Engine Backfire
 

I have a 96 velocity 26 with a 454 magnum 415hp mercury engine. Bravo 1 out drive. The boat has been in storage for almost a year with a 1/4 tank of gas in it plus a 1/2 of fresh gas. Wed night i took the boat out and it wouldn't't clear out over 3400 rpm would only do about 40 mph. Saturday I changed with plugs and the fuel filter. That night it ran great, cleared out would do about 67 and run high rpm, but by the end of the night it was back to running bad again. This Sunday I tried to use it again but kept popping, wouldn't't clear out or run over 4200 rpm. I was going to drain the rest of the fuel in the tank out but I pulled the gauge sensor out i didn't see any water in the lowest portion of the tank. Ran the rest of the gas out of the boat, filled it up with fresh 93 octane. and it still won't clear out, except if it's not under load. when in gear it eventually will get up to 4200 rpm spitting , sputtering, and popping. My question is , is this a carb problem or timing? Has anyone had trouble like this before , and might know what it is? thanks for your time. Joel H.

Re: Engine Backfire
 

Sounds like a "lean" pop to me. May be something stuck in the secondaries of the carbureator or a bad fuel pump.. That's kind what is sounds like to me without being in the boat and hearing it for myself.

Re: Engine Backfire
 

I'd check timing first. At least, try to turn the distributor without loosen up the bolt. If it moves, it's probably your answer. If it does not move, check cap and rotor. If ok, check your timing to make sure it's ok. If evrything is ok, then I would doubt the carb. By the way, are 100% sure spark plug wires are not taking humidity ?

Good luck.

Re: Engine Backfire
 

I a, with jeff, would also think it could be a problem with timing, You have already knocked out other possible problems. I would check to see if the dist is tight, put a light on it.

Re: Engine Backfire
 

The wires are newer MSD 8mm's, and look to be in good shape. Thanks for the advice but my next question is What degree advance should the timing be at ?Thanks Joel H.

Re: Engine Backfire
 

Did you replace the fuel filter again - ie: after removing all the fuel ? If not I would since this may have made it run better the first time.

Re: Engine Backfire
 

When the motor pops, does it pop back thru the carbureator or the exhaust?

In my experience to get a "pop" you would either have the ignition wires arcing, not enough fuel, too much fuel, bad plugs or a bad iginition module.

Depending on the size of prop 3500 is probably right about when the secondaries open. :rolleyes:

Re: Engine Backfire
 

both through the carb and the the exhaust. I'm about to check the timing this afternoon but i'm not sure on the degree of advance it should have, anyone know it's the 454 415 hp merc

Re: Engine Backfire
 

Is it the stock motor that came in the boat or aftermarket. i knowalot of the merc's say it right on the tag thast on the valve cover..

Re: Engine Backfire
 

If It Has The T-bolt Iv It Is 10* Initial And 24* Advance 34* Total.

Re: Engine Backfire
 

think about it...you had 1/2 a tank of turpentine and put good fuel on top... that didn't make your ond fuel any better... all that meant was that you had 1/2 a tank of garbage mixed w/ 1/2 a tank of fresh fuel.... and yet... still once you did that you got it to run right for a little bit.... now... what could have changed the timing or anything else between then and now ? answer.. nothing...

you have a carb full of corrosion and junk and varnish from the old fuel... it has plugged up some air corrector or fuel passage somewhere and that is your problem... take the carb off ... rattle it to pieces clean it like your life depends on it blow out all the passages with hi press air... throw away ALL the fuel that you still have in the boat along w/ the filters and start over and youll be fine.... never let a carb sit around w/ fuel in like that or run old gas thru the motor... even if it runs it was prob about 70 octane and could ping the motor to death.... that was a bad mistake and i sincerely hope you get away with it...

but the solution to your current problem is easy.

Re: Engine Backfire
 

This is the answer to this problem. New fuel and several fuel filter changes.

Re: Engine Backfire
 

thanks, I changed the filter ran all the old gas out I mean 100% out then filled it up with 93. I'll pull the carb out tomorrow and try changing the filter again. Checking the timing won't hurt, I know that the carb is a webber and is identical to the edelbrock carbs they sell. Can i just use one of their rebuild kits or should i try and get the merc rebuilt kit ? thanks for your time Joel H.

