I have a 40 Merc outboard, 4 cyl, twin carbs. At what approximate rpm do I set the jets? Been a while since I looked at them, are there two, or just one? It is somewhat difficult to set the high end, as you are bouncing along and trying to listen and turn the screwdriver all at once. Which carb do I set first? Or does one carb feed two cylinders, and it doesn't matter, and I just need to get them both close? Do I do it strictly by sound, or do I get them right, then screw them in and count the turns, then back them both back out to the same turns on both?
Steve, Some people think carbs are rocket science, they are not, but there are rules. First, you need someway of determining what your true air/fuel mixture is. You no longer can "read" plugs since the lead has been removed from gasoline The ash deposit, which used to be present is lead bromide and is no longer present. So the tool of choice today is a wide-band oxygen sensor. They take all the guessing out of doing this. If you think you can do this by the seat of your pants, only very few can and if you are asking this question, you are not one of them. The following procedure is essential, no matter how you measure A/F ratio, because of cross influence.
1) Main jet at wide open throttle only
2) Intermediate jet or needle height at mid-range throttle only
3) Last mixture adjustment is the idle mixture at idle only
4) Last carb adjustment is always the accelerator pump, if so equipped These steps apply to all carbs on all types of motors. If however, you have multiple carbs or throttle bodies, you must also balance the air flow between them. This is very important to make certain one cylinder isn't pulling along the other(s). This is either done with a vacuum meter monitoring manifold vacuum or with an air flow venturi gage (my favorite). This is where you adjust the carb linkage for equal flow or vacuum. There are no shortcuts here. You either have the tools and the knowledge or you don't do it. If you get it wrong, you can easily fry a piston. Steve
the jets? Been a while since I looked at them,
you are bouncing along and trying to listen and
feed two cylinders, and it doesn't matter, and I
them right, then screw them in and count the turns,
been removed from gasoline The ash deposit, which used
today is a wide-band oxygen sensor. They take all
your pants, only very few can and if you are asking
no matter how you measure A/F ratio, because of
multiple carbs or throttle bodies, you must also
cylinder isn't pulling along the other(s). This is
venturi gage (my favorite). This is where you
You either have the tools and the knowledge or you
Good post. All I'd add is that it would be surprising to find adjustable high-speed jets on an outboard. Idle mixture and idle-stop screws perhaps though. The image of a nitwit diddling with them at full power on the water is hilarious. It would be even funnier if he gets those or the balance linkage totally out of whack, and then has to pay a black mechanic to make it right.
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