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454 carb engine wants to backfire/die after 20 minutes, only under load

V

Viceroy

New member
Let's start with the details of the engine:

1989 454 Engine
Originally Quadrajet engine, now Holley Marine Spreadbore
One of two inboard engines, starboard side
Originally Thunderbolt Ignition, now Mallory distributor

The problem:

When running at high rpm, after about 20 minutes, starboard engine starts backfiring and stalling. Will eventually just drop dead. Will run all day without a single hiccup at idle speed, or "minimum wake" speed (1100-1500 rpm). Engine behaves perfectly and exhibits full performance up until the point the backfire/stalling begins.

Attempts that have been made on this boat to isolate the problem:

Original Quadrajet carbs/arrestors swapped from port engine to starboard engine (no change, and port experienced no similar issue)
Switched starboard's fuel feed from starboard tank/filter to port tank/filter (no change)
Fuel pump replaced (no change)
Ignition coil replaced, and turned upright as per MSD instructions (stalling still occurs after 20 minutes, but now does not backfire or die)

Unrelated Maintenance and Repairs performed that also made no difference to the issue:

Spark plugs replaced (Delco MR43T)
Thunderbolt distributors replaced with Mallory Marine distributors
New Spark plug wires (isolated in holders)
Quadrajet carbs replaced with Holley Marine Spreadbore carbs



I feel like I'm onto some sort of ignition problem, due to changing the coil having improved the situation, but always open to the fact that this could be a red herring.

Any help here? Known issues? Previous experience?

Thanks in advance.
 
I had initially thought the same thing, but I feel like I have eliminated that by the simple fact that this engine will roll out HARD like a champion, as long as 20 minutes have not gone by. I can run the boat on just starboard at full throttle, and the engine gives up nothing......until 20 minutes have passed. Something more than a little funny about that.
 
Tank vent clear? Sometimes mud wasps fill them up and you can't tell. This would cause these symptoms as the tank builds negative pressure and the fuel pump can't suck against it any more.
 
I used the crossover switches to run both engines off of the port tank and port separator. A tank problem with the starboard has been eliminated. I'm going to trigger the problem again and measure the temperature of the carburetor and also the ignition coil.

My prevailing theories right now are either the coil is overheating somehow, or maybe under high load, the exhaust crossover in the intake manifold is cooking the carburetor and causing vapor lock in the carburetor itself. I did notice one time after cleaning off the top end of the engine, the area the crossover under baked the water off of that area pretty darn quickly. It certainly would explain why this issue only rears its ugly head when the engine is being pushed under sustained high load. High sustained exhaust temps could be cooking the carb.
 
Personally. I'd install a fuel pressure gage before the carb and see if the carb is getting enough pressure (at least 5 psi at WOT). That eliminates a bunch of things. Got my doubts on the 'cross over' theory.

Jeff
 
as stated previously, (assuming the described conditions are accurate) this describes a fuel starvation issue.

Starting a new thread, with your specific details, may yield something worthwhile...
 
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