Jrock

This is what's going on. The motor run's good through it's rpm range, around 4500 for the first 20 minutes. After that, she falls on her face above 3000rpm's. Can't come on plane. But below 3000rpm's, she runs perfect. The motor is a 95 gen V 454/330hp. with a 600cfm single feed, vacuum secondaries. It's OMC.

This is what I replaced.
Fuel seperater & electric fuel pump.
Had the bowls off the carb. Everything looked normal. The electric choke was working.

Any suggestions?

Thanks.

formula31

It does sound like it could be a fuel issue. You really need to get a fuel pressure gauge installed right at the carb and see if it falls when the problem occurs. Another area that causes problems in the fuel system is the anti-syphon valve at the tank. If you go back to idle for a bit, does it then run ok for another 20 minutes or do you have to let it sit till it cools down?

Crazyhorse

Sounds like it's time to buy that new Black Thunder you've been dreaming about.

Jrock

Ok, this is what happen last time I had it out.
It runs good for 20-30mins. Then as I'm cruising (3500rpm) the rpm's start to drop, till the boat can no longer stay on plain. At this time the motor will run normal up to 3000rpm. Which is when it starts to plain. But, then it starts to buck and won't allow the boat to plain.
I'll sit there for awhile, start it up and it runs normal again, but only for another 20mins.
Could it be some kind of vapor lock? Most everything was replaced on the fuel system.
Can anybody help me?

Jersey Tom

A bad coil can also cause this when it gets hot and the problem will go away when it cools off. Just something else to check.