Re: Engine Backfire
 

This last weekend was on friends boat. Boat ran good friday night. Saturday was different story. Boat would not plane out. After we tried to get going 3 times he stopped boat and gave me a new rotor and cap. I removed the old one but it still looked good. I put the new one on anyway. After that we were off and running. I would have never have guessed that was the problem.

Re: Engine Backfire
 

To my knowledge, Merc has never made a 454 with 415hp.
A 1996 carbed 454 Mag is 365hp.


I would definately change the cap and rotor before I tried anything else.

Re: Engine Backfire
 

Sounds like a carb issue to me. Accelerator pump is probably bad. You can see if the clear return line from your fuel pump to your carb has fuel in it too. If it does the pump is bad. You can also check your accelerator pump while the motor is not running by disconnecting the throttle linkage or having some-one pump it while your looking down the carb and see if your getting a good spray on both sides. If you aren't then it needs to be rebuilt.
For timing, a good rule of thumb is about 8 deg. advanced but I'm not positive.
Any marine shop should be able to order you a rebuild kit for that carb whether it's a Merc kit or Sierra or what ever.

Re: Engine Backfire
 

Ok I've changed cap and rotor, points sensor, fuel filter, and cleaned and rebuilt the carb. I put the timing light on it dialed the gun to zero and turned the distributor till it was 8 degree's advanced. The engine will idle fine but once a load is put on it and you try to take off. the engine back fires through the carb and won't clear out passed 3,000 rpm. I don't know what else it could be other then that the engine jumped a tooth on the timing. I really really hope it didn't................. And thats a pain in the ass to fix. Last time i took the boat out I changed the plugs it was running great then progressively started running bad. I changed everything listed above but it's still running just as bad, maybe even worse. I'm going to change the plugs again, if thats not it I'm guessing the timing chain. What do you think. thanks for your advice.. Joel H.

Re: Engine Backfire
 

stupid thing but are you sure your spark plug wires are on the right cylinders? If so, I agree with you, (jumped a tooth on timing) Or last thing I could think of is the electronic box. I've had once a box that did not give the advance needed at higher RPM... Let us know what you find, I can't think of other thing...

Re: Engine Backfire
 

Where the fuel line connects to the carb. there is small filter behind the fitting, check to see if that is clogged. You mentioned points, what year did you say the motor was?? Merc. hasn't used points in a very long time. You may even have a bad ignition module or be using a 6 volt coil when you should have a 12 V. I've seen moisture get into the connection on the ignition modules and reak havoc.

It's really hard to tell with out being on the boat itself.

Worse case you could even have a wiped out camshaft but I tend to think it's fuel..

Re: Engine Backfire
 

good point, we also had once a coil that was not making 12v and engine was not powerfull and back firing. I forgot about that one. good point.

Re: Engine Backfire
 

I'm still confused though. A 1996 Mercury 454 Magnum doesn't have a points style distribuitor, it had Thunderbolt.

A few years ago I had a problem with a bad run of Ignition coils. The motor would break up the more RPM's I gave it. Swapped out coil and the problem was solved.

Re: Engine Backfire
 

Ok I have now replaced the cap, rotor, the electronic sensor inside the dist., the spark plugs twice, spark plug wires, the coil, totally cleaned out and rebult the carb, changed the fuel filter twice, the gas was totally drained and refilled with brand new 93 octane, plus checked that the carb working well through both stages. and the engine is just getting slowly worse and no won't clear out over 1,500 rpm and keeps back firing through the carb. I've come to the conclusion that it's something internal, like the cam being wiped or the timing chain having jumped a tooth, or some burnt valves due to the bad gas. I've replaced anything external that it could have been on the engine. which really sucks because now it's time to start pulling it apart.

Re: Engine Backfire
 

Did you replace/swap the ignition modules? Depending on what iginition system you have it will belocated on a plate mounted near your exhaust system or on the back of your distribuitor. The newer ignition systems have a plug in type of module, the old ones are hard wired into a square box.

You may also want to disconnect the grey wire from the negative side of the coil, this is your tachometer wire. I've seen tachs. short out and break up the ignition.

Check to see if your fuel system has a "check ball" in-line of the fuel tank. If it does you may want to try and remove it, I've seen it restrict fuel flow severly if it's stuck.

