The black plug could be water from a leak in the cooling system into the combustion chamber. A leak down tester can help, and it will check the crank seal condition. The right side is good for spooge. You can not look at a float valve and assume its fine. You do not even have a guarantee by leaving the bowl off and pushing the floats up, that it will work correctly. Just replace it yearly, or when you are having screwy problems. And stay with oem parts, keep it simple, you do not need more variables like crappier parts. An 89, does it have a power jet also? And for pete's sake, dropping the oil ratio to 40 is effectively increased the amount of gas! You made it richer yet? Premium fuel, factory oil ratio, factory jetting, replace the float valve, have a leak down test done.