Engine back-fires at wide open throttle. 7.4liter

[Sea~Lah]

Boat is new to me. Still working out bugs since I put it in the water a few weeks ago.
I drained the gas tank before launching, added fresh high test fuel.
New plugs,
new raw water pump impeller,
new water separator,
new elbow risers,
removed approx one quart of engine oil (it was over-filled by Po)
cleaned flame arrester
Sprayed carb cleaner twice.

With all that, the engine starts just fine and sounds pretty good. It's when I have it wide open that the engine starts to backfire. It occurs about 6 times in lets say in a half mile distance. The engine just prior to the back fire would also run fine, but if I took it from wot to mid speed, than gunned her back to wot she would slow down instead of speeding up, then she would hesitate to get back to wot. Then she would back fire again. If I slow her down a little she doesn't back fire. I've put in fuel stabilizer and Methal hydrate into the carb and also added to gas tank.
Sorry for the long explanation. I'm just trying to give you the proper history.
The engine has 450 hours, and hasn't run in approx 2years. I've gone through a 1/2 tank of gas so far.
Any suggestions as to why she backfires would be greatly appreciated!!!

 

[techmitch]

How old are the distributor cap and the ignition wires? these are my first two suspects.

 

[Quint4]

Time to rebuild the carb or get a new one I would guess. Spraying carb cleaner into the carb will do very little. I had similar issues last year and did everything you did and more (coil, wires, ignition sensor, reset timing, new anti-siphon valve, etc., etc.). I put on a new Edelbrock 1410 carb and my boat starts and runs better than ever.

 

[33ec]

Make sure your fuel filters are clean and your fuel pump pressure is good. if an engine runs too lean, it will backfire as well--I agree that you should start with basic tune up components and check your timing but I would bet on a bad fuel supply if it runs well just off of WOT. I had a similar situation on a fuel injected motor recently-turned out to be low pressure on the electric fule pump causing it to lean out under load.--simple fix if that is all it is.
good luck

 

[JediJD]

Can you tell if it is backfiring through the exhaust or through the intake? What do the new plugs look like? Just looking for clues...if it is backfiring through the exhaust, could be a sign of a very rich mixture igniting with very hot exhaust gasses or components but would show on plugs as rich burn. If it is coughing through the intake, it can be a really lean mixture pre-firing too early...or a bad advance system (at cruise you may be 20-30% advanced over idle and your secondary advance system (either vacuum, centrifugal or electronic) could be contributing (however, its more likely to be too little advance and less power than too much advance and intake backfire). Valves not seating or closing properly from a worn timing chain (common) or worn cam lobes (uncommon) are other possibles.

I like the distributor/wires idea as maintenance items but suspect that the basics are really in order...compression test and fuel pressure checks before tearing open carbs. New carbs are really cool though.

 

[Hwillcockson]

You may have more than one problem... As mentioned above, new cap, rotor and wires and check the timing. A backfire through the exhaust is unburned fuel igniting in your exhaust system.

You may also need to rebuild the carb's... (hesitation issue). Just my $.02

 

[Sea~Lah]

When it backfires it seems to be coming from the engine compartment.

 

[davefin]

Similar issue with a boat I once owned. Turned out to be a clogged gas vent in the side of the hull. Replaced it and all was fine.

 

[MrFL54]

Had the same problem beginning of the year Feb. Before the end of last summer I did a tune up, plugs, cap,wires ect. In Feb it was doing the same as you discribed, rebuilt the carb (Q-Jet) and now it is fine again. Another $0.02 worth :thumbsup:

 

[Captain Allen]

I had a similar problem. Rine fine at 1,000-1,500 rpm's, but would backfire when I tried to put her on top. turned out it was a weak fuel pump. Replaced it and it ran fine. Could be fuel pump or carb, either way, sounds like it is not getting enough fuel.