1994 merc 4.3L no comp in 3 and 4, does not want to idle but runs great above 1000 rpm ?

94 Rinker 280 fiesta vee, starboard 4.3L engine slowly lost power at 4000 rpm, took two weeks to get it fired up again, finally jiggled throttle controls and runs great at 1000 rpm or higher. Syncs and planes perfectly. Mechanic shows no compression first in 2,3 and 4. Now just in 3 and 4.
Old spark plugs were wrong for the motor and 2,3, and 4 plugs were burnt. All 6 new plugs look great after running. All fluids, temp, etc are fine.
Can just 3 and 4 not be running and boat still plane out and sync up at same rpm and run straight as an arrow (from 1000 up to 4000 rpm) now ??
Any ideas would be helpful.

Re: 94 merc 4.3L no comp in 3 and 4, does not want to idle but runs great above 1000 rpm ?

no compression, usually means valves, maybe head gasket, got water in the oil ?

Motor overheating ?

I don't know about the sync question cause i have seen motors run great for 20 years with no compression on one cyl.

See i think you got them sucked in and one popped back out.

Really only one way to tell pull the heads.

Re: 94 merc 4.3L no comp in 3 and 4, does not want to idle but runs great above 1000 rpm ?

Thanks for the quick reply.
No overheating, no water in the oil.
The old plugs were wrong for the engine and they were burnt in the bad cylinders.
New plugs installed look great after running engine for 2 hrs.

Re: 94 merc 4.3L no comp in 3 and 4, does not want to idle but runs great above 1000 rpm ?

...4.3L no comp in 3 and 4...

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That would mean both head gaskets or an intake manifold warped or gasket blown. Spray some carb cleaner around the intake manifold; if the engine idle smooths or RPMs increase then the gasket or intake is defective.

Re: 94 merc 4.3L no comp in 3 and 4, does not want to idle but runs great above 1000 rpm ?

starboard 4.3L engine slowly lost power at 4000 rpm, Old spark plugs were wrong for the motor and 2,3, and 4 plugs were burnt.

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Ayuh,.... My Guess is,...
There's Crud in the fuel filters causing a Lean condition...
You didn't deal with it at the begining, instead runnin' it...

Now you've Detonated the motor to somewhere between Broken, 'n Anchor status...

With No compression in some cylinders,...
I'd pull the motor,... It's easier to work on, on a stand, rather than in the boat...