Hello Everyone,

I just helped a friend with a boating project, putting 2 supercharged 468's into a 36 foot Apache. This is the first time I have played with supercharged engines. These engines are topped with B&M blowers with two 650 holleys each.

Both engines were rebuilt at the same time, both with identical equipment. Ignition is GM/Mercruiser electronic circa 1993.

To make a long story short, Engine Number 1 runs just fine at 14 deg. Initial, 34 degrees full advance at 3000 rpm. We started at 28 degrees total, but the engine just did not want to run there.

Engine Number 2 runs the best at 16 deg initial, 36 degrees total at 3000 rpm. However, this engine has a cold start backfire. She will caugh through the carbs on initial cranking, but once she fires up, the condition goes away. If you start the engine hot (well, hot for a boat is about 150 deg.), she starts just fine - no backfire. It only backfires on the initial cold cranking startup. We can make the problem better if we crank the timing up, but I just don't think a blower motor should run any higher than what I have it.

It's hard to hear detonation with two motors roaring at 5000 rpm with the engine hatch off.

Any thoughts?

This may sound like a real pain, but you could try swapping carbs from one engine to the other to see if the problem follows the carbs. You didn't mention if these engines were run in on a dyno before they were installed. Firing two blown engines with four carbs for the first time together is more than I think I could handle. By the way, ask your friend what it's like having that much money. Sounds fun. :D

12run,

Well, as far as the money part goes, he got a deal on the boat because both motors and trannys were out of it at the time, and the dealership thought one engine was damaged more than it really was. Allot of blood and sweat was really all that was required. I bet he has less money in the boat then most of us have in our cars.

Swapping the carbs may be an option, I just figured someone here would know this condition and have a quick answer. All my tuning and carb resource books have very little information on supercharger applications.

We basically have gone off the pretense that the engines ran when they came out, so we have not touched the carbs at all, save for setting the float levels and idle. It really went much smoother than I ever thought. Once warm, they run awesome. I just would feel better if I could fix the cold start cough.

I will say, 200 gallons of gas does not last that long in this thing.............. I thought filling up my truck at the pump was bad - Ouch!!


Thanks!