Panther

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.

speeddemon651

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.

Panther

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..

1BIGJIM

iTrader: (1)

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.

Panther

No, it wasn't mentioned. Good point!

wild mike

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

axapowell

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

speeddemon651

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.

axapowell

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

jeff32

speeddemon651! Did you find your problem?