tube size/idle feed restriction/number of turns on idle screws: rule of thumb ?

[Frank400]

   I'm not chasing a problem here.  I've done two complete carb rebuilds with Cliff's parts and following Cliff's instructions and book.  Awesome results in both cases, one being a stock older motor and the other a mild performance motor.  I was just wondering if there is some rule of thumb like if you're more than x turns out with the idle mixture screws, maybe you should increase the size of the idle tubes and idle feed restricitions ? 
  Also, then deciding to go a little bigger on those two (tube and IFR), should one go bigger first let's say with the IFR and then if more fuel needed, going bigger with the tube size ?  Just trying to learn more and more, I just love that kind of info.  Frank.

[Cliff Ruggles]

The type of idle mixture screw plays a big roler here as well.  Many later emission carbs have long tips with a gentle taper.  1979 and later carbs have fine pitch metric threads as well.

It's difficult to get a lot of fuel into the engine with either set-up without backing out the mixture screws considerably.

When opening up idle tubes and down channel restrictions, keep in mind that the idle airbleeds also have a MAJOR impact in how hard the engine vacuum is pulling on them when the throttle plates are closed.

One must also consider that the idle fuel system feeds the main system as throttle angle is increased.  It does this thru the transfer slots, so be conservative with idle tubes and down channels when setting up your carburetor. 

Adding a lot of idle fuel to get control with minimum turns on the mixture screws may make the carb too rich right off idle.......Cliff

[Frank400]

 

Adding a lot of idle fuel to get control with minimum turns on the mixture screws may make the carb too rich right off idle.......Cliff

   so do I understand that off idle, fuel is supplied by the idle system AND the main system ?  If so, that makes sense on being conservative on the idle system.  Thanks again for the great info.  Frank.

[Cliff Ruggles]

No, the idle system continues to add fuel right off idle via the idle transfer slots.  The main system comes into play early in the game, but not at idle speed, and very low throttle angles........Cliff