Reply To: 3.6 HP Mercury carb questions

[David Stanton]

FYI: I learned some more things about the carb that I’m mentioning in case any of you ever have the misfortune of having to work on one of these:

1. The choke (which is a small piston that moves to open or close a small port to the carb bowl gas) will not work unless the throttle piston (yes a vertical piston in the carb throat instead of a butterfly valve) is at minimum setting – and the word “start” on the case opening by the throttle lever puts it there. If the throttle is open farther, there is not enough suction at the choke gas port to draw gas through the choke-piston port. So the throttle has to be near minimum or you will pull and pull and pull on a cold motor!

2. The link from the carb throttle to the motor’s throttle lever (the vertical threaded rod out the top of the carb) has to be adjusted so the piston in the carb throat just gets to the top of the throat (max open) at WOT, and not a bit farther. First time I put it on a boat and went to WOT, the throttle piston went up a bit too far and jammed, and, although the motor slowed down some with the throttle lever, that was due to retarding the spark, and not backing off the carb throttle. Adjusting so it just barely gets to the top of the carb throat at WOT cured that problem.

3. It has high and slow speed, spring-loaded to keep in place, mixture screws; but the high speed screw appears to be no more than a jet that adds a slight bit of fuel to that already provided by a fixed jet that hangs in the carb bowl, so you can turn that adjustment several turns, including all the way in (lean-off) and the motor still runs pretty good. It was subtle, but between 2 and 4 turns out, it did run slightly faster. The slow speed mixture screw was similarly not very responsive either, but if it got adjusted too lean, it did eventually cough at idle. On the good side of all this is the fact that the motor ran pretty good at just about any setting of those mixture screws.

Now that I’ve learned a bit about this motor, I hope I never see one again!
Dave