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Old 03-15-2013, 05:55 AM   #7125
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Joined: Aug 2007
Location: Idaho
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Originally Posted by crobox View Post
So I've been thinking more about how carbs work, since your great explanation, Dave. I do have a few more questions....

Regarding the air mixture screw, this regulates the amount of air that gets mixed in with the fuel in the low/pilot jet, right? So it only applies to idle, right? And I guess the more you screw it in, the leaner the idle mix...

And second question... mostly just our of curiosity.... The bottom of the main jet sits a little lower than the bottom of the pilot jet. Are both of these actually submerged in the float bowl fuel? Or is the main jet submerged and the pilot jet just above the level of the fuel?

Just thinking about it all and trying to understand how it works...

Regarding the pilot jet/ slow jet, fuel mixture screw and circuit. As you can see there are several names for it but that is not unknown in the mechanical world. The mixture screw can be either an air or fuel mixture screw depending on where it is placed in the idle circuit. Usually if it is in front of the slide, down wind side or nearer the engine it is a fuel screw. if it is ahead of the slide on the air cleaner side it is an air screw. It works on the same principal as the needle jet/main jet where it limits the flow of fuel or air in the idle circuit. It is also a tapered rod that you either retract or push into a fixed orifice by screwing it in or out. Understanding that concept it is easy to see that if it is an air screw, screwing it in richens the mixture by limiting air mixed with the fuel. The opposite is true with a fuel screw obviously.

The pilot jet mainly affects idle, and is adjustable for best idle purposes. There is a little bleed over effect into the low speed range as with any circuit of the carburetor. It takes care of duties when the main circuit is closed down due to the needle for the main circuit and low venturi effect essentially shutting it if. So for a very brief period of throttle opening the slow circuit helps transition to the main circuit as the needle is pulled out of the main jet. There are other factors involved there too such as slide cutaway which is used to promote venturi action for the main circuit, and some carbs have accelerator pumps too (pumper carbs they are commonly called) for the off idle transition.

Both main and slow jets are submerged in fuel all the time, I do not know why the slow jet is shorter officially but I suspect it is to keep it above the flow of the main jet and out of potential crud in the bottom of the float bowl since it is smaller than the main jet and will plug easier. They could in theory put a slow jet in a place where it could use the main jet circuit as a source of fuel drawing though the main jet and metering the mix at idle with the mixture screw. I can see potential transition issues that way so I think they are separate for a good reason. You do see jet sharing sometimes in CV carbs where they use a rubber plug to seal the passage after you put the jet in and it draws it's fuel then through a larger jet but that is a whole different kind of carburetor.
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