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M330 backfires above 2000 rpm after warm up

4

440x2

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Configuration is two 1977 440s

Configuration is two 1977 440s in a Uniflite with Pertronix. Problem just started happening after a year of running trouble free. Port engine starts and runs fine for the warmup in the harbor then once out the gate runs fine at 2500 rpm for a few minutes then begin to hear a popping noise (think carb backfire). Backed down to 2000 rpm and all is well for hours. Run back up to 2200 rpm and the backfire resumes. Already replaced cap and rotor. Appreciate any advice.
 
is it holding RPMs when poppin

is it holding RPMs when popping or does if give up a few hundred RPMs. just curious as I had that problem and wonder if it's related.
 
"It seemed to drop rpms but si

"It seemed to drop rpms but since I didn't want to dig a bigger hole I backed off the throttle as soon as the popping starts so I don't bend things. Today I went after the distributor and think I found the problem. Under the plate inside the distributor where the centrifugal advance weights and springs are it was totally rusted up. Somehow water got in the distributor and since it had been so long since I ran the engines hard, the rust had time to finally freeze up the advance. It was so bad you could not move either of the weights so I had the distributor bead blasted and electric greased the advance mechanism. Ran the engine and it ran a lot better at the dock but won't know if the "popping" is gone till I take the boat out for a spin. Maybe tomorrow. I'll post back in here the results."
 
"Good work, but if the problem

"Good work, but if the problem doesn't go away, start looking for fuel flow problems: Dirty filter, water in gas, etc.

Jeff"
 
"Fixed! The advance was stuck

"Fixed! The advance was stuck/frozen therefore going above 2000 rpm the distributor couldn't advance consequently the timing was off enough to cause the backfire.

Thanks for the advice re fuel but I ruled that out early since the two engines share the same fuel supply.

So, if you're running good below 2200 rpm and backire above 2200 rpm, your advance is highly suspect."
 
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