Silly question, when you replaced the plug wires, did you replace the coil wire?

What about the fuel pump?? Have you verified your fuel pressure with a mechanical guage?

Finally, have a good mechanic run the boat with you.. :rolleyes:

Re: Engine Backfire
 

I did not read the entire post.

Was valve springs mentioned?
Did you pull the valve covers?
If there is something wrong with the exhaust valve train you will backfire from the carb.

Re: Engine Backfire
 

No, it wasn't mentioned. Good point! :cool:

Re: Engine Backfire
 

i had the same problem, everyone said broken valve spring , i put new motors in so i haven't had a chance to look at the old motors yet

Re: Engine Backfire
 

How about the barbed fitting on the fuel tank pickup? You ran the fuel low, maybe you picked something up from the bottom of the tank.

There is fitting which feeds your carb on the sending unit mounted to the tank. In the fitting is a spring and ball. This is an anti-sipone valve. If it is plugged, the engine will stumble and back fire. You can drive the ball and spring out after you remove it from the sender. It maybe a long shot, but you should check it before you tear into your engine.

Dave

Re: Engine Backfire
 

fuel wise, it's good you can lean over the carb and see all the nozels flowing fine and getting the right about of fuel. it's with out a dought getting the proper amount of fuel. Someone mentioned the spark plug wire, i did replace thoes. The only thing i haven't done is the ingition module, I am going to try and pull off the valves covers to see if thier is any sign's of loose rockers, or damage of any kind and go from there.

Re: Engine Backfire
 

When I discovered my problem, it only backfired and quit while under load. May still be worth the look see! Don't forget volume and pressure are both needed.

Dave

Re: Engine Backfire
 

speeddemon651! Did you find your problem?

Re: Engine Backfire
 

Any updates?

Re: Engine Backfire
 

How many hours on the engine? I had to trouble shoot an engine this summer that was doing pretty much the same thing. So I ran it at idle and pulled the spark plug wire off the distributor one at a time to see if all cylinder were firing. 2 weren't. So I checked the compression, I found that those cylinder were low. I pulled the heads off and took them apart and it was the valves that were "mushrooming". Some of them started to cave it and when measured, my worst one was 0.239" longer than the shortest one. The engine had over 700 hrs on it.
So my advice to you would be to start the engine, run it at idle and disconnect the spark plug one at a time and listen to check if the engine sound change and if you find one or more, check the compression.
If you're lucky, it'd be valve fatigue only. If not... :(

Re: Engine Backfire
 

Install a fuel pressure gage, that will tell if it is fuel starved. I would guess the pickup or anti syphon valve if pressure drops.

If its a lobe on the cam it will tick intermittantly then quiet. If you have ticking, cut the oil filter open and look for the flakes of death. I highly doubt the timing chain jumped.

As far as ignition goes, double check everything. Pay particular attention to the rotor and assure it doesn't have a greasy paw print on it.

BT :cool:

Re: Engine Backfire
 

Don't know if your boat is equiped with a rev limiter, but if so, check that. I have been tracking down a problem with one of my engines breaking up and finally tracked it down to that. Mine is a MSD and when I disconnected it, it ran great. Only took about 2 months to find it. :rolleyes:

Re: Engine Backfire
 

power valve in the carb

Re: Engine Backfire
 

are there any non-mercruiser wires connected to the coil?. A compression test should tell you if you have other mechanical problems.

Re: Engine Backfire
 

need to pull valve covers and check cam or valve springs,sounds thats were your likely going to find your problem

Re: Engine Backfire
 

I had a similar issue and after several months of troubleshooting, replacing this and that, it turned out to be a leaking intake gasket on the bottom side which caused a lean condition on two cylinders.

Rick

Re: Engine Backfire
 

What was the outcome?

Re: Engine Backfire
 

I have a similar issue above 4000 rpm and after a season of electronic and fuel troubleshooting, have narrowed it down to either a leaking intake manifold gasket causing a lean condition on two cylinders, and I also suspect some valve float due to either not enough closed spring pressure or coil binding. I have the intake off and see the bad gasket and the springs are suspect because I changed the camshaft to a slightly higher lift and did not change the springs accordingly.

Rick