1993 Johnson 521cc rough start, rough idle, keeps coughing

Hello!

I have a 1993 model Johnson 521cc. It's the 20hp model, but carb has been changed to a 30 hp one.
After last winter I've got some starting problems..
starts actually easy on 3rd or 4th pull, but then it coughs and stops. Another pull, same happens.. this keeps happening a few times, then it starts to run on idle.

Full throttle runs fine, pushes boat fine, when slowing to lower rpms, doesn't want to run.. it doesn't shut down but almost seems to do it.. like at times it's not getting enough fuel.

So where to start to fix this? I'm not a mechanic but can do most basic things.. these engines aren't that complex.

did it ever run good with this carb?is the gas new?run some seafoam thru it...you can do this a couple of ways..stick you gas feed to the carb into the can and let it suck enough in to kill the motor...let it set overnight and connect the gas hose back up normally and start the engine..let it smoke til it gets rid of the seafoam...the other way is to mix the seafoam liberally into your gas and run a few gallons and see if it improves...

is it sync'd correctly? maybe rebuild the carb...

About sync: how do you check / adjust it? It run good last summer with this carb.. maybe I'll give this seafoam a try. Gas is new, there's was a few litres of old gas left, but i've used all that by now. Over winter I had put some winter protection oil through carb in to engine, and burn't away all gas from hoses etc, so there wasn't gas in the engine over winter.