Motor: 502 with 250 blower and duel Holley 850 carbs

Problem: when I start the motor it runs for about 20 seconds then the front cylinders cut out. I know this because the front headers are cold and the boost gauge needle is shaking back and forth at with little vacuum. If I let it sit 10 min. it starts fine then the same thing over and over. Two week a go it was doing the same thing and finally it started to idle correctly after about 6 starts!!

Things I've checked: compression check is good, CMI's don't leak.

Things I've swapped with other motor: front carb, ignition system (ignition module, distributor, sensor and coil) and fuel pump.

I've noticed a little moisture on one of the front spark plugs and checked the exhaust port at the header and there was about of a spoon full of water. I figured this was reversion from when the cylinders cut out.

What could it be?

it could be those cylinders have the valves adjusted to tight and when the oil pressure pumps the lifter up one of the valves is not seating properly.

what brand plug wires and how old are they,i have seen msd plug wires cause problems,and cyl 1 and 2 are the longest wires.

vacuum gauge not steady I would agree with Mike possable valve train issue.

Merc wires probably 7+ years old with NGK R5673-6 plugs.

Why would it idle good at the start and then go bad if it were the wires or plugs?

Gauge is steady at the start then gets shakey when cylinders cut out.

Remember compression is good.

Adjusted the valves today and no change. Removed CMI and all noticed small amount of water each header. Could there be a leak in the CMI that I can't detect when It's off the boat. it seems like the water is what makes it start idle bad after 20 seconds. Where is the water coming from?

Try running it with the exhaust water supply disconnected and dumping into the bilge. Process of elimination would be my path to glory...good luck

ttt

Vacuum leak? Try forcing choke closed on front carb and see if the fronts start firing again?