Racing Fuel Systems

Here's my question. At cruising speed say 2400 rpm should a holley carb still be on the idle circuit or the main circuit?
A quadrajet carb at 2400 rpm cruise is on the main circuit.
Would annular boosters having a higher gain be on the main circuit at 2400 rpm?

I've been told this chart isn't the most accurate

While there might be an rpm range involved [ from xxx to yyy rpm ], I doubt that it is solely rpm related. Many other factors involved. Engine size, engine load [ what gear the car is in diff ratio, etc ],towing a trailer, carb size.

2:73 rear gear ratio, turbo 400 auto, 60mph, 2400 rpm, 454BBC, 3900 pound car

This is not a 1-size fits all situation and the chart shown is fairly accurate in the general sense.

rgalajda: GTO Geoff is correct, however you can perform a few tests and determine for yourself. A vacuum gauge, an AFR/lambda gauge and some trial runs will help get you dialed in quickly. A gopro style mini camera mounted above and into the top of the carb (air cleaner removed) would also be very beneficial.

In the short term; remove the air cleaner top and watch the boosters while slowly increasing the engine rpm until the boosters are activated. It will be apparent when fuel is discharging from the boosters. In most cases, annular boosters will activate quicker and stronger than d/l boosters.

In your case; I would guess that the idle, transfer slot, p/v and main circuits are activated to varying degrees at 2400rpm. Does the engine run rough at this rpm?

I used to power brake the car in first gear and raise my hand when the boosters started flowing. We did it mainly to compare main circuit startup to launch rpm in an effort to get rid of a slight stumble. Float level and MAB size seemed to move it too, if I remember correctly. I'd be real interested in hearing if I was totally out to lunch with my findings or my procedure.

This was a 468 BBC and a 400 trans with I think a 4800 stall and a 9375.

3310-1 780 Holley. Carb works excellent.I am not having a problem myself. Just curious why so many comments say you will be running on the idle circuit at cruise.
But the go pro camera sounds like a great experiment. Will try that in the spring.

Just a general discussion.

When I tune carbs (Holley style) on my own engine I observe AFR at steady speeds - 40, 60, 70, 80, 100kp/h as this is relevant to a street and even street/strip deal, the target AFR’s are a bit different. IFR is first to get in the ballpark, main jets are already an educated guess 2-4 sizes lower than factory spec. The effect of changing IFR can be seen all the way to 100kp/h which for my car gearing is 3250rpm. When I settle on an IFR size I then fine tune the lower constant speeds with iab, keeping in mind that they also affect AFR at that 3250rpm. Mains are definitely working at that rpm because mab changes can be observed on the gauge and emulsion to a degree observed but more ‘felt’ with the right foot.

Is the effect of IFR and IAB changes at that kind of cfm through the carby @ 3250 rpm due to addition or subtraction of idle/transfer fuel or the effect it has on the main well or both? Maybe Mike can answer that one?