Mercury 4HP 4-Stroke Running problems


New member
23 Jan 2005
Can anyone help? I have a Mercury 4HP 4-Stroke 1999 vintage which is not running properly.. I bought it in the summer 2nd hand with a boat and seems hardly used (seller said 10 hrs and it looks like it). The snag is it won't run without choke and even then the idle is too fast and it isn't giving full power. Compression & fuel flow to the carb are fine and I have a new spark plug and oil. Having tinkered with the carb I can get it to run nicely at idle with one of the 2 'brass' screws on the carb body open about 1/2 turn and the choke now home- this is not the main jet (it is flat ended and has a hole drilled through the shank) and it there is no spring/ friction to set this screw to anything other than fully home. Anyway, even with this screw open (held with screwdriver & the choke now home) I get even less revs than before. Whilst I am here what setting should the main jet be- its currently 1 1/2 open.


9 Oct 2003

Sounds like the idle jet is blocked or the float in the carb is stuck. Which will cause all the problems you have described. The only way to cure this is to take the carb
of the engine and remove the float chamber (4 screws) then clean the idle jet and check the float is moving ok. A tin of carb cleaner will also be useful.

Andrew /forums/images/graemlins/cool.gif