thank you I will try that this week end,when I rotate the props what can I expect the boat to do and can I run it wot?

same issue on my 2001 353 fastech
 

I just re-installed my motors after repowering the 496 ho with upgraded pistons 5 lb procharged etc,etc,etc

I am not sure if I still have the rpm difference like before doing the upgrade ...my feeling always was that the fuel pump or screen was getting clogged or blocked at higher rpm I had a 200-250 rpm difference mine was at wot or above 4600rpm I know in the past 496 motors had issues with debris clogging the fuel pump because the filter is after the pump..496 have 2 fuel pumps 1 (boost pump l/s of engine in front of fuel water seperaotor)that draws the fuel from the tank and 1 that is on the r/s of engine which sends fuel to the rails.

It will handle differently, spinning out vs. in... overall slower, but you should be able to see if the lower rpms follow the props, or remain the same.
Carder

PS: did you check the magnets in the upper drives for any bearing or gear material sticking to them?

are we sure you just don't have a cam lobe leaving town ? or as happened before here, that you didn't get the firing order wrong on that motor somehow before all this started...

the firing order is fine when I replaced the plug I do one at a time also use a scope to see how it was firing.

could be a bad throttle cable, they can get stretched over time