Backfire Thru Carb
 

Let her warm up for a few minutes at idle speed, came on plane and heard a pop, opened the hatch and saw a backfire thru carb going on plane, checked the oil for cam parts, no water, metal and clean, started again, got on plane and pop. Ran for a few more minutes and went to 4000 rpm, no pop. Is this moisture in the distb? It was either that or timing, this has never happened and the boat was drenched in a downpour. Then it was covered for a few days and the terarium effect took over. I think it is ok and was just a little moisture slinging around and throwing things off. I will put the timing light on it to be sure but think it is OK. What do you think? Am I on track?

take dist apart (cap off)or may not dry soon enough. If timing is not retarded,that is most likely it. My .02

I was about to do that but the thunder started again so I closed her up. I think you are right, that is the next step for the next time. Thanks. Can timing go away after only a few outings? I have about 10 hours this year....and don't say "yes it can and that is a sign the cam is going away" I don't want to hear that:D

Are you carbed? Could it be flooding? I had a boat do that to me once,kept loading up and backfiring even though timing was on. One carb was just leaking all the time.

That is a good possibility, I seem to smell gas in the engine compartment when it is closed up and this engine uses more fuel, alot more fuel as found out by me when it ran out of gas 1/4 tank from the other. I may have a float that is stuck huh?

Hey Mitch,

try swapping carbs and see if the backfires move to the other engine. might be the easiest way to trouble shoot.
you could also monitor fuel and see if that other engine now burns excessive fuel

sure hope it is not the cam.

Rob

and yes. i want the shore power cord. we'll talk

and i got a deal. I 'll trouble shoot your carb problem on the water and you replace my starboard seawater pump impeller.

Rob, no way on the pump, I don't have 4 hands and can't stand on my head that long. I am sure the cam is OK, when my last one went it was a pop out the exhaust, this is clearly out the carb. I will run it again but think it is moisture as this was running perfect when I shut it down last time. The previous owner said he did nothing to the carbs on the rebuild so I bet they are ready for one.

Mitch just replace both the dist caps its cheap insurance. I do every year,since I had one go bad....

If you poped the carb and it didn't have power valve protection, you could have blown your power valves. This would show up as richness at idle.

Also, if it was really wet and your plug wires were questionable, you could have had a cross fire between two wires.

I'm sure that this isn't your problem, but a bad cam, springs or valve could also get a back fire out of the carb. Those items cost too much money to be your problem though.

If you have a vacuum gage take some readings. Will tell you a lot about the valve train condition.

BT :cool:

carb. 500 hp's are known for this after idleing for a long time!! And since it did this you'll need to replace the power valve's in the carb.'s:rolleyes:

I don't know if this applies in your case, but in cars, the first thing I suspect with backfiring thru the carb is lean fuel mixture. Second thing is the timing off. The third is crossfiring.

This was my first though as I was reading all of the posts. If pop is when you press down on the throttle the engine is going lean & thus the backfire. I will say you have bad or sticking accelerator pump or something in that area.

If it's backfiring under 2000 rpm, when trying to go on plane, it's possible you're puttin too much load on the engine.

When you lug a engine too much at a low rpm, it WILL cause a slow constintant backfire.

If it is a rapid backfire, start lookin toward the ignition for the problem.

These are some of the problems I have encountered in the past years. And I am assuming the problem goes away once you're on plane.

Try: trimming the drives a little more, when going on plane, so the props slip a little more, and see if that helps. If it does, you might have too much cup in your props.
Just speakin from experience, Frank.

I would agree with a lean fuel mixture if it had happened at a higer rpm, but the fact that it happened at 2,000 would lead me to cross firing.

I would check your plug wires and change the distributor cap. Was it a really loud backfire, or a small pop. If it was a small pop your power-valves are probably still ok.

When she does start is she running rich?

IF YOU ARE RUNNING MSD DIST.& MSD BOX. MAKE SURE THE DIST. WIRER'S ARE NOT CROSSED!!! PURPLE GOES TO GREEN. I DID THIS AND SAME THING HAPPENED. RB

Re: Backfire Thru Carb
 

[QUOTE=Krumbsnatcher]I would agree with a lean fuel mixture if it had happened at a higer rpm, but the fact that it happened at 2,000 would lead me to cross firing.
QUOTE]

Nope, lean mixture will cause backfire from just off idle & up

Re: Backfire Thru Carb
 

Mitch, give me a call. If you've got a Miller Lite or a Mick Ultra for Barb we can stop over and check a few things out. 738-0474 or 403-3962