IS IT POSIBLE TO PUT THE K&N FILTER SYSTEM AND A LESS RESTRICTIVE EXHAUST ON A 502 MPI WITHOUT CHANGES TO THE CPU SYSTEM WITHOUT HURTING THE ENGINE.

This is not first hand experience. But, I was told that a K & N filter and performance exhaust system will make the 502EFI run better(leaner). They are a little rich from the factory. Not much in performance gains though.

I've done those exact mods and ran to lean send it to Arizona speed and marine I got an oso discount cost me 300 www.azspeed-marine.com

I'm running CMI elbow tops and the K&N with no problem at all. I didn't pick up much speed though. I was told that Merc sets them up at the factory to run rich.

if it isnt running leaner you havent made any more power, if its running to lean you can install an adjustable fuel regulator and bump up the fuel pressure till you get a good reading on your plugs

I've done all of these mods, including the fuel pressure regulator (set at 48psi per Arizona Speed's instructions). What do I need to look for on the plugs to make sure I'm not running too lean?

run her at WOT then bring her bak to idle and shut her off , pull a plug it should be tan- brownish color

Better safe than sorry one wot run to lean could be bad news