Check the timing for sure. A few degrees could cause that 200rpm difference. Make sure both engines are advancing at the rate and have the same total advance. They should both be at 35* total advance at 4300rpms. The stock merc balancers should have a mark on them for 35* also. Timing at idle should be 13*.

When running at 3800 both (on tach) I can ear that it's ok for the tach.
But I have to check the timing ( i will do it this saturday and tell you after)
Cables and fuel separator seems to be good point too, if i can i will change both before saturday.

Thank you guys, I will let you know what happen saturday !!!

Ok, so after checking cables and timing, we have made a sea trial, and still nothing different...
after talking with a mechanics, he told me to check the blower boost, but the gauges are not ok.....

So I have put a smaller top pulley only on the engine that wasn't running 5200, and made a sea trial yesterday....
Everything is ok now, both engines are running 5200 rpm when wot.

So my question is:

what was the problem?

Blower need to rebuild ?
valve problem?

what else...?

I would put the same pulleys back on each motor. Then swap the blowers. Really shouldnt take more than 1/2 hour to that. If the other motor is low on rpm, than the blower needs a rebuild. Which sounds like it could be the problem.

However, it still can be anything from the engines having different camshafts, to valvetrain issues. So i'd verify the blower situation first. Good Luck and keep us posted

It is common for belts to slip as they age on a 10 rib setup. Did you try a new belt as I suggested earlier?

I have ordered a pair, and I am waiting for it now...