Dropping RPMs, BOOMs and smoke

[TuxedoPk]

Had my prop balanced and refinished- ran great at about 4900 rpm for about 45 minutes yesterday then dropped down to running only about 4400 rpm.

Took the boat out today- problem was gone for about 20 minutes running at 4900 rpm (WOT) then I noticed it started dropping 4800, 4700...down to around 3800. Idled for a while then raced it again at 4900... sure enough startdd dropping rpms again. Then BOOM. I'm not sure if what I heard was a misfire, metal clanging on metal or what- but it didn't sound pretty. Also noticed smoke coming out the rear exhaust.

This has happened to me a few other times (smoke/BOOM).. never when the mechanic is around though.. This was the first time I noticed the dropping rpms though... but it might have been been doing it w/o me noticing.

All the 2" guages read normal...

What's changed since the problem began?
I had the wiring harness replaced and more significantly I was told that my Crane Ignition was defective so a new one was installed. Also the spark plugs were replaced.

The battery is strong (Purchased at Cumberland). According to the manual that came with the Crane ignition it states that misfire at high rpm is usually due to coil failure, including internal arcing or arcing at the high voltage terminal as well as arcing across spark plug boots or the distributor cap. They also suggest routing all magnetic trigger connections away from any other wiring especially CD unit coil cable and any high voltage sparg plug wires. They rectify correcting misfiring problems by replacing spark plugs and checking that the plugs are the proper type, heat range, and gap size.


** Does the dropping rpms and noise sound like an intermittent misfire? What else might cause this to happen?

I was really happy with the new prop until this crap started up.

[RvR]

The “BOOM” sounds like raw gas in the exhaust. I would pull the plugs and inspect them to see if they are all burning the same. You might be able to narrow it down to one cylinder.

[erikshube]

Im having the same problem (Low RPM @ WOT and backfiring) . Im slowly working through it. The boat is almost 30 yrs old so Im rebuilding/replacing a few things a month. So far new fuel pump and filter, carb rebuild, check timing, check compression and still having issues. New distributor and coil coming on Wednesday. Ill let you know if I finally figure it out. Maybe it will help you.

[Planetwarmer]

If you are having firing problems, then you would have a loss of power. That would cause your rpms to drop. A backfire can be caused by air/fuel not being combusted in the chamber and being pushed out to the e-manifold, then when another cylinder fires it will ignite the fuel and EXPLODE (backfire)!

Another possibility is your alternator isn't putting enough amps to charge your battery at high rpms, however your battery would probably be dead or very low when you tried to start it unless you idled long enough to charge it before you shut off the ignition. In that case, you wouldn't notice it.

The fuel pump could be weak or your fuel filter dirty.

Just some possibilities

[VetteLT193]

What color is the smoke? Can you tell if it comes out of both exhaust pipes or just one?

Backfires usually have a distinct sound because generally it is the backfire combined with a vacuum / sucking noise.

From the way you make things sound so far the problem seems engine-wide. It could even be something like a carb float getting stuck open or fuel pressure too high... that overloads the bowl, fuel starts dumping into the engine, the engine gets a rich condition so the RPMs drop, gets even more loaded and gas ignites in the manifold or exhaust pipe. This is another cause of what planetwarmer described.

Usually an ignition problem results in a rough condition at least when you slow down.

[MOP]

When was the last time it had wires, they don't last real long in a damp bilge.

[Pismo]

Ignition - cap, rotor, wires, plugs.

[Sweet little 16]

holley carb?? power valve??

[smokediver]

if you are going through coils ... probably an exhaust manifold . do you have the old style one piece manifolds ?