new 496 Stroker MIV motors with 731 cams edelbrock Aluminum heads ..... Holey 850's (4187) ... No Chokes ...... First run Yesterday everything seem ok .. Second run ..one motor stalls everytime you put it in gear ... other motor is ok ... Checked tot make sure nothing was wrapped up it the prop and then check for water/contanimation in the fuel ... All was ok .... A little history on these motors .. They seem to alway have starting issues even on the test stands ... seem easy to flood .... quite a few small fires ...not good ....
The issue with the stall seems also to be too much fuel .. thanks .....m

Check your float level, could be over running into the bowels.

Accelerator pump shot too big, too quick, etc?? Power valves ok? Float levels? Excessive fuel pressure?

I think the key (to this problem) is the fact that it ran fine on the first pass - then a change. number one culprit - dirt in the fuel system, number two - loss of manifold vacuum allowing power piston to "dump" too much fuel right off the idle mark.
Does it idle ok?

Dirt or rust in the fuel would be my first guess.

My carbs generally need retorquing after the first day of use following rebuild - a vacuum leak will make your mill run instantly rich off-idle, but fine at idle. It can cause stall, cough, fouling until your rpm pulls enough good air to offset the rich mixture, and then it'll run lean at wot - look for higher temps at wot - may indicate vacuum leak.

Has anyone been inside these carbs before or are they stock? They sudden stall sounds like a lean at idle condition which means you need to go out on the air fuel mixture screws. Almost 3/4 to 1 turn. If its fat and burns your eyes when it idles there might be one of the prior mentioned problems involved. Wrong gasket, bad power valve, stuck needle & seat any of these. If you can get it started and idle push the accelerator pump down and see if the engine dies or revs just slightly. This will help on figuring out whats going on.

You said "quite a few small fires"
That would be caused from backfires and would also blow the powervalves. That creates a very rich idle and would do exactly what you are seeing when put in gear.

the idle mixture screws were all over the place ....one was closed! .... Set em all at one turn and while I'm sure it can get better its damn close ..(but what do I know?) ... Run so much better .... idle great...slip right into gear .... Even seem to run better over all the range ... starting is bump the key and it fires right up ...(warm) ... I still plan on some Jetting and fine tuning but they are 100% better ....
I did read where there are 2 idle speed screws? A primary and a Secondary ? Thanks for the help .....m

Joe .. Our first guess was water in the Fuel ....its only been 2 years .. ... But I did pump all the gas out and then another fresh 10 gallons pumped out ....But we drained the filters and no water .. tiny amount of trash ... very small ..