keep fouling plugs
 

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.

hey all thanks for the input, im unsure of whats in it for a cam, but will certainly look into locking the distributor..........cheers

hey stevesxm, i posted the last reply at the same time as you .......was thinking the same thing about the cmi exhaust and adding mufflers but 2 late now as cmi's were sold. its just hard to believe that the silent choice affected it that badly, there has got to be alot of guys who have had to do the same swap with little or no issues.........i cant afford another "episode" like last yr-the boat flooded itself out in the middle of the st lawrence shipping channel and yes with a freighter hard on the airhorn for me to move the hell out of the way

I would agree with the theory of what you changed is probably the cause. I also agree that it is likely a reversion problem. Your cam may be to agressive to use silent choice.

just out of curiousity why would you think :

1) a cam that didn't revert before as a function of centerline and lift would suddenly do it now ?

2) why you would think that reversion would gas foul plugs ?

if the thing was sucking water back in i think you would see a whole raft of other and more serious symptoms ( and evidence) and if it was the cam then problem would not now just materialize after it had been fine all along.

when i repowered my boat a long time ago it had some sort of switchable system that dumped into a y pipe that existed under water ... i took them off and " threw them away " .

the guy that bought the one good 454 from me put it in a cruiser and couldn't do thru hull... so he used std manifolds and little exhausts and the y pipe. and he couldn't keep the plugs alive no matter what. so i convinced him to ( as a test run a old pair of gills i had with some cobbled up top exits.

boat ran perfect.

you motor is a big motor. when you restrict the exhaust like
( i am theorizing ) you have, what is happening is exactly what i would expect to happen.

you don't mention it but i assume that the boat runs fine when the exhausts are " open" . assuming that is the case, the only conclusion is that when they are " closed " they choke the ex enough to cause the issue.

nothing else makes any sense.

I am not an guru on the topic. Perhaps that is obvious. However, he said he added the silent choice and he has the problem when he idles. Seems to me with the water dumping into the exhaust stream before the silent choice and with the silent choice closed it would be more likely to revert than when it is open.

Reversion would not cause the plugs to foul. An increase in backpressure would. Chances are that he was right on the ragged edge of fouling the plugs before the switch and the increased backpressure pushed him over the edge. The 850 carb was on the large side to start with. An 800cfm is more than plenty and on the large side as well.

Without actually replacing the carb, I would do several things. Double check the float adjustments with the boat in the water. Next is start playing with the idle air bleed adjustment screws. Turn them 1/8 of a turn at a time for testing.

What is the idle rpm in neutral and in gear under load???
What spark plugs are you running???

hi all, yes the boat runs fine when the exhaust runs open...it even idle fines around the docks with it closed, this situation has happened a couple of times the last time i had to wait in line for fuel maybe 20 mins or so(idle speed only) fill up starts fine, leave docks get out to the open water...open exhaust get on the throttle to plane and no power, back fire up thru the carb and then dies on me. i did bring it to a qualified mechanic who replaced plugs,wires,distributor, etc he rejetted the carb because they were way to "fat" he told me his instincts tells him the motor mustve have had a supercharger on it and thus the "fat jets' he also did a comp check and all were around 150ish...............just trying to put as much info out there.........really appreciate all the feedback

check your fuel pressure,i run mine at 6 to 7 lbs psi,anything over 8 on a holley will not allow the needel&seat to fully close.at idle,the engine does not need much fuel,this is when the problem will be noticed the most.i also agree with locking the dist,a 9 to 1 compression engine will have no trouble starting with 34 deg of initial ig timing.i run my blower motors locked and they idel nice and clean,and i run farely large cams.i also agree with griff,unless your cam is grossly wrong,reversion wont foul plugs,the water in the cylinder just gets pushed past the rings.

Changing jets has ABSOLUTELY nothing to do with fouling plugs at idle. If a carb is too large, it needs bigger jets because of a weaker vacuum signal at the carb.

What spark plugs are you running and what is the idle rpm???
The float levels need to be set with the boat idling in the water.

Fixx
 

what everyone else said!, my old 365 hp had a 800cfm Q-jet on it so i don't think a 450hp 454 with a 850 cfm Holley would be to big..Chit im running a 750 cfm on a 355 in my donzi and it loves's it..and what griff said the jets have nothing to do with the idle circuit in the carb..you also mentioned it back fired or popped,you may have blown your power valve's if you don't have the protection circuit in the base plate. do you have a 4 corner idle circuit carb? start out at 1/2 turn and dont go no more then 1 full turn out..turn the screws in a little at a time and wait for the smoke to clear and see if this helps..you may also have the wrong power valve in the carb,also if you turn the idle to high on the primary circuit on the carb your throttle blades are past the idle circuit and will load up..
ALWAYS set the idle with the secondary throttle blades or you will get a stumble off idle..

Also what power valve are you running ? the increased back pressure changed the manifold vacume and depending on the power valve caused it to open to soon...that would kill plugs quick..