Booster Signal

[JETFAST]

I drilled a main well bout 5% looking for fuel one time... And flow dropped, loss of hp and tq with no other changes. AD 4150.

It's all about the multiplication factor's.

[rgalajda]

Below are some pictures of booster modifications I did to a Holley 3310-1 carb I run on my 454. These modifications were noted by David Vizard.

“Removing a little metal from the booster’s inner form ( sharpening the booster ID edge ) produces an increase in signal more than what you would expect . It does so because the sharp edge causes more air to go through the booster rather than be deflected around the outer diameter. A little detailing can result in a 10-20 percent increase in booster signal."
Next I converted the downleg booster to a stepped downleg booster.
“ The step does little to to increase the booster signal , but it does bring about better fuel shearing at the discharge point , thus bringing about better fuel atomization. "

[Right hand drive]

[rgalajda]

[rgalajda]

These are the tools I used with a variable speed drill . Thats all. 1/2" cylindrical end cut bur bit.

[Right hand drive]

[GTO Geoff]

Not all carbs have the idle cct taking fuel from the main well [ jet ] as Shrinker describes.
Some Weber models, & the Competition Series Thermoquads take idle fuel directly from the float bowl. There may be others.

[Right hand drive]

[HarrysTaxi2]

[rgalajda]

A lot of theory but not much answers. I’ll add to this and if any one wants to correct me than just pipe in. I’m asuming he is not talking about building a custom carb.



“What happens is the manifold vacuum (the horrible cam combos etc) requires a certain response from the idle and transfer circuit. Now the idle and transfer is fed from the main jet. The first thing you have to achieve is idle then transfer function. A very big part of that is ignition timing and strength. If the timing especially is not optimal it is necessary to supply incorrect inefficient amounts of fuel. Now after you have sorted out the timing and fuel for the low load stuff you have to introduce the main booster correctly. If we think about this a bit we realize that the fuel exiting the booster has to happen at a rate that matched the rate of slowing of the transfer and idle discharges. Now the fuel discharging the idle hole does so in response to the vacuum at the hole, thats pretty simple concept but the vacuum of the engine is dependent upon the combustion efficiency and that varies with principally the timing and the fuel supply for what we are talking about. OK so now we don't know how much fuel is coming out the idle hole at say 1500 rpm? Now move to the transfer circuit. Here we have a fuel supply that is sharing the same jet etc as the idle but the vacuum at the outlet is more sensitive to the butterfly. When the butterfly moves past the slot its obvious that its limiting the fuel exit area underneath the slot and the air leakage area above the slot. So there is effects happening there. Then when it moves totally above the slot its exposing the slot to a airspeed/vacuum condition. The mains have most likely not started yet. Thats important to see. “

Timing. Minimum fuel consumption ( good vacuum ) at idle usually requires manifold vacuum pulling in about 20 degrees more of additional timing. This was common in the 1960’s before emission standards. The optimum idle advance is typically about 30 degrees for a short cammed ( performance ) street engine and as much as 50 degrees for street/strip engine.
I personally run 12 degrees initial plus 20 degrees manifold vacuum for a total of 32 degrees idle timing in gear.

Idle Mixture. All idle mixture issues add up to the fact that as manifold vacuum decreases , the mixture ratio needed gets richer and the total amount of air required for best idle increases.Idle air/fuel required can range from 14.5:1 to 13:1 or 12.5:1.

4 corner idle. For a short cammed street machine , the idle circuit of a Holley style carb need only be on the primary side of a 4 barrel.
This works fine when there is plenty of vacuum ( 12 or more inches ) Anything that reduces idle vacuum further ( such as a bigger cam/tight lsa ) may require 4 corner idle. I have run a Quadrajet,, a carter AFB , a 2 corner idle Holley , and a 4 corner idle Holley with 12 inches vacuum in gear. Now the Carter 750 AFB was way off when first installed , requiring the idle jet to be enlarged and the Idle air bleed reduced to get a 750 rpm smooth idle in gear. Thats with a 227@.050 cam and a tight 107 LSA on a 454 BBC.

“The important point is that the IFR jet ROBS the fuel from the MAIN JET. There is the conundrum. Alter the IFR fuel flow rate by any means(ign timing, exhaust, plenum spacers etc), Do anything really and you have stuffed your mains calibration. Start again buddy. “

I find this vastly over stated . First of all , you calibrate the idle/off idle system first, then move on to the main jet. So there is no reason to start over again. Now of course using a AFR meter is almost mandatory these days. If you can afford to build a custom engine than you can afford a Wideband AFR Gauge for $300.

“So e-bleeds need to be moved up, down, changed size etc. And of course all the other tuning issues change as well, its never ending. “

If you chose the proper carb for the situation in the first place , then e-bleeds will not likely have to be dealt with.

“Its not the carburetors that are faulty its usually the combos. There is different ideal applications for different brands etc because of their directional philosophy.”

This statement is true.