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Port bogging down, Stbd backfiring...

M

magickjc

New member
Engines: 1984 350 HP, 454 long blocks with 1800 hours.

Single tank, no valve or metal screen filters on tank. Water separator for each engine. Fuel flow indicators before water separators. Fuel lines replaced and are in great shape.

Problem(s):

Port engine: Starts and runs fine but it won't go above 2200/2399 RPM's, it 'bogs' down when you try to accelerate, the RPM's will fall back and then it can come back up to 2200/2300.

Stbd. engine: Starts and runs fine but at 2800 RPM's it will backfire, intermittently enough that it can't be run any higher than 2800 RPM's.

Been through a couple hundred gallons of fuel this season.

Removed and cleaned both carbs (and filter on carb) first, carbs were pretty gunked up but the exact problem was found. Then I replaced water / fuel separator and fuel filter on both engines, same exact problem.

Lots of advice from fellow dock mates now thinking that it might have to do with timing/distributor.

Searched the forum but couldn't find anything close, anyone been through this before or have any advice? If I left any details out that might be needed, please let me know.

Thanks in advance for everyone's help.
 
magickjc said:
Single tank, no valve or metal screen filters on tank. Water separator for each engine. Fuel flow indicators before water separators. Fuel lines replaced and are in great shape.
Click to expand...

So, the only common item with both engines is the last fuel delivery and the tank, correct?
Is there no filter before the flow sensor?

magickjc said:
Been through a couple hundred gallons of fuel this season.
Click to expand...

Did the problem start on both engines with the last fuel delivery?
 
Tank, fuel water separator, (I had it wrong initially) fuel flow sensor, engine filter, carb filter...

Ran fine early on but didn't really run it fast until early June.

I've taken on one hundred gallons on two occasions and the problem occurred after the first fueling.
 
Well, the symptoms could be consistent with low advance, and it's a easy test. I would test advance idle to 3200rpm in neutral. That will get that out of the way.

The engines could be fuel starved. Are there an anti-siphon valves at the tank? Remove and inspect them. If timing and ASV look good, i'd hook up a 6 gallon outboard tank to one engine, fresh fuel and try that. Hook in right to the input of the fuel pump on the engine. Don't forget to bolt and clamp the loose feed hose.
 
Do you mean just running the rpm's up to 3200 when in neutral? If so, I did that it and is fine at the dock. Just has the problems in gear while running.

Good idea re by-passing the fuel tank with another one. Certainly will eliminate bad fuel.
 
Using a timing light, to measure where the spark occurs relative to the crankshaft's position.

The ignition timing needs to "advance" as the engine rpm increases. If it doesn't, the engine won't develope full power.

If this is a strange concept, find a buddy that can help. If NA, hire somebody to check - watch, take notes, ask questions and learn.
 
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