I installed cmi headers this weekend and took it out to testand it is missing under load and will not go over 55 at 4500, it usally does 68 at 5050. I thought I might have broke a sprk plug taking off the heavy stock manifolds, so I replaced them. The boat ran fine 4 weeks ago, mabye bad fuel? spark plug wires? Any knowledgable info would be appreciated.

When I changed to CMI's, I had the same problem. What I seem to have done, was smash the plug wire boots when taking the old manifolds off. I am sure it happened when I was wrestling with them. Not dropping it hard enough to break a plug, but enough to cut the boot. It was only on one plug, but with the hatch up, you could hear it arc to the header. By the way, the longer I ran it, the worse it got. A lot of black tape helped get me back to the dock, but never cured it. I bought a new set of wires, just in case I might have slightly damaged others.

Hope this helps,
Bill

I'll swap them tomorrow, where did you get your wires, did it fix all of the miss, was it mainly under load?

Any other ideas?

Definitely try wires.

I would change the pugs to,,, your right there any way,, you might have cracked the porecelin(sp?) on one of them.

Ahh, May the force be with you. I have a pair of those 502MPIs...
You know what fixed mine? A good hour ride on the trailer should fix it right up. One of my motors wouldn't light. I towed it for an hour on I-95 to my mechanic. It fired up as soon as he hit the key.
Do the plugs and wires anyway.
You may need to remap the ignition with that high flow exhaust. It's not your problem with the miss, but the motor may be lean after you get it running OK.

It started out only under a load, but I think after the spark found the path of least resistance it steadly got worse. As far as where I got the wires, I ordered them from jegs or summit. I can't remember. I hope that cures it.

You shouldn't need the couputer remapped. I had mine done, and the wrong program was put in somehow. I won't name any names, but I had to run such high fuel pressure to keep the plugs looking good that at idle it would load up in no time. I had Whipple reprogram it and have loved it ever since. After saying all of that, if yours is stock I am sure it runs rich now, so I would check the plugs after a few runs and see what happens. I was not smart enough to change one thing at a time. I did the computer, flame arrestor, fuel press reg, and exhaust all at once. And for the next question, the top end may have increased by 2-3 mph, maybe. I always run my bravo 1 which is not as fast as the mirage plus which liked to blow out. The Bravo 1 likes to cruise about 5 mph faster around 3200 rpm. Anyway, where I did notice most of the difference with these mods was in coming on plane, and acceleration. Not to mention how much louder it is. That's the best.

What boat and power do you have? What is a safe color for the plugs? I was told it would run to lean with the headers and K&N. PM me and let me know about your excperience


Thanks

Bill 3 please post your results here. Doing the same thing to a 27 Fountain with the EFI Mag.

Ron

A friend of mine changed the exhaust and flame arrestor on his '99 502MPI. The motor did run lean and need to have the computer remapped. That cured it.
The boat came out of the water better, and pulled about 2-3 Mph on top (hits rev limiter).

My stock 502MPIs do run rich. I'm suprised that the exhaust and flame arrestor changed his motor soo much. But it did.