omc 4.3l idles great but misses/runs very rough above idle.. no clue where to go now?

So i will try to include as much detail as i can as to what ive done so far. I bought an 89 seaswirl sierra classic with a blown up 4.3 motor. Engine is a 94 vortec motor that had been swapped in ten yrs ago or so. The engine had somehow broken a ring in cyl. #2 and the piston melted/came apart. I rebuilt the motor and it idles perfect. Compression test shows 170 across the board, vacuum was at 20 in hg, all seems good. Anything above idle it misses, sounds like running on 5 cylinders almost. I set the timing at 8, put new plug wires, plugs, cap and rotor, had the coil and electronic spark advance checked and they were good.i eliminated the ignition system. I did the test of running boat with filler cap open, no change. Ran it up and dripped extra fuel into the carb while running, no change so i dont think its a lean issue.. it has older gas that had been stabilized but i am going to put new fuel in today, and i am running with no thermostat right now also until i buy one so boat isnt going over 100 degrees..also getting a new water separator filter today.. im stumped as to why the misfire??

Re: omc 4.3l idles great but misses/runs very rough above idle.. no clue where to go

I should also add when im running the boat on land it wont go over 2200 rpm it sounds like theres a limiter.. higher the rpm goes the worse the miss gets.

Re: omc 4.3l idles great but misses/runs very rough above idle.. no clue where to go

Check your carb to be sure it's bolted nice and snug. They like to vibrate loose, developing a vacuum leak you might not notice. Mine ran similarly to yours (not a vortec, but still) and I didn't notice how loose the carb was until fuel actually started to leak from underneath it. What does the exhaust smell/look like?

Re: omc 4.3l idles great but misses/runs very rough above idle.. no clue where to go

duke3478 said:

Check your carb to be sure it's bolted nice and snug. They like to vibrate loose, developing a vacuum leak you might not notice. Mine ran similarly to yours (not a vortec, but still) and I didn't notice how loose the carb was until fuel actually started to leak from underneath it. What does the exhaust smell/look like?

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I checked it and was nice and tight. No leaks around it. I am about to toss in the thermostat now, the local dealer said that since im not even breaking 100 degrees op temp when i should be at 160 thatll be a big part of it probably.

Re: omc 4.3l idles great but misses/runs very rough above idle.. no clue where to go

The exhaust actually smells alittle rich. Definitely not a lean smell. Smells like a car first thing in the morning cold start on a 40 degree day. Hopefully a t stat will help alot

Re: omc 4.3l idles great but misses/runs very rough above idle.. no clue where to go

Well update, put in the new 160 thermostat and cleaned out the fuel water separator, primed it with fresh gas, and then disconnected the fuel line from the tank and hooked it into a gas can i just got of fresh 87 gas, ran it, it got to temp quickly and held there, and free running it on muffs i was able to grt it to turn 2700 now instead of 2200 rpm and the shudder/miss seems alot less up to that point. Not sure what the rev limit is with a 14.5/19 prop pitch but if i tried to go anywhere past 2700 it shook the boat pretty violently. Silly question here for the boat newbie but when on muffs in the driveway, and i run er around 1200 rpm or higher it does somewhat have a vibration throughout the boat.. should it do that since theres no load on the boat and i wouldnt feel it in water? Or should it be smooth?