The oil cooler is too small. That much hp should have a cooler with a 3x12 core inside. Total length of about 18 in. I agree that there is something wrong causing the low idle vacuum. First make sure the carb is right, the needle and seat doesn't leak, the float bowls aren't set too high, the idle mixture screws are between 1/2 and 2 turns out. Then make sure the ignition system is working correctly, 16min. degrees of initial timing and 34-36 total. Last check the inner workings of the engine with compression and leakdown tests. I still don't think it's water temp related. Like I said, I run a crossover with a 120 thermostat and I don't foul plugs. These engines aren't rocket science but they can be frustrating.

I did a compression test, prefect. I will do a leakdown.
Cooler, I would like a termosate setup. With cold lake water it is HARD to get oil temp, and then once it is hot.........

Vacuum, I will look at.
Carb is coming off, and I am going to build it to HP 500 specs with 2 sizes bigger jets........ does that make sense?

RT

If your AF numbers were correct on the dyno there's no need to change your jets, in fact you'll just make the engine run richer which may exacerbate your problem.

I'd start with getting a visual down the venturi and see if fuel is leaking past the booster at idle before going any further.

When you say fouling plugs, do you mean they're just darker in color or do they actually foul and misfire? It takes a fairly decent amount of fuel to foul a plug.

You could always put a #6 heat range plug in, I think I read that you're running a #7.

But the dyno had dyno headers, no flame arrestor, no pullies.

When I say fouled, I mean......... NO FIRE. Put new plugs in it will run fine for a bit. IF you idle more than a few min they are toast. If you run at 3000 or more they slowly go to toast.
The boat goes from 80- 60's in about an hour, new plugs back to 80...... always the front 2 plugs...... always.

RT

Dyno headers run leaner than manifolds, the engine will be slightly richer in the boat. Also, running without the flame arrester will make it run slightly leaner than with because it restricts airflow to the carb. The pullies are just a tiny bit extra load on the engine and carbs pull more fuel when they are put under a load. So if you jet up you'll be even richer...

If you're fouling plugs that quickly you're dumping fuel, has nothing to do with water temps....

A V6-14 module on a V8 engine will give about 10* of advance, all in by 2500rpms.

Your problem is being caused by the low vacuum. The low vacuum is making the carb powervalves open and making it run rich at idle. You need to figure out why you have such low vacuum.

Fix the problem, not the issues caused by it.

Been trying to say that for the beginning...you say it much simpler than I can... :drink:

what sort of vacuum should I have on a 9.8-1 compression 502 with Edlebrock oval port aluminum heads, a Victor Jr. intake and a Crane 741 with an 850 holley?

1. make sure you are checking the vacum in the right place on the carb. 2. what block is this mark 4 mark 5 mark6 if it is a 5 or 6 i hope you used the head gaskets that block off the front water passages in front of the cly. bore if you don;t the water will not move through the block like it should. i built a pair of 496's ,not the merc 496 used the 741 no thermostat at all i used the air gap with 1050's idles at 800 never loads up and made 640 at 5800

Thats basically my motor RBT has described. Odds are its the Vacuum leak. Should be 10-12 inches of vac. If it has a vacuum leak that bad then the carb might be adjusted over rich to compensate for the leak just to keep it running and therefore taking out the coldest plugs. If thats really the correct vac reading from the manifold and the intake gaskets are not leaking then I would try a different carb, just to see. Was it a new carb? I have had used carbs that just never ran right no matter what I did, swapped out to a new one (or different) and problem was gone.I run NGK #7 in my motor with no problem.#9 fouls out with extended idle. Make sure to try plugs that are a bit warmer. BTW What was the compression PSI when you checked?
So try warmer plugs, make sure there is no vac leak, buy or borrow a carb.