Boatnt has real experience with this, I don't. I'm just helping with parts look up.

Look here : http://www.mercruiserparts.com/Show_...ents%28NXT1%29

#31 is Gear Indicator Switch Assembly

and #15 Shift Interupt Switch Assembly

now disconnect 31 the gear indicator and try it,the gear indicator is a input to the ecu and controls the rev limiter in reverse,,if bad or not adjusted correctly the ecu thinks its in reverse and activates the rev limiter,
when disconnected it thinks its in neutral and will run fine

but can it make motor run badly ?

ok will try .tks

i just got off the phone with the mechanic and he says he tried both plugs off and still it wont run right !

any more ideas ?

tks alot , i appreciate it .but still no solution ???

Try taking the IAC off of the motor that is running right and put it on the other motor it's only two bolts and a plug. Really simple to eliminate this part. I'm guessing you're not getting any faults when you scanned the ecm or alarms

Vapor lock and bad ECM issue? Are you running fuel with ethanol?

Have you talked to the techs at Mercury Racing ? Have your serial number for them and if you give them enough good information, they can be very helpful trouble shooting.

I am overseas and we do not have access to Mercury Racing Techs here ! We have a useless local dealer .tks

I meant call them on the telephone. 920-921-5330. Give them your serial number, describe your symptoms. They have the most experience troubleshooting issues such as yours.