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Miss at WOT, I'm stumped

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OldMercMan

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Received a FREE 80 HP 1979 Merc inline 4. Since it was free, and all the wires had rotten away in the 15 years it sat, I did the following:

New Stator, trigger, rectifier, powerhead wiring harness, mercontrol wiring harness, and key switch.

Rebuilt carbs (Welch plugs, both jets, new floats, needle and seat, idle screw).

Pulled powerhead and replaced exhaust baffle and water jacket gaskets, new lower spacer gaskets and water tube rubber sleeve as well.

New water pump and lower unit oil seals and shift shaft bushing/seal.

Link and sync on the water as per the merc service manual. Fantastic hole shot, forward idle at 600 rpms.

Ran like a dream, except, approaching 5200 rpms it starts to die. Great right up to 5,199 rpms, crossing to 5200 it loses power and, if I don't throttle back, it will fall right down to 2000 rpms. If I throttle back and give it 2 seconds, I can get right back up to 5200, then it starts to drop rpms and die again. WOT should be 5200-5500.

I took a stab in the dark and replaced the power pack, plug wires and all four coils. No better approaching 5200 rpms.

Recomfirmed today that max spark on the water is 27 BTDC. Plugs are slightly oily. Even did a full throttle kill, pulled plugs on the water, plugs slightly oily, no sign of water.

Im at my wits end, what am I missing???
 
You MIGHT have too much spark advance--27 degrees seems a bit high with today's crappy fuel. I'd try a few less degres.

Recommend installing Autolite 303 spark plugs (that you can read) and try a WOT color test. To wit: Run it a while to get the plugs 'broken in', then run WOT for at least 10 seconds. Shut it right down--no idling allowed--and pull the plugs. You're looking for a medium brown color.

Jeff
 
fastjeff said:
You MIGHT have too much spark advance--27 degrees seems a bit high with today's crappy fuel. I'd try a few less degres.

Recommend installing Autolite 303 spark plugs (that you can read) and try a WOT color test. To wit: Run it a while to get the plugs 'broken in', then run WOT for at least 10 seconds. Shut it right down--no idling allowed--and pull the plugs. You're looking for a medium brown color.

Jeff
Click to expand...

We did try 25 degrees yesterday, but it didn't improve anything so we put it back to 27. I have only run shell 93, but that still has ethanol in it. If you leave the throttle at wide open - when it begins to slow/die it does pulsate/lurch a bit. I'm leaning toward a fuel restriction - going to increase line size after fuel pump and remove those tiny screens at the carburetors.
 
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