Stalls at cruising speed after 30 minutes

I have a Sea Ray 233 mercruiser with a 5.0 from a 888. This setup runs great idles great, great low end and runs 4300-4500 rpms at 46 mph. Maintenance Completed. Fuel System: Rebuilt carb (except pump)-new fuel line- new fuel pump-new antisiphon valve-carb adjusted per manual including choke-tank cleaned. Ignition System: New coil-points-condenser-wires-cap-rotor-voltage tested at coil, resistor good, coil only warm at cruising speed. Outdrive: New water pump- shift cable-bellows-shift interupter working great. Engine: Cooling system pressure tested and flushed-new thermonstat engine runs in the green (cool).
I've taken my boat out several times with the same result. I'm at cruising speed for 30 minutes or so running great, the vacuum gauge starts to jump and then I lose power to stallout. It will try to start back up but I cant keep it running. When the boat cools down it will start right up like nothing ever happened. Any Ideas>

Re: Stalls at cruising speed after 30 minutes

Is the fuel tank vent clear? To check, just remove the fuel cap when the engine starts slowing down. If you hear air sucking in and the engine picks back up, you have a vent problem.

Re: Stalls at cruising speed after 30 minutes

I can blow through the vent inlet with ease to the tank with the cap off.

Re: Stalls at cruising speed after 30 minutes

Can anybody think of something else? Maybe vapor lock or something in the carb heating up. The reason I have replaced so many things, this is a total rebuild from the stringers on up. This boat had one foot in boat heaven, and the other foot was standing in the water at the boat ramp Thanks to everone in this forum you have got me to this point and I appreciate your time.

Re: Stalls at cruising speed after 30 minutes

Davinski said:

Can anybody think of something else? Maybe vapor lock or something in the carb heating up. The reason I have replaced so many things, this is a total rebuild from the stringers on up. This boat had one foot in boat heaven, and the other foot was standing in the water at the boat ramp Thanks to everone in this forum you have got me to this point and I appreciate your time.

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I've taken my boat out several times with the same result. I'm at cruising speed for 30 minutes or so running great, the vacuum gauge starts to jump and then I lose power to stallout. It will try to start back up but I cant keep it running.

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Ayuh,... Right at That point,... Do yer diagnostics... Is it firin',..?? Gettin' Gas,..?? plugs wet, or dry,..?? Color,..??

Re: Stalls at cruising speed after 30 minutes

Vacuum gauge is measuring what? Engine vacuum I would assume? And how long does it need to sit to "cool" down? My engine is still measuring 140 almost 2hours after running. Vacuum when it does "jump". Is it going from x value higher or lower? Or just gets erratic?

Re: Stalls at cruising speed after 30 minutes

It measures the difference of atmospheric pressure and absolute manifold pressure. My setup and elevation stays at 12 hg. When the problem starts the value bounces down and back for a few seconds signaling to me that its about to get alot quieter. When the engine quits the vacuum gauge reads 0 or absolute atmospheric pressure. Not sure how long it needs to sit to cool off before I can start it. When I get the blang thing home, the last thing I feel like doing is working on it.