1977 Mercruiser 233 suddenly running rough, frequent stalls

Hi Everyone,

I'm a brand spanking new boat owner as of two and a half weeks ago and so far I have heard the "bust out another..." more times than I can count, haha.

I purchased a 1977 Sea Ray SRV 220 CC from a friend of a friend. The boat has been very well maintained for its age. It hadn't been run in 2016 so when I brought it home I had a little trouble starting it but once I got it started it ran like a champ. It was smooth, held steady RMPs, and was quieter than I expected. Everything was great.

Other than being tough to start when it was cold, it ran without issue for the last two weeks but suddenly, Saturday night, the engine started running a little rough and eventually died. It left me stranded on the lake in the dark. No better time to get to know the coast guard, right?

I got it back in my driveway the next day and started tearing into the motor. I cleaned the points and adjust the gap, adjusted the timing, and completely disassembled the carburetor and cleaned every little hole, nook, and cranny I could find. I put it all back together last night and fired it up. The motor starts reliably every single time now, which is great, but it still runs rough. It stalls during idle, the RPMs bounce around, and it peters out when going high RPMs. I have adjusted the screws in the carb in and out and it doesn't seem to make much difference, other than killing it. I can see fuel being sprayed onto the lower butterfly valve (throttle?). Opening and closing the upper butterfly valve doesn't seem to do much either, other than killing the engine if I close it all the way.

Do you have any other suggestions I can try? I'm not an expert mechanic but I know enough to make sense of what I'm looking at if it's explained to me.

1977 Sea Ray SRV 220 CC
Mercruiser 233 V8 (Ford 351 Windsor)
Rochester 2-barrel Carburetor

Engine
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Engine Tag
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Points
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Carburetor Linkages
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Carburetor Top Down
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Carburetor Top Down
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Opening and closing the upper butterfly valve doesn't seem to do much either, other than killing the engine if I close it all the way.
That would be the choke plate and will kill the engine closing it. The idle mixture screws are just that ,mixture screws. too tight and the motor stall from being lean, too wide ,it runs rich at idle speed.Adjust them for maximun rpm at idle .

start with a compression test on all cylinders,followed by a spark test

Did you run the boat with old gas???? IFSO, dump the fuel filter into a jar.... gas should be clear... Wait 20 mins and see if it stratifies. IF it does... water in the gas. Reinstall filter and see if all ok. IFSO.. get a new filter. Unless you ran a tight fitting wire thru all the jets, its not clean. Air hose and solvent not good enough these post ethanol days.