or a sticking choke or throttle linkage. Check your plugs for a clue. If the carbs check out okay, ignition is the next target. Check your timing and advance curve. New coils, wires and plugs would be next. If you're still running points, consider changing to a Pertronix electronic unit. Worth every Canadian dime!

"
 
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