Author Topic: Do large main air bleeds delay the main system?  (Read 1995 times)

Offline novadude

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Do large main air bleeds delay the main system?
« on: November 04, 2013, 08:41:29 AM »
On a single main air bleed carb, does increasing the size of the main air-bleed delay the main system operation?  How much does main air bleed size affect the "cut in" point of the main system.

Just asking because I was under the hood watching the boosters whild working the throttle.  I was a bit surprised that, in neutral, the main system doesn't really come in until ~1800-2000 rpm on my carb.  There are no "holes" so I guess the transition slots / idle system are providing enough fuel until the main system comes in.  Also, I'm sure the main system would come in at a lower RPM under load when the throttle angle was greater.   

Carb works fine - .080 MAB, .050 idle bypass air.  Throttle plates seem to be in the "right" position at curb idle, as it doesn't take much  throttle movement at all to start seeing vacuum on the "ported" line going to the distributor. 

With all this talk about nozzle drip, I would have thought the main system would have started moving fuel sooner.  That got me wondering if you had a carb with "nozzle drip" if you could adjust the MABs bigger, and up the jetting to maintain proper fuel curve.  Would this delay the "cut-in" point of the main system?  As an example, going from .050 MAB and 73 jet to .080 MAB and 76 jet (SMAB carb).

Offline Cliff Ruggles

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Re: Do large main air bleeds delay the main system?
« Reply #1 on: November 05, 2013, 03:37:24 AM »
Throttle angle is significantly difference when the engine sees load, so fuel will come in much sooner than a static test in neutral or park.  The main airbleeds in the SMAB units are indirect, so they wouldn't have much effect on when the system comes in, or one wouldn't think so.  They do have an impact on fuel delivery from the jets with any given jet/rod combination, but those models aren't super-sensitive to metering changes like the earlier designs....Cliff