1990 454 Merc 300hp w/ Thunderbolt ignition / aprox 1000 hrs. While cruising last week, eng began losing power, 200 – 300 rpm, and began misfiring out exhaust. Appeared to be fouled plug(s) as confirmed by replacing them along with cap and rotor (which were all last replaced in Feb 02). Also found and corrected the shaft circlip inside distributor, bent and not properly seated allowing shaft to move 1/4" up and down which I believe can cause timing to vary. After performing above work, engine runs beautifully again (responsive along with plenty of power). I felt plugs became fouled due to my accidentally neglecting the spark arrestor for nearly 2 seasons which finally got so clogged (my bad), the engine began choking while cruising 4 wks ago - cleaning took care of that problem. I had also spent 5-6 hrs idling this season with the above condition. So my assumption was that the dirty/clogged spark arrestor along with 6 hrs of idling would cause the engine to load up and foul the plug(s). Yesterday, I performed compression check: 1-148#, 3-129#, 5-119#, 7-138# /// 2-134#, 4-109#, 6-130#, 7-142# (did not perform "squirt oil in cyl" test to verify if low numbers were ring or valve related).<br /><br />My question is, with compression numbers as low as these and the biggest difference between the high and low cylinders being 39# or 26.4%, can anyone speculate how much longer the eng will run well before having to do either a valve job or eng overhaul and did it possibly contribute to the fouling of the plugs?
