I had both motors and drives pulled over the winner to fix oil leaks and clean up the bilge. Fixed laundry list on the boat of miscellaneous things that needed to be addressed to put it back in top mechanical condition. Nothing on the engines were changed except spark plugs, oil pans, seals, gaskets and timing covers.The boat is a 1996 Merc 350 Mag Gen+ coupled to Black Hawks 31p props. They are both doing the same thing now drop them into gear and one out of two or three times dies. When I was looking for a P-29 to buy I wanted all factory. A lot of people had trouble putting carbureted motors in front of the Blackhawks and dying when you put them in gear. I knew that a factory power set up what's trouble-free engine package and thats what I wanted. I'm waiting in line to get back in the shop to get the rest of these bugs worked out but this one has us scratching our heads. By asking here I'm hoping to save some trouble shooting and go straight to the issue. Thanks for any help...

try the neutral safety switch common merc problem

also called the shift interuppter switch

Staging on the carbs might be the issue too. Had that prob on my cig. Put it into gear and the carb wasn't adjusting to keep fuel level up under load. But kinda weird both are doing it at the sane time. Even with the neutral safety switch. Should be both going out.

I heard those blackhawks were giving fits going into gear.If it can be fixed he will .It's in good hands. hf...

Engines are MPI.

I would be looking at the shift interrupt. It should momentarily ground ignition coil out to let it go in and out of gear. Immediately after it engages, the rpm should bump up a bit so stalling does not occur. If engines were out, and both doing the same thing, maybe some plug in connection for both was missed? It is also a possible cable adjustment issue and due to play/misadjustment, the interrupt and actual shift are not sequenced correctly.

I don't believe a Blackhawk drive uses a shift interrupt switch since its a Bravo based drive.

Check the ignition timing.

Black hawks don't have a shift interrupt switch. That's only on the alpha drives
The neutral safety switch will not cause the engine to stall. It only prevents the engines from starting in gear

If the engines do not stall do they run normal and pull proper wot rpm?

Everything else is normal. Thanks for everyone's input so far. I'll be dropping it off soon to straighten it out. I've been very pleased with the service I've received it's just dotting an I and crossing a T to finish it up.

MPI engine with bravo based drives. My guess would be the IAC motors are not plugged in. They are in the same position when the engines were shut off