40 horse merc cutting out at idle

[baldav]

Despite catching a 22' smallie, last Saturday was a frustrating day. In addition to my boat having a small leak and my starting battery loosing its charge, my Merc was cutting out at idle speed. It operates fine in WOT, but while pulling back trollin and shifting in between neutral and forward the engine would cut out. At first it happened every 5 minuets are so, but then frequency picked up to cutting out nearly once a minute. Other then this new phenomenon the engine works fine. Any ideas on what is causing this and how it might be fixed? Thanks in advance for your help!

[Valv]

I would start with replacing new plugs and fuel filter.

When is the last time you replaced water pump impeller, checked compression and had carbs cleaned ?

[eurolarva]

Like Valv said. I would bet it is the low speed circuit of the carb is gummed or has a bit of grit in it. Not sure on a merc but there is probably a mixture adjust screw. Typically there are small holes inside the carb that is part of that system that a hole or more can plug up and give you the problems your are experiencing. What is the model and serial number for the motor?

Check out part number 24. This is typically a certian amout of turns from clockwise like 1 1/4 turn or so. Part 33 is a welsh plug that covers the holes I mentioned. Turn part 24 clockwise till it lightly seats counting the turns. Then unscrew is all the way out counterclockwise. Shoot some carb cleaner in there with a can of carb cleaner with a straw. Put the screw back in and reset it the way it was before and see if that improves it.