88 Cobra 350 Backfiring on Acceleration

[Chris5280]

Back again... Replaced my shift cable at the end of last season as it was shot and stalling the engine. Got her in the water once after that and she shifts great, but there were some definite carb problems. Didn't have the time myself this spring, so I took the boat in to the shop for a tuneup.

Finally got her back this week (crazy) and put her in the water yesterday. Idles great, shifts great. Accelerates fine to about 1800-2000 rpm and then develops a bad misfire. Tried to ease past it, but it just continued with additional throttle. Shop that did the tuneup says they can't get her back in until October, so I'm on my own.

Seemed like maybe running too lean, so I pulled the fuel filter, which only got 15 minutes of use last season. Couldn't detect any water. Checked the fuel pickup tube/filter, no issues. Fuel pump only has a few hours on it as well. Carb was just rebuilt in the tuneup.

I pulled the plugs, which were not replaced in the tuneup, and they have a black carbon buildup which would seem to point to a rich, not lean, mixture. Replaced them with the NGK platinums and ordered new marine wires today as well. Just did the cap/rotor/condenser last season.

What am I missing?

 

[jerryjerry05]

Check the voltage at the coil? around 9v going in, then see if the coils getting excessively hot?? you should be able to touch it and not burn yourself, it'll be hot but no too bad.
Then check the tiny switches on the shift mechanism, once you get to a certain point the cable might be making the switches activate??
I'd find a different shop if they don't stand behind their work

 

[Chris5280]

Check the voltage at the coil? around 9v going in, then see if the coils getting excessively hot?? you should be able to touch it and not burn yourself, it'll be hot but no too bad.
Then check the tiny switches on the shift mechanism, once you get to a certain point the cable might be making the switches activate??
I'd find a different shop if they don't stand behind their work

Thanks. Voltage seems about right, and the coil is hot but not uncomfortable to touch. I adjusted the ESA switches when I replaced the shift cable last year, and she shifts smoothly now, so I think the interrupt switches are adjusted properly.

What's my next step? Am I looking for causes of a lean or rich fuel mixture?

Unfortunately, I'm in Denver, and there are not a ton of marine repair shops. This was my first visit to this shop as it was the only one I could get an appointment with. Typically shops are booked up for the summer season by the beginning of June and won't schedule any more appointments until the following season...

Thanks!

 

[CaptnKingfisher]

Backfiring is a sign of a lean condition. Sounds like your whole ignition system is basically new and your carb was freshly rebuilt. But who knows if the shop that installed this stuff did it right. If it were my engine this is what I'd do ..start with the easy stuff. Get a manual cuz you're gonna need to know some specs to check these things
1: verify good compression on each cylinder
2: verify coil current is around 9 volts with engine running
3. Check timing belt deflection. Should be between 1/4" and 1/2" deflection
4: test for spark
5: verify dwell on coil matches specs (not sure what it is on your engine)
6: check timing
7: check spark plugs are gapped correctly

At this point if all these tests are good then youve definitely got a fuel problem. I'd start by verifying your idle speed is set correctly.. check your idle rpm matches specs. Once that's good you can make adjustments to idle mixture screws. Lmk if you need the procedure. Next I'd reclean the fuel filter, blow it out and blow out whatever fuel lines you can with an air compressor. Now that you've got the fuel hoses off you can take your time and pay attention to making sure they all go back on tightly and the hoses aren't damaged. You've already said you're fuel pump is pretty new, so now that leaves one bugger left. .. carb rebuild (again). Before I do that though, I'd try hooking up a 6 gallon outboard tank with fresh, good gas and see if the problem occurs running on that tank.