New 460 engine stalls when shifting into fwd / rev

I have a new remanufactured 460 Ford engine in my boat. I'm having trouble with the engine maintaining a
constant 750 RPM idle. When it gets up to operating temp. it will idle ok for awhile then it starts to stumble a bit and will eventually stall. The fuel pump is new, it has a new Mallory single point mechanical advance
distributor, new Autolite 24 spark plugs, the 600 Holly has been rebuilt. I'm really leaning toward the carb.
at this time. It seem like the only thing left. Can someone offer any advise as to what the problem could be.
Thanks........

You do not mention year/model.
The 460 OMC was always fitted with the KC drive.
The KC was available in both Dog Clutch and a later Cone Clutch version.
If you have the earlier Dog Clutch, you will also have an ESA.
ESA = Electronic Shift Assist.
If this is malfunctioning (as-in not adjusted correctly), it may be part of all of your problem.

If the later Cone Clutch KC..... I belive that the ESA was eliminated.

.

and if it's dog clutch, a failing lower shift cable can cause the ESA switch to stay closed, stalling the engine

pull spark plugs when it stalls and make sure they aren't getting quenched with water from a bad riser gasket or manifold

The Year is a 1989 KC. I have a clutch dog and the lower cable is brand new and rigged to the manual.
The ESA is brand new and is working as it should.
The manifolds are new. Risers ok.
ESA micro switch is bypassed with a momentary switch for manual operation at this time up at the shift lever.
When the button is pushed the idle drops to about 400 RPM. I can hold the button in and the engine will not stall. It only stalls when shifted into gear and the button is released.
It seems whenever a load is placed on the engine.
I ran the engine numerous times out of the water and the shifting is fine. When the prop is in the water is when I have the problem.
Hope this info helps.
Thanks for the quick replies....

OK... perhaps you may not be understanding the ESA.
There should be no issue going INTO gear with Dog Clutch gear engagement.
If there is an issue, then something is not quite right. Cable? Linkage? Actual drive? Shift spool? Sliding sleeve? etc.

The issue is in coming OUT of gear, back into neutral, and this is due to the propeller thrust causing the Dog Teeth to remain engaged.

Try going into neutral while traveling at 8 mph or so.......
She'll usually come right out of gear into neutral, because the hull speed is greater than what the prop itself could produce at idle RPM.
I.E., reduced prop thrust = reduce dog teeth pressure during this momentary reduction in RPM.
(don't try this near the shore or dock! )

The momentary stumble of engine RPM, is to momentarily relax this propeller thrust, in order for the Dog Teeth to let go of each other.
Quick, concise, deliberate shifts are mandatory, if you want this to work properly.
IOW, don't dog a shift... do it quickly and concisely.

.

Thanks very much,
I'll try it out.....

the new esa has a 3 second delay. Just letting you know. It is the esa with no overstroke.

I won't even ask what that motor cost to rebuild and where did you get new mani's and such?

The coupler new also ?

Me if i had to rebuild the 460 i wouldn't. i would have gone with a bbc.

Good luck !