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Discussion Starter · #1 ·
I'm going to be rebuilding a carb later this week, Never really messed with carbs so its all pretty new to me. I've done a search and read my manuals and found out how to replace parts and set the carb up. But what i want to know is what and why am i adjusting these things and what is the carb doing when i adjust the screws.
Idle mixture screw and the Bypass air screw.
I take it these two screws somehow control the fuel/air flow. But i can't find info on whats happening ? I mean for ex - If i turn the idle mixture screw in, is that allowing more air and less fuel ?
Sorry to be a pain but i just want a little understanding of what I'm doing and why.
 

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On older carbs the tickover (or idle speed) was set by propping the throttle slightly open with a stop screw.

On the bypass carbs the throttle stop screw is set so the throttle is completely shut. The stop screw is there to prevent the throttle butterfly from closing too far and getting wedged in the throttle body.

So to answer your question - the small mixture screw increases the amount of petrol at idle (richens the mixture) as you unscrew it.

The larger air bypass screw increases the amout of mixture you are letting into the engine as you unscrew it (speeds up the idle speed) with the throttle shut.
 

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For about 8 or 9 days, until last night, I was figuring this out myself!
I got to the point where I just wanted to follow the instructions and make things work, and was willing just to accept that they do without knowing how.
The little one is called the volume control screw. The big one is called the bypass screw. The bypass screw controls how much air gets into the carb's idle circuit. The volume control screw controls how much fuel gets into the carb's idle circuit. So you are going to be using these two screws to set your idle mixture and idle speed.

Have you see this?:
http://type2.com/bartnik/carb.htm
 

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Discussion Starter · #4 ·
For about 8 or 9 days, until last night, I was figuring this out myself!
I got to the point where I just wanted to follow the instructions and make things work, and was willing just to accept that they do without knowing how.
The little one is called the volume control screw. The big one is called the bypass screw. The bypass screw controls how much air gets into the carb's idle circuit. The volume control screw controls how much fuel gets into the carb's idle circuit. So you are going to be using these two screws to set your idle mixture and idle speed.

Have you see this?:
http://type2.com/bartnik/carb.htm
Thanks, Just read the link and its very helpful. Keeps refering to the Bentley manual tho and i don't have one so thats next on the want list. I've got an understanding tho now of whats going on.
 
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