OMC 120HP suddenly backfiring when accelerating

[iflyboats]

Hello all,

My 1980 OMC 2.5L (120HP), Rochester 2GC carb has recently developed a backfiring issue. I was out on the lake cruising when it suddenly lost RPM and started backfiring through the carb when trying to give it throttle.

I've since replaced the distributor cap, rotor, points & condenser. Have taken apart & cleaned the carbs too many times to count, everything seems spotlessly clean. Compression seems OK with one cylinder slightly low (120,110,120,120 - book spec is 120). I've tried setting base timing everywhere from the 4 degrees specified in the manual all the way up to around 12 with no change.

In the driveway with water hooked up, if I hold the choke half closed, I can gun the throttle and it accelerates strong & cleanly through 3k+ rpm. Without holding the choke, it starts up & idles beautifully, but even the gentlest acceleration will cause intermittent backfiring through the carb (usually 1-4 sporadic pops until the RPM stabilizes).

Accelerator pump is pumping strongly, fuel pump is pumping plenty of volume.

I've run wire through every passage I can find in the carb, blown with carb cleaner & compressed air.

I'm at my wits end with this damn thing. I tried removing one of the main jets, it solved the backfiring issue but obviously was rich as a pig. Carb seems to be running lean but I can't find the cause.

Any ideas of what else I could try?

 

[racerone]

Remove valve cover.----Inspect valve rockers.----Check valve clearance.-------

 

[iflyboats]

Remove valve cover.----Inspect valve rockers.----Check valve clearance.-------

Earlier today I pulled the cover and cranked the engine - all valves appear to be opening fully. Engine has hydraulic lifters & I had just reset the adjustment on those last month. No pushrods look bent. Being that it runs so strongly at half choke I’m hesitant to think it could be a valve problem, but at this point I’m open to anything. Couple months ago when I was going through the engine I had lapped in the valves and compression is no different now than it was before the issue popped up.

 

[racerone]

Some will argue.----I think you have a valve problem.

 

[Bondo]

if I hold the choke half closed, I can gun the throttle and it accelerates strong & cleanly through 3k+ rpm. Without holding the choke, it starts up & idles beautifully, but even the gentlest acceleration will cause intermittent backfiring through the carb (usually 1-4 sporadic pops until the RPM stabilizes).

Ayuh,..... 'n this says it's runnin' Lean,.....

 

[kenny nunez]

When you have a timing light pointing at the marks, does the timing marks hold steady when the backfiring through the carburetor happens?
You did not mention replacing the spark plugs, are they new?

 

[iflyboats]

Spark plugs are only a few months old.

I seem to have fixed the backfiring issue, however I'm still not getting any power. Took it to the lake today for a test run, and I can't get over 2k RPMs. Power peaks at about half throttle, and any increase just causes a deep sound from the carb with no increase in power. i'm starting to think it's the fuel pump. Few months ago when I first got the boat it was running just fine, but each time I take it out I get a lower max RPM. Fuel pump is original so pretty likely that's the cause - I've inspected the diaphragm and its in good shape so I'm not sure how else they can fail. I'm assuming there is some sort of check valve somewhere to build pressure that must not be working - I noticed last time I disconnected the fuel line to the carb there was barely any residual pressure (carb float needle is new & not leaking).

 

[kenny nunez]

Try the 6 gallon outboard motor tank trick. Connect it to where the line from the tank is connected to the fuel filter. Of course be sure that the filters are all new including the one that may be in the carburetor inlet fitting.

 

[iflyboats]

Well, I think I solved my problem.

I pressure tested the fuel pump, had 4PSI from idle through 2.5k so I ruled that out.

I had previously tested for spark at each plug while cranking and all 4 were good, so I assumed my spark wires were good.

Finally decided to pull them all and ohm test them. The coil wire read 15K ohms, as did cyl #4. Put on new Sierra wires from Napa, and after running at 2k on the muffs for a bit to clean the plugs the RPM suddenly shot up, and when I pulled down to idle it was now at ~1200 instead of 600. Seems I had a dead cylinder all along, can’t believe I couldn’t tell it was misfiring. Now accelerates smoothly and strongly. Can’t wait to get out on the water and see if that solved the issue!

 

[Scott Danforth]

while at it, file the points, set the dwell with a meter and set timing.