9.9 2 Cycle coughs

Hi

I have a 2000 9.9 2 cycle. It's previously run absolutely fine but has now developed an annoying cough. It starts perfectly and when cold will cough at idle, it's a bit like a sneeze, running fine and then ah-choo! Once warm it seems to idle fine. However when in gear (forward or reverse) at idle it does it again. My guess is that when warm at idle RPM it's OK but when the revs drop slightly by being in gear the problem appears. Most of the time the motor continues to run but occasionally it will stop and needs a pull to restart. First thought was maybe a gummed up carb so I tried Sea Foam. Lots of smoke as advertised but no change in behavior. New plugs fitted - no difference. I took it to a dealer who said he couldn't find anything wrong which led me to suspect it might be fuel as he was using his fuel and his tank. So I put it back on the boat with fresh fuel, and for good measure, a new tank. No difference. I'm about to also replace the fuel line, however I'm pretty sure that will make no difference. My logic goes like this. It runs fine when opened up under load so it's not a problem caused by lack of fuel (e.g. an air leak or constriction in the line).

It's on a 25 foot sailboat and the danger is that as I'm approaching the dock I'll go into neutral ready to reverse and it'll cough and die at the worst moment. Anyone have any experience of this type of behavior and the cure?

Has it been worked on lately or has anyone adjusted the carb. If not the carb needs to be rebuilt and cleaned I think you have a partially plugged low speed jet thats causing the lean sneeze.

Thanks for that. As I said, it's been in to a Mercury dealer however apart from their raising a bill for over $200 I've no way of knowing what if anything they actually did (as opposed to what the bill says they did) because the motor still behaved the same when I got it back from them. Unfortunately shortly after it was refitted on the boat I went abroad and while I was away someone else had a loss of control leaving the dock and got swept down on to my stern, breaking the swivel bracket so I wasn't able to take it back to the dealer. I've only just finished repairing it. I'll pull the carb and take a look. Thanks for the suggestion.

Otherwise try opening the low speed jet about 1/8-1/4 turn more open. There is a hole in front of the air silencer might have a plug in it for a small screwdriver the jet is behind that.

Thanks for the suggestions. Just got back from the marina. A combination of setting up per the book (slow running speed and mixture screws) and a bit of judicious tweaking of the timing advance screws got me to a position where everything ran sweet while tied to the dock. I'll try it on the move at the weekend. I think the timing may have been a bit too far advanced.