Looking for some ideas on the probable causes for a progressively lean condition on a stock Mercruiser 420.


Progressively lean, as in, Cylinder 1 - Acceptable, Cylinder 3 - Lighter, Cylinder 5 - White, Cylinder 7 - Melted Electrode.


Even today, Cylinder 3 shot out the porcelain spark plug from the plug body.


As mentioned, Stock 1990 Mercruiser HP420's, Factory Gil Offshore Manifolds, TRS Tails - dry to tip, Dual Plane Mercruiser Intake (~380 Hours).


Here is the history, purchased last year - during first or second run, Port Engine shot out cylinder number 2 spark plug porcelain and after inspecting for damage (camera, compression test/leak down) I replaced the plug continued to run all of 2020 Season without any major issues (~30 Hours). During winterization, the Port Engine was noticeably ticking and popping through the carb. Fast Forward to Spring 2021, I complete a compression test on the engine and find that Cylinder 2 and Cylinder 6 are low. Pull the heads and find a crack between the intake and exhaust valve on those two cylinders. Picked up a set of used 188 casting heads from an HP500 and have the machine shop go through them and switch over the valve springs. All is good and I reassemble the Port Engine using all Fel-pro gaskets, applicable torque specs, etc.. Boat runs down the river for ~40 Minutes and I lose a cylinder. Find that Cylinder number 7 has melted the electrode from the spark plug, all other cylinders are seemingly OK.


Inspected for damage, none found, replaced spark plug and took her out for a short cruise down the river (~40 mins). Cylinder 7 melted the electrode again. Both times, Cylinder 7, Cylinder 5, and Cylinder 3 we found to have their spark plugs loose once I shut the engine down. They were all tightened to the proper specification and they are the correct type (AC MR43T). Took a slow motion video of my carb and notice a very odd spray pattern on the odd bank primary circuit. Removed odd bank cylinder head and sent to the machine shop, found one bent valve so they repaired, cleaned, and decked the head 4 thousandths because it was slightly warped.


Rebuilt the carb, reassembled the port engine, and hit the Chesapeake Bay for the shootout, boat ran great all day (~3 Hours) but at the end of the day it sounded like it was staving for fuel when returned to the ramp, shutting off/stumbling. Replaced fuel pump and separator this morning and went for a sea-trial. I ran the boat ~1 hour today, mainly at 3400-3600 with a couple short blasts to 4600-4800 RPM. Then I noticed that I was losing RPM on the port engine again. As soon as I noticed the RPM distance, the port engine shot out Cylinder 3's spark plug. Upon removal of the plugs, I found that Cylinder 7 melted the electrode, Cylinder 5 was super lean, Cylinder 3 blew itself apart, and Cylinder 1 was on the light side of acceptable. I also pulled the timing back to 10* from the factory 14* to save from detonation. HP-V8 Thunderbolt IV ignition. I have checked for intake manifold leaks with both starting fluid and propone, no change in idle. Carbs are stepped up one jet size all around from the factory configuration (80/91 now)


So, in summary, new heads - new fuel pump - rebuilt carb - new head gaskets - two sets of wires - new spark plugs, still having lean odd cylinder issues. Keep in mind, starboard engine hasn't missed a beat, she is running like a sewing machine and also that it is only the ODD cylinders having the issue, even cylinder have been fine. This port engine has been an issue since purchased.


Any input would be great!

Odd Cylinders, Cylinder 7 to 1 as seen left to right




Even cylinders, Cylinder 2 to 8 as seen left to right

I dont know what those plugs look like new but they look like they have been very hot. Any chance there is a water circulation issue causing that side of that engine to run a lot hotter than the other ? You changed the wires but how about the cap and rotor ? Any chance you are getting some cross-firing there ?

Thinking out loud

Do you have a pic of that bank’s piston tops when head was removed ?

Thanks Bill, any input is appreciated at this point! Wires are new, replaced after each failure. Coil, cap, and rotor are new as well.

Temperatures seem uniform across the water system but I was double check for a blockage.

SB,

I thought I had one but couldn’t locate it. The first tear down, the piston tops had an expected amount of carbon. The last time it was apart, the piston heads on 7, 5, and 3 seemed to be cleaner.

SB,

I thought I had one but couldn’t locate it. The first tear down, the piston tops had an expected amount of carbon. The last time it was apart, the piston heads on 7, 5, and 3 seemed to be cleaner.

Not sure why that posted twice.

Josh, you sent me this at the first teardown. You can see number 3 here. We’re they all about the same? It looks a little different than the even side.

Did you try cleaning off number 3?