Re: 5.7 mercruiser 1988 engine runs fine to working temp
"Runs like crap" is pretty vague. We are not there, your best observations and descriptions of what happens and when is all we can work from.
Look at the spark plugs. They will show if it is too rich or too lean. Too rich and I would suspect the choke is working backwards somehow. Too lean and we are looking at fuel delivery (float level, filters clogged, fuel pump output not in spec, air leaks on suction side, yada yada).
Could also be clogged gas tank venting.
Has the mechanic been out on the boat and witnessed the engine's performance at operating temp? It is one thing to set the carb in the shop, quite another to verify the operation at temp.
Observations: Can you smell raw fuel? Does a lot of smoke come out the exhaust? Have you looked at the carb with the spark arrestor off to verify the choke plate is totally vertical?
Does the engine spit back thru the carb? Does it backfire out the exhaust?
Enquiring minds need to know...