A little help from my friends............

[Donzigo]

Boat: 1989 Z-33 with 1996 Gen V 454's, Holly 600 CFM carb, electric fuel pumps.

Problem: The engines are fine in the beginning and putting on the step, then at 3,000 to 3,400 and after a minute or two, the port side starts popping and back-firing, then the tach goes nuts, then the engine either dies or goes down to idle. I can immediately bring the starboard side down and go right back through the same excersice. It's driving me nuts trying to figure this out.

Heres what I have done so far:
a. Swapped Holly 600, carbs (& both are freshly refurbished)
b. Swapped electric fuel pumps
c. Disconnected the Hobbs oil pressure switch
d. Swapped coils
e. Swapped electrical modules (next to exhaust manifold)
f. Disconnected port tachometer wire (grey on negative side of coil)
g. Swapped fuel filters (no debri on either when poured into glass container)
h. Swapped fuel lines to the opposing sides (extended port line to reach)
i. Checked for fuel tank anti-siphon valves (don't have them)
j. 140 gallon fuel tank is one year old ($900 tank)
k. Installed new distributor sensors (updated type) on both engines

It's driving me nuts. What have I missed?.......

Richard

[gold-n-rod]

An electrical ground issue?

[MOP]

Try a hot wire to the coil, also the key switch may be on its way out. You may chase your tail a bit more, a corroded hidden connection will not carry good current and break down under load.

[CosmoKat]

Putting on the step?

Does that old model have a step hull?

Sounds like a head gasket issue......maybe water is leaking into one of the cylinders at higher RPM's

[Ranman]

and after a minute or two, the port side starts popping and back-firing

This happened to me when I got water in the fuel. When running a Merc water seperating fuel filter, if the filter gets too full it sends the water up to the motor causing a lean condition resulting in backfiring and popping. The popping would only occur under load and the motor would be fine until it hit a "threshold" RPM (like 2500 in my case. Below 2500 the motor would seem fine. what confused me at the time was that I'd dump the filter, re-install it and immediatly have the problem again. This was because I had ingested a lot of water into the tank somehow (bad fill up I think). I ended up getting a Racor filter that will shut down the engine completely when its full. Also, it's easier to tell when the Racor has water in it vs. the Merc. I had to drain the Racor many times before the issue was gone for good, but that indeed was the problem.

Anyway, is there any chance you have water in the fuel or a full filter on the one engine?

[RickSE]

Crank down your battery connections. Mine was cutting out, and would occasionally pop, last summer at high RPM's and drove me nuts for several months until I finally checked the battery connections. Mine cables were hand tight but apparently not tight enough since the problem went away after I put a wrench on the connections. I swore I had a fuel problem but in the end it was electrical. I had changed batteries the winter before and apparently did not tighten the cables enough.

I also had a tach go bad once and did what you describe but sounds like you've already addressed this.

[Donzigo]

Thanks to all ..................GoldenRod & MOP got it right.

Yesterday, my buddy & I went out into Boca Ciega Bay and started eliminaiting things. We put in on plane (CosmoKat, that's what I call putting it on the STEP) and checked the butterfly on the Holly, thus eliminating the carb. Then, since the breaker was tripping, we thought it was a grounding issue. But, we dismissed that and went about eliminating things: The Panel, Gauges, Tach, Breakers, Engine Syncronizers, Depth guage, Lights, Ignition Switch, Panel, Alternator, Choke, & a few more. Ok, now we checked the Engine Plug-In harness (no corrosion, MOP, good call though). Then, we wired the engines together and ran the engines off of one Ignition switch. BINGO, the problem turned BOTH the engines off. So, we knew it must be on the Ignition Circuit (as MOP suggested). So, we wired a direct positive fromt he battery to the coil and she runs great. Since the Igniton Switches are OK, it's got to be in the PURPLE Ignition wire from the Panel to the Coil.

Now, today's Saturday project is to run new 16 gauge Purple wire from the ignition to the coil and it should take care of the problem. Stay tuned.

GoldenRod: on the right track
MOP: predictably on the money
CosMoKat: Those old Z-33 don't have the step technology; but she's heavy, comfortable, and tough as hell in the ocean and has handled anything I've thrown at her............and I've taken a few over the bow, for sure.
Ranman: Thanks for the reply Randy. I was chasing the fuel issue, and thought that was it, changed and switched everything. Yeah, I thought it might be debri or water, as well, until I took a guy out who knew what to look for.
RickSE: I checked the batterys first thing. Like you, I figured it must be a battery issue and that was my first stop.

[Cuda]

Glad to hear you found it Richard.

[MOP]

Richard thanks for the kind word, was a shot in the dark glad it helped to kill the Goose!

Phil