GO4BROKE

I have problems with my stock 97 454 mag 365hp. It isn't firing on #3 at all and loses #2 intermittantly. I have done a tune up including caps,rotors,plugs,wires,fuel filters. Was good for 2.5 hours of run time then acted up again. #3 plug black, not firing. #2 plug very rich, firing sometimes, all others OK. Did a compression test, all 8 within 2 psi of 160psi (150psi is merc spec). Swapped hall effect switch and coil from my other engine, no change. I suspect cam or valve spring problems now. Pulled the valve cover and rockers to check the springs, saw no broken coils but I don't have any of my valve spring compressors at home, so I have to wait until I can get into work monday to actually remove and inspect them. I am also going to measure valve lift with a dial indicator to check the cam. I have read that mobil one can cause lifter rollers not to roll and therefore waste the cam. I have run mobil 1 since 20 hours, now at 203 hours. My thought on this is if its too slick to turn the roller, it should be too slick to wear the cam? Does anyone have suggestions as to other things I should check besides the springs and cam?

Vinny P

Did you try a spark tester on the suspected cylinders? That will tell you for sure if you are getting a good spark or not. Is that motor e.f.i. ? If so, it is possible that the injector is hanging open causing the plug to foul.

GO4BROKE

Its a carb motor. No I haven't actually tried a spark tester, but literally evey ignition system compenent, except ignition module, has been replaced with no changes. I will hook up a spark tester tomorrow though, just to cover all the bases.

Offshore Addiction

what kind of exh,those center cylindres love to quit firing when there starten to revert water,and you cant even trace the water,try another compression tester,thats a little too close on your test

GO4BROKE

I used a different compression gauge tonight, no change. I also removed the valve springs for #3 cylinder, no problems found. I also put a dial indicator on the push rod to measure cam lobe lift. #1(which fires) is .273 I and .288 E, #3 is .278 I and .282 E. Spec is .300" for both intake and exhaust. All are a little low but that may be from the push rod and lifter and dial indicator, you're supposed to mic the cam when its out of the motor to be accurate. If #1 lift is good enough to fire then #3 should to by my way of figuring. I also checked spark with a spark testor just for the hell of it, it was strong. I also did a cylinder leak down, passed with flying colors.
OK, I have spark, compression, fuel (plug was black), shouldn't this thing fire??
If I had enough reversion to keep #3 from firing , wouldn't the exhaust port be clean from the steam? I have stock exhaust by the way.
Any ideas?????

GO4BROKE

I don't know if this is related but this motor lost its sea water pump a week or two before the miss started. The temp gauge never went above 210 degrees before I shut it down. Also the miss has went away twice . Once after replacing fuel filters, plugs, and caps and rotors for about 1.5 hrs run time, then started up again the next day. I found a burned plug wire and replaced all the wires. After that the miss stayed away for 2.5 hours of drive time. Now the miss is always there. I'm at a loss.

Intolerant1

Could be a bad power valve in your carb if its a Holley. Its dripping and the way your boat sits, the fuel pools and runs to those two cyclinders your having problems with. Just another thought.

GO4BROKE

Good thought, but its a stock Weber/Carter/Edelbrock. And I rebuilt it this spring. Thanks!

Offshore Addiction

base gasket under carb,or carb,you need to put a vacume gauge on it,or you may have to lifters that dont want to deflate under load,but I keep thinking carb......if it was ignition youd have white plugs,what color are all the other plugs?

GO4BROKE

Plugs:#3 black
#2 very rich
All others just a smidgen rich
I'm going to swap #1 and #3 valve springs tonight and put it all back together to see if the skip follows the springs.