My buddy took his boat out for the first time this season and it ran perfect until he shut it off! When he restarted it, the boat would not get on plane. We drained the fuel, rebuilt the carb, inspected the pickup, tank vent, replaced the fuel pump (which seemed to have questionable internals) and spun on a new fuel filter. Off to the launch and no improvement. We did a full ignition tune up and although the engine now idles beautifully it still lacks power to plane off. There is some noise coming from the carb when the engine is under a load and the transom has a small accumulation of soot after a water test. At this point we are unsure where to go! Leakdown test? Verify timing advance? Any suggestions would be greatly appreciated this has gone on for weeks now!

Check the lifters. Had this happen to me years ago. Chased everything (fuel, carb, ignition system, etc.). Turned out to be a broken lifter.

A bad lifter would usually make a ticking noise. Has anyone inspected the prop to see if it has spun the hub?

Thanks for the tips, i should have mentioned that we cant get the engine to rev above 2500-3000 rpm. It is just not making any power. On the garden hose it revs fine, as soon as a load is applied it wont tach out.

Mine was a head gasket. Same conditions. Good luck

Today we got out the timing light and it appears that the ignition is advancing. From the 1 o'clock position the mark advances to approximatley 12 o'clock at about 3k rpm. Does this sound like enough? By my calculations that is about 30 degrees. I think this engine may have real problems.

Sounds like the choke is sticking closed
Wire it open and see if that helps

small blocks are notorious for sunken valves and will start backfiring through the carb as load increases.do a simple compression check or a leak down if needed

Carb or efi? if a carb which model?? Webber?

I got my hands on a snap on timing light which supposidly will tell me how far the ignition is advancing. Will test with this tomorrow and see what the results are.
It is a weber carb and I verified today that the choke plate is open. Thanks again for all of the suggestions. We'll see what tomorrow brings.