OSO!!

I'm struggling and looking for suggestions to get the port engine in my 42’ Sportfish to run smoothly below 1500 RPM, and at idle.

The boat is powered by a pair of ZZ502 short-blocks installed last spring, paired with new AFR 265 heads and Holley HP TBI EFI. The starboard engine runs flawlessly, while the port engine has a rough idle and a noticeable miss up to 1500 RPM. Once throttled up, both engines run smoothly above 1500 RPM, make solid power, and burn equal amounts of fuel at cruise (3500 RPM).

What I’ve done so far:

• Replaced intake gaskets
  Smoke tested intake system – no leaks found
• Swapped throttle body and distributor with the starboard engine
• Replaced spark plugs
• Replaced ignition coil
• Re-adjusted valves
• Verified compression (175–180 psi across all cylinders)
• Confirmed both engines are running the same tune
• Temporarily installed a carburetor with distributor disconnected from ECU

Above changes had no effect on my poor running engine.

Could this be a lifter issue? Has anyone experienced problems with GM lifters in similar setups? I usually work through these kinds of problems myself, but this one has me stumped. I don’t have a problem buying a set of lifters if you think it is possible they could be issue.

Fingers crossed that the collective wisdom here on OSO can help me figure this out—before I pull out what’s left of my hair!

Thanks,
Steve

Can you tell which bank has the miss ?

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try pulling 1 plug wire at a time to see which cylinder has the miss? The plug wire that does not appreciably change the running condition is probably the cylinder that's wonky.

I suspect I'm getting a miss from each bank—at least, that's how it feels. It's hard to test by pulling plug wires because the exhaust exits below the waterline at idle, causing a lot of bubbles to smack the bottom of the boat. This makes it difficult to tell whether the sound is just water turbulence or an actual engine miss.

With good compression across all cylinders it seems lifter weakness is questionable, but the fact that it cleans up around 1500 revs points back to oil pressure or weak spark at lower rpms ?

sounds like a stupid question but what plug wires are on it? just finished battling a problem sniper setup on a 455 olds that kept having emf problems. it idled strange but ran good in the upper range. found that all 8 of the plug wires at the distributor end were assembled with only about an 1/8" of core exposed and not wrapped up and under the terminal end so I had about 3/16" air gap from the terminal to the wire. I never measured the ohms of the wires as they were new MSD wires but now that I fixed the wires it is all working like it should and idles way better.

It has great oil pressure, and wires, cap, and rotor are Sierra Marine. I've had great luck with them over the years. I have not tried the wires from the other engine. I will do that!

Well, swapping cap, rotor, and wires from other engine did not change anything.

Not sure where to go from here. I'm seriously considering installing a set of lifters. I don't know what else to do.

I have readjusted valves twice using 1/2 turn from zero lash. Would more or less preload be worth trying?

Id say your preload is correct as long as you are on base circle of the cam for that cylinder, I run them in the firing order......TDC #1 then 1/4 turn of the engine set #8 1/4 turn and so on as long as they are standard firing order ?

These were factory assembled plug wires? Or custom assembled??

Thanks