How does a carb work?

[Whitakerla]

I've recently rebuilt my carbs and finally discovered where the high speed jets are on the carbs of my '73 65hp Evinrude. I guess I have a learning desire to understand how everything works that I dismantle and I can't seem to understand how the high speed jets work in a carb compared to the "low speed" jets (if there is such a thing). More importantly, why are there 2 pick up tubes that drop into the bowl and how do they work. So far I've figured out that the high speed jet allows fuel to pass through it(which I assume atmospheric pressure pushes it into the jet) where it is then directed up into one of the pick up tubes, but which one? The big fat one, or the thin tube inside the big fat one and what do each of the tubes do? I understand the venturi concept (learned from youtube), but don't understand the "jets concept" completely. Can someone break it down "barney style" for me? Thanks.

 

[iwombat]

Re: How does a carb work?

Here's a reasonable explanation of how a carb works.

http://www.bikerenews.com/Stories_Archives/Carburetors.html

 

[kfa4303]

Re: How does a carb work?

great link iwombat. you might also want to look of the Bernouli effect, which is the driving force behind carb function. They're essentially a fancy "straw" that you motor sucks fuel and air through. For 2 stoke motors, it also serves as the oiling system as the oil is premixed with the fuel, making proper function even more important. The most often fail due to build up or blockages in the small passages within the carb. Thankfully, rebuilding and cleaning a carb is generally pretty easy and inexpensive. There are often two "jets" or needles as you mentioned. The HI speed needle is used to control fuel flow during start up and high end performance, while the lo speed needle is used to dial in the motors idle. Older model OmC motors had adjustable hi speed needles, but they became fixed around the mid sixties, or so. Once correctly adjusted, thee needles should not need to be adjust again or quite some time. Here's a handy link that can explain the proper way to dial in your carb(s) after a rebuild.

http://forums.iboats.com/showthread.php?t=167352