Ok, I have a 29 Mach 1 with a carbureted (93) 502 stock everything except the carburetor which is a 800cfm Holley 4011. The Holley 4011 is not your standard holly configuration but this setup has run flawlessly for the last 19 years so I have no intentions on swapping out the carb. And by flawlessly I mean that it started every time without ever having to pump the gas even once, idled great and ran out without a glitch.

The problem that I am having is that a couple of weeks ago my boat started to run rough and stall when idling or coming off of plane so I did a minor tune up by replacing cap, rotor, magnetic pick up, plugs, wires, fuel separator, oil/filter and at the same time my carb was taken off and cleansed with carb cleaner and then bolted back on with no changes other than getting cleaned up.

Last weekend down at the Perry party she let me down again by stalling out whenever I was trying to dock all while blowing black smoke out the back, it looked like my diesel truck while pulling the boat and passing other cars.

What do I need to look at and where to start?

sounds like it is time to rebuild that Holley carb of yours.. 19 years old, i'm sure the internals are shot

Yep, time to clean/rebuild the carb. Ethonal gas is a BIG problem with boats. I had to rebuild a holley carb on the dock last year. Your floats are most likely sticking causing way to much fuel in the carb at idle. At 1/4 to 1/2 throttle it probably runs better cause there is enough air to mix with the amount of fuel in the carb.

Until two weeks ago it never let me down and the only thing its doing now is just idol problems. I don’t know and that is why I am asking but wouldn’t there just be a single part someplace that has failed and not the whole carb? The carb has always been maintained by professional shops until I bought the boat 3 years ago. I would go to one of those shops now but where I live I just don’t trust the boat shops.

It's not that the carb itself has failed just that the internal moving parts are most likely gummed up from years and years of normal operation. As mentioned ethanol isn't the most friendly fuel out there. If you are not comfortable with the local boat shops then take your carb off and have it rebuilt by an auto shop of your choice. Carb kits are relatively cheap.

Do you have power valves in the carb?Some carbs have only one in the primary metering block, some have two, both bowls.If blown, it would let too much fuel dumped in causing black smoke indicating over rich condition and causing bad idle. Cheap to replace before investing in a new carb.While you have the metering block off to check for a bad float the PV can be inspected as well.

I honestly don’t know what it looks like inside because its not your normal Holley carb I didn’t want to pull the top off until I could get a new gasket set. This Holley doesn’t have the normal bowls and mitering plates hanging off of both sides, instead it has a lid that comes off of the top.

it sounds like a needel&seat problem,it is true that ethanol is hard on them.

it is a Holley that looks like the old autolites. one big long casting with removable top cover. whole idea is no bowl gaskets below fuel level. internally is is essentially the same as standard holleys. no metering plate per say. power valve is in bottom. tear it down, soak it, blow out all passages. get a power valve. kits are kinda pricey for that one as it is scarce. about 50 bucks apiece...now i gotta go look at my torn down spare in garage. this was from memory. edit; the power valve is accessed from under the carb. little housing next to (and looks like) the accel pump housing. 1 on each end. valve housing for rear barrells was empty-passages not drilled. T-20 bit for the screws. both gaskets survived when i pulled them. you might be ok with cleaning out idle passages and replacing the power valve. power valve doesn't come with most kits anyway. nother edit; be sure it is a 4011. mine are 4010's. diff is 4010 is holley square bore pattern. 4011 is q-jet spread bore. the top cover is fatter at back barrellson the spread bore. gaskets don't interchange. but both are for sale on ebay cheap. seen some 4011 gaskets 3 bucks and change each...in fact search 4010 and 4011 holley on ebay and look at the pics. will tell you a lot.

Thank you guys for the help!