95 Merc 5.7L bluewater

[Csantino]

Ok, I'll try and keep it short. Port engine issue backfiring after about 15-20 mins but never stalls, it is sluggish compared to the starboard engine. Starboard will be pushing 4000 rpms as port is just breaking 3200 rpms. Throttles fine in neutral. What I have done so far. Carb rebuilt beginning of May, Cap and rotor and plugs, replaced both fuel water separators, replaced ignition coil. I ran port engine off of the starboard fuel tank to rule out syphon valve and tank vent. Not sure what to look at next.

 

[Rick Stephens]

Sounds like carburetor still. Running lean and puffing back through the carburetor. Scott always calls that a 'lean sneeze'. If you doubt it, swap the carbs and see if it follows to starboard. Only other simple problem that will cause a backfire is timing to far advanced.

 

[Scott Danforth]

Go back to the carb

 

[alldodge]

Agree most likely the carb, but could be fuel pump. Check fuel pressure at the carb with a Tee fitting, or swap carbs and see if it moves