Engine Chugging

I have a 1990 Ski Nautique. Last year it would almost stall coming off the start and had the accelerator pump changed. Works great this year off the start but every second or third time out in the boat it will really chug at low speeds (10-25 mph).....but it is hit and miss....sometimes it runs great all the time and seems to run well at higher speeds. Any suggestions?

It is similar to when you first start driving a standard car and if you let the clutch out to quickly the vehicle chugs......same feeling in my boat where it gets power then loses power then gets it and loses it but very quickly in between gaining and losing power so it is "chugging". Chug is large enough that passengers and myself are physically beig moved by the chugging. Just was out skiing my son and it started great (he is only 9 so I ski him at about 24 mph) and then half way through (after about 15 minutes) started doing it again. Then I skied at about 34 mph and was generally fine.......could it be a spark plug in cylinder 1-4?

The carb was done this year (Bought a carb kit and local boat store did it) but the ignition has never been tuned up. Is this a big job?

Re the plugs thanks for the tip! I know there is 8 plugs but 5-8 were apparently new plugs when I bought the boat and 1-4 were a couple of years old.bwas goon to change them anyways but didn't think this was what causing the issue.

I am not sure specifically what the shop did with the carb and whether they did all the things you note below. I don't have a timing light or dwell meter but I could get one. Yes it does feel like it is running rich and occossionally when i turn the engine off and then go to restart it is like it is flooded and need start it like a cold start. Re the chugging I don't do anything to clear it currently......is there something I could do to get me through the next week?

No, but will......is there anything that I can do to clear the chugging when it occurs to get me through the next week or two?

When it is chugging and I stop and turn off engine and then restart (where I have to get it going like a cold start) the exhaust has been black. If I look for fuel dripping and if there is anything I should do?