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2003 5.0L V8 mercruiser backfire

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Keith1109

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Last summer (2023) engine wouldn’t start. First replaced distributor and coil, but turned out to be ignition module which was also replaced. At the same time I had the 2brl carb rebuilt professionally. Used the boat 2 or 3 times after and it ran great, best it has in years. After winter storage in the spring upon first start up this year, it took a little to get it started but eventually did. Took it out on bay (in NJ) and it ran great. Next time out, it was running fine, had to back off throttle to negotiate a swell (bay is very choppy) and upon accelerating from nearly stopped it backfired. It only did it that one time for the rest of the day, and ran fine. Took it out this weekend. It took longer than usual to start, and once it did, it barely kept idle. But eventually warmed up and ran fine. Took it out and the initial acceleration to on plane was fine, no issues. Was running for about 10 or 15 mins on plane, encountered a swell, decelerated and upon reaccelerating, it backfired. Turned the boat around (I was probably 5 miles from the dock) and had to negotiate other boats and swells, and each time I had to decelerate and reaccelarate, it would backfire and now cut out. It would restart each time, but this time to try and accelerate to plane it backfired and cut out. Restarted and slowly accelerated, no backfire. Tried accelerating fast again, it backfired. Got it back to the dock, on the trailer and back to the marina. Was running water through the ear muffs, and pushed down on the throttle quickly, no backfire, revved strongly. Engine only has 160 hours as I only run it on weekends in the summer, and total 5 to 10 hours each year.
 
I'd be inclined to think you have crud in the tank and the motion stirs it up and it's collecting on the pickup screen...
 
I looked it up. I'm going to replace the antisyphon value and see if it makes a difference. I'll make sure hte screen at the end of the tube is clear of crud.
 
Keith1109 said:
I looked it up. I'm going to replace the antisyphon value and see if it makes a difference. I'll make sure hte screen at the end of the tube is clear of crud.
Click to expand...
Ayuh,..... I'd start at the fuel filter,..... If a canister type, dump it into a clear container, 'n look for crud, 'n water,....
 
Did you fill it up any time between when you replaced the fuel filter and when it began showing problems? Any water in the fuel filter?
Backfiring is classic symptom of lean condition. You have to figure out why there's a fuel starvation issue. I would suspect the following: carb accelerator pump, float level, needle and seat, clogged jets, weak fuel pump, blocked supply line. If those all check out, then the next thing down on the troubleshooting chart will be timing and distributor. If you have points type distributor, then check the gap and replace the condenser.
 
In my original post, Last year replaced distributor, coil, and ignition module, and had carburator professionally rebuilt. This year replaced solenoid and ignition switch before taking it out the first time. I had a full tank of gas when I replaced the fuel water separating filter and it ran great after. A couple of years ago, I had water in my tank and know what that feels like on the boat. It just dies and has a very hard time restarting. Once warm, it doesn't have a problem starting. Since the carburator was rebuilt and the boat ran great last year after, I'm thinking it's more likely fuel supply. In this case, it runs from original start on plane for a while, and when I hit rough water and have to power down, and and then upon powering up it backfires. Also, as I mentioned, when I started it up this past weekend (it sat for a week as I only use it on weekends), it had a hard time starting. And when it did start, it barely idled for a bit before warming up and running fine. From the help here and reading online, I'm thinking the best cause is the antisyphon value getting stuck as the boat rocks and the gas is splashing inside the tank or it's the fuel pump. Not saying its not a carburator issue, but less likely given it was newly rebuilt and ran great both last year and the first time out this year.
 
Keith1109 said:
In my original post, Last year replaced distributor, coil, and ignition module, and had carburator professionally rebuilt. This year replaced solenoid and ignition switch before taking it out the first time. I had a full tank of gas when I replaced the fuel water separating filter and it ran great after. A couple of years ago, I had water in my tank and know what that feels like on the boat. It just dies and has a very hard time restarting. Once warm, it doesn't have a problem starting. Since the carburator was rebuilt and the boat ran great last year after, I'm thinking it's more likely fuel supply. In this case, it runs from original start on plane for a while, and when I hit rough water and have to power down, and and then upon powering up it backfires. Also, as I mentioned, when I started it up this past weekend (it sat for a week as I only use it on weekends), it had a hard time starting. And when it did start, it barely idled for a bit before warming up and running fine. From the help here and reading online, I'm thinking the best cause is the antisyphon value getting stuck as the boat rocks and the gas is splashing inside the tank or it's the fuel pump. Not saying its not a carburator issue, but less likely given it was newly rebuilt and ran great both last year and the first time out this year.
Click to expand...

The antisyphon valve is a spring loaded check valve. It is closed until the fuel pump vacuum opens it.

It could be suspect but you need to conform NO gas in carb first. Then determine why. Could be pump related, could be as others suggested, tank/pickup related.

lots of things needed to be checked before swapping parts...
 
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