dawezel

Guys:

I recently started having some serious backfiring on my port 454 Mag. I'm hoping that someone might be able to give me some ideas. Here's the history of the situation....

After sitting for a few months this summer, I wasn't able to start the engine and discovered that I wasn't getting any fuel to the carb. I replaced the fuel pump and filter. The old fuel pump was a Carter mechanical and the new one is supposed to be the Carter replacement model for the one that was on it. After replacing, I took the boat up the river for about 20 minutes and it seemed to run fine. Next time I took it out in the Gulf and after about 40 minutes of run time (20 minutes at idle and 20 minutes at cruising speed) the motor backfired a few times. I slowed down to idle and the motor seemed to load up and cough and wouldn't stay running. I started it back up and every 30 seconds or 1 minute it would load up again and cough and stall. When I gave it throttle, it would seem to clean out a bit and run longer. After a few of these sequences, the motor finally died and wouldn't start at all. Got back home on the starboard engine and was able to restart the port motor once on the lift. It then seemed to idle fine.

My mechanic came out the next morning and we started the motor. It idled fine and didn't load up at all. Didn't have a timing light, but it sounded good. We hooked up a fuel pressure gauge and I think it was a consistent 8 pounds. The motor has about 450 hours and it's the original quadrajet carb. Distributor cap looked clean. My mechanic hooked up a vacuum gauge and it was reading a bit low - 12 - 13 pounds. I took the boat out again that afternoon for another test run. Seemed to run fine at idle and then after about 5 minutes at 3500 RPM's it started to backfire violently. I then idled it back home and it continued to idle well. Not sure if the vacuum is causing the issue, but my dad seems to think it would run poorly all the time if it was. One thing I plan to try to disconnecting my powershift. I have a poweshift mechanism that was only built my mercruiser for a year or two and runs off vacuum pressure. I'm going to plug the hole and take it out of the loop in case there could be a vacuum leak in there. Other than that, I'm not too sure what I should try.

I'd try new plugs and wires, but there's not much room in the engine compartment, so it's a big job and I'm planning to pull the motors for rebuilds in couple of months anyway. Just hoping there may be something simple that one of the OSO experts can point out that can put me back in the water for the rest of the season.

Thanks.

Griff

Start with the basics first. Full tuneup with new plugs, cap, rotor & fuel filter. If that doesn't work, switch carbs from engine to engine and see if the problem follows the carb.

CcanDo

you may also try swapping ignition coils.To bad its hard to change the plug wires.However,the wires may be a good place to start.Also,the coil wire.Further,you may want to add fuel stabilizer.

Thunderstruck

Maybe your needle and seat is sticking intermittently and/or the carb fuel filter in the inlet line is clogged. You started with a fuel problem, I would not switch to electrical issues until you are sure the fuel system issues have been completely ruled out. Carefully look in the carb with the flame arrestor off at idle speed when you are experiencing problems. Look for fuel running into the venturis which is an indication of a stuck needle and seat. Also, clean the screen filter inside the fuel inlet to the carb.

Then you need to check plugs for fouling or just replace them. The fuel issues may have caused fouled plugs.

Edward R. Cozzi

I'm with Thunderstruck on this one.
Was the boat in storage? Was the gas drained from the float bowl before storage? What about the fuel tank vent?
Once I saw a clogged vent cause a vacuum in the tank after a while of running. Easy to check, just loosen the filler cap.

MILD THUNDER

iTrader: (3)

Check for a blown power valve. If it wasnt bad it might be now from backfiring. Does it backfire thru the carb or exhaust? A blown power valve will cause it to load up. A cold engine will want to run rich, a hot one will want to run leaner.

With the low vacuum reading like that, somethings wrong. With the engine idling, put your hand over the choke plate opening to seal off the air intake. If there are no vacuum leaks, the motor should die right away.

I had a similiar problem with my old 454 mag carb motor. It started popping thru the carb a little. Mostly when i cracked the throttle sharply. I chased my tail trying to diagnose it. It began to pop worse and worse. Finally it got to a point where I could hold the rpms at 2500 and it was a steady pop,pop,pop,pop. I pulled the valve covers and let it idle. I noticed the exhaust valves on two cylinders were not barely moving. I pulled the intake and sure enough, flat cam lobes. Put a new cam in it and it ran great. I did however, blow 2 power valves on the carb from all the popping. Something to think about.

Griff

No powervalves in Q-jets.

MILD THUNDER

iTrader: (3)

OOPS, Sorry, so used to these holleys' Ive had.

Wally

iTrader: (1)

I'd also check the fuel water sepperator to make sure it doesnt have water in it

dawezel

Thanks for all the replies. There's some good ideas there I hadn't thought of. Wish me luck.

Dawezel