Marine Power 5.7 misfires after 1 hour

[Nickburf]

Hi all, new guy here, thanks in advance for any ideas. I recently bought a Alura 35 with twin Marine Power 5.7 straight inboards. Issue: port engine misfires after an hour of run time. I did full tune up (no other issues just for baseline) including plugs, coil, caps, wires, rotors, oil change and raw water impellers (FWC engines) they both start easily, idle well and run smoothly but after 1 hour port engine misfires/backfires etc. a bit less after tune up but still an issue. Temp is 160 oil pressure 60, I’m out of ideas for temperature related stuff thanks!

 

[Scott Danforth]

Its never the coil

Are the motors carb or EFI?
which ignition system?

Backfire is usually running lean. So look at the fuel system

 

[Nickburf]

Thanks for the fast response!
Carb engines, Mallory ignition (no points)
Boat doesn’t have fuel crossover valves but I can probably try to switch the rubber hoses. Any idea what the fuel issue can be? Filter new and looks good, I dumped fuel out into glass to examine, nice and clear no water
thanks

 

[Scott Danforth]

Go thru the carb.

 

[Scott Danforth]

BTW....welcome aboard

 

[kenny nunez]

One other thing to check is the voltage at the +side of the coil, try to hook a volt meter to it when you first start the engine and leave it on, then see if the voltage drops when the backfiring begins. Your engine’s wiring harness may have had breaker points originally and the resistor wire could be the problem.

 

[Scott Danforth]

Malory dizzy needs a ballast resistor or you fry the module..... (ask me how I know this :facepalm: )

 

[kenny nunez]

I agree that they require a resistor the main thing is that the resistor is not “breaking down” after the hour or so of use. Experience has taught me that you cannot assume anything, these may be Petronics conversions. These engines may have been Marine Power replacement power heads without the electrical systems.
This is a shot in the dark but have you ever tried replacing the rotor, sometimes they can also cause these problems when they heat up.

 

[Nickburf]

Thanks guys, as soon as the weather improves I’m on it

 

[aimlow]

One other thing to check is the voltage at the +side of the coil, try to hook a volt meter to it when you first start the engine and leave it on, then see if the voltage drops when the backfiring begins. Your engine’s wiring harness may have had breaker points originally and the resistor wire could be the problem.

This. A coil replacement modue must "see" 12 volts. However, the coil must be ballasted (resistor), either internally or externally. A very common problem. A non-resisted coil draws too much current and overheats. Bottom line, the coil and the breaker module must be matched.

Most backfiring is electrical, or in rare occurrences mechanical. In my 50+ years experience, lean backfire seldom happens without other symptoms being present.

 

[QBhoy]

Does it just missfire at low rpm or all way through revs ? Just one engine ?