Mercruiser 4.3LX/Thunderbolt IV won't advance past 2600 RPM

My brother has a 1993 Mercruiser 4.3LX/Thunderbolt IV that will not accelerate beyond 2600 RPM and 9 mph. It ran great, and we shut it down to troll with the kicker for 6 hours and when we restarted it, we couldn't get it to run for more than 15 seconds. It bogged down and would not run. We rebuilt the weber 4 bbl carb, and replaced the Fuel/water separator, replaced the distributor cap and rotor, and set the idle to 650 RPM. It runs smooth and accelerates without hesitation but it hits 2600 RPM and just tops out there. It doesn't stall or miss, it just won't run higher than 2600 RPM. We set the floats according to the directions in the kit. I suspect that we might need to recheck those. I'm also wondering if the ICM could be causing this. How about fuel pump on it's way out, and not able to supply enough pressure at higher RPMs? We aren't mechanics and would love to take it to a mechanic, but mechanics around here just want to sell you a new motor for a bazillion bucks, so we're gonna figure this out.
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is the timing advancing, as the engine RPM increases?

verifying that the fuel pressure is adequate would be good to know.

makomark said:

is the timing advancing, as the engine RPM increases?
verifying that the fuel pressure is adequate would be good to know.

Click to expand...

Like I said we aren't mechanics. I assume the timing advance would have to be with a timing light while on the water and accelerating? I believe my brother ordered a new fuel pump. Thank you for the reply!

you will need a timing light...but you can do it in neutral, preferably with the boat in the water...you can burn up the impeller doing it on a hose. best to use an advance timing light if one is available....the timing should increase with and MAX out at 2500RPM at about 22 degrees BTDC.

If the timing doesn't advance, the engine won't make any power...the advance is in the module .