TOPIC: engine frustrations???

Here is my power plant vitals:

New 351 Ford Windsor by Ford for Mercruiser
Engine has roller cam, GT-40 heads, re-built Holley 600cfm Carb
Presto-lite distributor with new electronic ignition
New Distributor cap with copper electrodes
New Irridium spark plugs
New 8mm wires

Issue: I took the boat out today with some pals to Sturgeon Fish in the Columbia River at Astoria Oregon. When we first got it up on plane the engine ran fine at a steady 4500 rpm. Within 5 minutes the engine began to lose rpm and vibrate somewhat. After coming off plane and running at 2500 rpm for awhile it would then get back up on plane for 3 or four minutes and then falter again. My water temp has been 160 deg. I also have a new fuel filter between the tank and a new spin on fuel/water separator filter. Just can't think of what I may be missing so I share this with you. I have never had a vapor lock problem on any of my equipment so I do not know what the symptoms of vapor lock might be. Any of you out there experience this problem??? The engine has 66 hours on it with four oil changes to break it in properly. NOW THIS??????

first question

How old is the fuel?

Those GT-40 heads are not in a car. Marine engines operate in a completely different environment. Valve seats and valve guides are two things I can recall being different between car and boat use. Others with more experience should chime in soon, or you could call an engine builder that specializes in marine applications to get the specifics.

New and non-ethanol

Thanks, this engine was purchased from a Marine engine builder in Florida called Rapido Marine. It ran beautifully last fall.

Tom

The engine needs air, fuel and fire. If you can assume that the air and the ignition setting is a constant, then fuel is the problem. (I'm assuming that your electronic ignition is A-OK. Personally, I always use Flamethrower distributors and coils.) Anyway, the first thing that comes to mind is a plugged fuel tank vent. The pump can't suck fuel out of a closed tank. Make sure mud wasps haven't moved in. If the vent is free and the fuel is new and of sufficient octane rating, I'd be ready to blame the fuel pump. It sounds like the pump can't keep up with the demand at 4,500 RPMs but manages at 2,500. If not the pump, then there is probably some other fuel restriction...filter, clogged lines, carb issue, etc. If none of the above, it could be that you're sucking air somewhere...pin hole in the fuel tank dip tube. Whatever it is, it's a fuel feed issue.

I agree 100% with Naut. Sounds like you're not getting venting to your fuel tank. Had this happen a few time on me, one with an I/O and the other with an outboard. Don't know about the upper west but we have mud dauber bees/wasps that just love boating and pack mud into everything including the vent tube. Make sure since there is not a kink in the vent tube.

Thanks Nautilus and 63 Sabre,, when I was fueling yesterday I had a large volume of air coming out of my tank vent which is located on the side of the boat below the fuel filler neck so I think I am good there. I just put a new dual diaphragm marine pump with the required return hose to the carburetor. I have another pump with a single diaphragm that is not coast guard approved but worth a try putting it back on and giving it a try since it worked before. I do appreciate your time and comments. Best Regards, Tom

Sounds like either fuel delivery or fuel to me.

Naut and I are still on the same page. Just for a trial, is there some way you can bypass the filter? Run a hose from the pump to the carb. Sounds like fuel delivery to me also. If it runs good at 2500 and not 4500. Size of the fuel line/hose? Once had a inboard tank that had the dip tube leaking around the top where it connects to the outlet portion. The sealing compound or glue was just old and brittle. Frustration led me to completely tear down the fuel feed system. Who wudda thunk. It's just something to check.