ok so heres the deal i switched my exhaust from cmi(maybe my 1st mistake)to a silent choice system....ever since when left ideling with the silent choice on i keep fouling 2 or3 plugs. i no the quick answer would be shut it off but this thing is a beast when the silent choice is off and around the docks it scares the livn bejeesusss outta people. ive had the carb taken apar(850 holley marine) and rejetted but prob still there . its a built 454 500ish hp with msds ignition. only other thing i was told was to switch the ignition anybody else have this issue....there gotta be fix thanks in advance

850 carb may be to big.

You most likely are having reversion. It means the camshaft overlap is sucking water back in the exhaust and into the engine. Post your camshaft specifications and you will get more information from the board. I would suggest pulling the rear sparkplugs and look for evidence of water.

This may sound crazy but I dealt with that same thing for a month last year and it turned out to be connection on a coil. I have msd's as well and tried everything I could think of to fix my port engine. One rainy day last year I was so sick of it I was about ready to sink the dam thing so crawled under the hatch and started switching things from one engine to the other. After about an hour of that I got to the coil and when I switched them from one side to the other they both ran fine. I have several hours after that and not one problem.

Jetting has absolutely nothing to do with the idle circuit. An 850cfm carb is too big for a 450hp 454.

As stated toooo big of a carb.

carb is a little on the big side, you could try running a hotter plug as well.

what cam ? how much initial timing ? if cam is fairly big, locking the dist and setting it at 32-34 will clean the idle up and help that...Rob

I agree with Rob, locking the distributor at 34 degrees was the best thing I did for my idle quality.

3rd basic rule of engineering

if everything was fine and you changed something, and now its not, then what you changed is the cause of the problem.

if the carb was ok before, it is now

if the cam was ok before it is now

if the plugs were the right heat range before, then they are now.

so... what you have done is introduce an exhaust configuration that is roughly 800 % more restrictive than the one you took off.

what happens when you stuff a rag in the tail pipe of your car and try to start it ? it won't and the plugs get wet.

same thing here. the exhaust configuration you have now is going to require a different jetting regime pretty comprehensively i would think. certainly at idle.

its the unintended consequences deal.

i think i would put the cmis back w/ some mufflers and you will be way better off in all respects.