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backfire through carb once every 40 minutes or so

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Old 07-28-2018, 02:24 PM
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Default backfire through carb once every 40 minutes or so

There are PLENTY of threads on backfiring through carburetors, but none seem to fit the situation I'm looking into. One of my twin FWC 305 CI 220 HP Crusader's backfires through the carburetor during cruise, around 3000 RPM once every 40 minutes or so. I hear one pop, see no noticeable RPM loss...nothing...and it keeps on running smoothly. To me it sounded like water in the fuel, so I swapped out the fuel water separators. There was next to no water in the fuel I poured out of the separators (as in one..maybe 2 little bubbles). The engines did seem smoother at idle after the change. I ran them for 3 hours at mixed speeds and just as I was approaching the harbor feeling that I had fixed it, the starboard engine backfired through the carb. Same as before....just one pop and back to normal. Given that it's the same engine and both now have new fuel water separators, I'm pretty comfortable it's not fuel contamination. The next thing I'm planning on looking into is the ignition timing, but again...since it happens so infrequently, I'm wondering even about that. Any insights? The oil is clean, there's no sign of water getting into the engine, and the boat is run 3 times a week...so it's not a boat that sits with old fuel..and I use Startron in the fuel just to avoid fuel problems anyway. Ideas?
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Old 07-28-2018, 02:42 PM
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Originally Posted by ThursdaysChild View Post
There are PLENTY of threads on backfiring through carburetors, but none seem to fit the situation I'm looking into. One of my twin FWC 305 CI 220 HP Crusader's backfires through the carburetor during cruise, around 3000 RPM once every 40 minutes or so. I hear one pop, see no noticeable RPM loss...nothing...and it keeps on running smoothly. To me it sounded like water in the fuel, so I swapped out the fuel water separators. There was next to no water in the fuel I poured out of the separators (as in one..maybe 2 little bubbles). The engines did seem smoother at idle after the change. I ran them for 3 hours at mixed speeds and just as I was approaching the harbor feeling that I had fixed it, the starboard engine backfired through the carb. Same as before....just one pop and back to normal. Given that it's the same engine and both now have new fuel water separators, I'm pretty comfortable it's not fuel contamination. The next thing I'm planning on looking into is the ignition timing, but again...since it happens so infrequently, I'm wondering even about that. Any insights? The oil is clean, there's no sign of water getting into the engine, and the boat is run 3 times a week...so it's not a boat that sits with old fuel..and I use Startron in the fuel just to avoid fuel problems anyway. Ideas?
Detonation . To much timing or a lean fuel mixture .
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Old 07-29-2018, 12:04 AM
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It sure sounds like a bit of water in the fuel.
Are you using E10 or Rec fuel. A gallon of E10 can hold 3-4 teaspoons of water in suspension before it begins to phase separate/precipitate. Fuel/water separators will work well with Rec fuel without Polar solvents. Ethanol laced fuels will inhibit their function until it gets saturated with enough water.
Double check your fuel tank fill cap and sender for possible water entry.
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Old 07-29-2018, 08:13 AM
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Thank you both for the input. I am doing a full tune up, checking the timing and will double check the fuel. I would think both engines would be acting the same way if the fuel were the problem since they draw from the same tank, but given how infrequent it happens, it's still a possibility. I'll keep you posted. Thanks again!
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Old 07-29-2018, 08:19 AM
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Is the distributor cap flat on top with the wires on the side?

Probably electrical. Most injected engines are crank fired ignition, so timing cannot be adjusted.
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Old 07-29-2018, 01:14 PM
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Put a some Marvel mystery oil in the crank case and run it before your next oil change.
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Old 07-29-2018, 03:27 PM
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Two things that can cause a carbureted engine to "pop" are bad ignition wiring that leads to cross-firing and a burnt intake valve. Start with inspecting the spark plug wires for cracks in the insulation, replace if at all questionable.
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Old 07-29-2018, 08:10 PM
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Was thinking a burned valve as well. Maybe check compression. Pull a spark plug wires off one at a time, while idling and see if it goes away for a particular cylinder?
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Old 07-30-2018, 05:36 AM
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A loose ground will do it. It's the equivalent of turning the engine off for a second and turning it back on.
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Old 07-30-2018, 09:21 AM
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Originally Posted by Stand By View Post
Was thinking a burned valve as well. Maybe check compression. Pull a spark plug wires off one at a time, while idling and see if it goes away for a particular cylinder?
I would think a burned valve would backfire more frequently...every few seconds.
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Old 07-30-2018, 09:35 AM
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Originally Posted by billinstuart View Post
I would think a burned valve would backfire more frequently...every few seconds.
I haven't had the experience of a burnt intake valve in a boat, but I've had several in cars. The backfiring can be infrequent or regular, depending on how burnt that valve is.
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Old 07-31-2018, 06:17 AM
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Originally Posted by yarcraft91 View Post
I haven't had the experience of a burnt intake valve in a boat, but I've had several in cars. The backfiring can be infrequent or regular, depending on how burnt that valve is.
Good point. Since most valves rotate, maybe 2 slightly bad spots on the seat and valve line up. I'm still thinking it's electrical...we haven't found out what distributor he has. If it's flat topped, I'll guarantee it's the distributor.
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Old 08-06-2018, 04:07 AM
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I always remember what I was taught about carburetors, that 90 percent of carburetor problems are ignition-related.

Go through the ignition first. Timing, plugs, wires, etc.
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Old 08-07-2018, 07:54 AM
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Hello, just my o2 cents worth, check the spark plugs and HT leads at both ends to the plug and distributor, pull the plugs too and have a look at the colour of them and the plug gap, one more thing, there maybe a mixture of spark plugs, it´s rare but ive seen the hot rated plug used where not necessary, but I would expect more backfiring than at random, just some thoughts, hope this helps.
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Old 08-10-2018, 04:39 PM
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Thank you for all the ideas. I tended to lean towards it being an electrical problem too, so I did a full tune up and replaced the plugs, wires, caps and rotors and checked the timing.....and that took care of the problem completely. The plugs didn't look too bad, and the timing was dead on, but the distributor cap on the engine in question was clearly shot (Prestolite electronic ignition with screw down cap)...and the other one wasn't far behind. Easy enough fix and it's running as smooth as silk throughout the RPM range consistently now! Thanks again.
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