I have a 496 H. O it runs fine until it gets to around 4700 rpm's then it starts to fall back on rpm's sometime as much as 500 not gradual but all at once. One time it would not run over 4200 rpm's. The tech guy ran a scan and the computer did not show any problems. I checked the filter and there was no dirt or water in it. He thinks it is the fuel pick up. I can't check it until I burn some fuel out of it. Is there any other things that can cause this? I assume the computer would show any engine problems. Thanks for any help.

Try an alternate fuel supply. Put a hose from the water separator into a 5gal can of gas and try it. Eliminate the gas tank. If it fixes it then you know you tank is clogging. Check the tank vent too.

Hm.. sounds like fuel starvation. If it is not the tank you may have another fulfilter closer to the fuelrail or alternatively it´s the fuelpump or the fuel pressure regulator

Check The Screen In The Boost Pump (the One By The Water Separator)

I'm having the same problem with new procharger motors i just had built. runs up to about 4800 rpms and a little after secondaries kick in it starts dropping off, back up 2 clicks and the boat takes off. fuel pressure gages read fine 7lbs start and climb with boost as there suppose to. Brand new holley 850cfm carbs with 92 primaries and 97 secondaries. running 91 octane. checked all plugs and all look good. the leanest one was 95% covered with soot all around. what else could it be except fuel

Check the vents for blockages or try taking the fuel cap off. My buddy worked as a Marine Mechanic for many years, and he told me that spiders will get into the vents and build nests in them, blocking the air from getting in.

I have the same problem on my old Nova in the right side tank. I have narrowed it to the sock on the pickup in the tank. I have been running Lucas Tune-up in a Bottle through it, and it has been gradually loosening it up. I would think that you would have noticed some crap in the filter though, I sure did.

If you do not have the inline fuel filter added, you can bet the inlet screen of the boost pump, is clogged, as GOODT mentioned. Unfortunately this is before the main fuel filter. Most people are adding additional inline filters to prevent this. Mercury added a throw away inline filter that was a little pricy. I added Racors.