Hi guys I have got a on going problem for a year now with no answer or any ideas left. My starboard engine (454) is dry (carbon) fouling spark plugs in cyl. 7 and 8. It will foul MR43T's in minutes MR44T's in hours. However it seems that if I am hard on the throttle it is not near as bad, if I cruise at 3500 RPM they're toast. I have changed the rotor, cap, wires, and coil. I checked my compression and its fine. I swaped my carbs thinking I was over fueling no good. I have no oil loss and no smoke from the engine. The thing will misfire and run like crap I will replace those two plugs (7&8) It will run like a raped ape then I slowly loose rpm's till it misfires I'll replace those plugs and go like hell again. By the way It has a prestolite ignition. Any help would be great. Thanks, Barry.

Maybe try going to the next heat range... like 45's? You've tried the colder plugs and it fouls in minutes, then the next step hotter and it takes hours... now go one step hotter.

How does it idle?

Do you have a picture of the intake mainfold, plus one of the timing cover ?

It idles fine it's tougher to start than the port engine. I do not have any photos but they are 330's with stock intakes. better known as pancake intakes

I think we need to get more info. what type of carbs, intake, plug wires. Has timing been checked lately? How old are the distributers themselves? Let us know your set up and maybe we can point you in the right direction.

Thanks

Do not increase the heat range. That is just a band aid. You could try some AC Rapid Fire #1 plugs. They are the same heat range as a 43 but have better, extended tip and don't foul as easy.

Find the real issue causing the problem and fix that.

Griff, here is the reason I say a hotter plug... no it's not the fix, but it could be a potential reason. This was taken straight from NGK's site....

Keep in mind the insulator nose length is a determining factor in the heat range of a spark plug, the longer the insulator nose, the less heat is absorbed, and the further the heat must travel into the cylinder head water jackets. This means the plug has a higher internal temperature, and is said to be a hot plug. A hot spark plug maintains a higher internal operating temperature to burn off oil and carbon deposits, and has no relationship to spark quality or intensity.

Having said that, it could be a number of issues: weak spark energy from dist. or box, carb issue at low rpm dumping too much fuel (ie... booster issue or air bleed clogged). Those intakes on those motors have horrible distribution paths and could exaggerate the smallest issue with the carb.

I'd try all the cheap and easy (read= free) stuff to see if ti makes a difference. You could put new plugs in and swap carbs and see if the problem still exists. If it does on that one motor (and not the other), then you know it's not the carb. You could do the same with the dist.... narrow things down before spending any money.

griff is correct. if it was a systemic problem or a a heat range problem as suggested then all the plugs would show the same problem. changing two plugs on two cylinders isn't going to fix anything or tell you anything. bad wires, cracked cap ... maybe and certainly the easiest and simplest things to look at... and thats assuming that this is a weak ignition issue... which i am sceptical of...

i might leak those two cyls down and maybe have a look at the valve springs, after i had looked at all the easy stuff. and for sure the easiest deal is to just swap the cap and wires and carb from the good motor and see if the problem stays or follows those parts.

You guys are right, I misread the OP... I thought all plugs were fouling....just the 7-8 were the worst.

Now I am second guessing the weak ignition theory of mine, as the engine still runs hard without a miss at high rpm.

I like the leakdown idea... not sure about the valve springs, as weak ones would surely cause a miss at high rpm... but it is easy to check, so I would definitely do that too.

I am kind of stumped now too.

I was going to tell you to check the plug wires, but you already changed them. Sorry