Spark Plugs for 540
 

So with nothing better to do over the winter, I am daydreaming about going boating and planning for the spring. I purchased a boat last year with a 540 in it, has super victor intake, 1050 dominator carb, cmi headers. Internal wise, the PO claims it is eagle forged rotating assembly, srp flat top pistons, roller cam and approx 9.7 to 1 compression. I am thinking it is around the 600hp range, it has fun my 87 Baja 240 sport up to 67mph GPS without tabs, and probably not the cleanest plugs in those front cylinders, so it seems reasonable.

Anyway, with all of the background dispensed, what I am wondering about is spark plugs. The motor is tuned a little rich, and running at idle I tend to foul the front 2 cylinders, which from what I understand is normal for a BBC anyway, but once up on plane, the boat really runs well. So, my thoughts on this are to move up a step to the next hotter plug and see if that solves it without any real tuning, I am good with motorcycle carbs, but this 4 barrel monster is a bit beyond me. The problem is that I am having is that I am currently running ACDelco RapidFire #2's and I can't find any comparison information on the heat range of those plugs to know what to step up to. I was hoping that someone had this info, or for that matter had a recommendation on spark plugs to try for that application.

what cylinder heads does it have?

I am not sure, previous owner said it had ported marine heads, but that is all the info I have. Unfortunately it is rack stored so I can't look at it until April. All I can do is sit around and think about boating.

Put a multi-spark ignition system on to clean up the idle.

Generally, the front and rear cylinders are a little bit leaner than the center ones.

Need to know heads in order to help with plugs. If it has an MSD distributor, lock the timing out on it and will really clean up the idle. Might need a start retard with your compression and your total timing being its naturally aspirated.

It's just the stock distributor on it. I was hoping knowing the current plugs I could just find the next hotter. But yea, it would benefit from actually tuning it, might look into MSD.

What exhaust are you running?

CMI elbow top

my guess is the spark plug heat range is not the problem.have you adjusted the idle mixture screws?

Any decent welder can add an O2 bung to those, or better yet send them back to CMI and they’ll do it for you, and then install a wideband. It’ll save you years of grief and highly likely an engine.

I didn't even think of doing that, if I end up keeping the boat through next season I may very well do this over the winter. Would be good to have them inspected and pressure tested anyway.


I haven't messed with them yet. It is a 4 corner adjustment, so I considered taking maybe 1/4 turn in all the way around and see what happens.