Let's assume your engine is in perfect operating condition, and you are convinced your carb(s) need some attention. FIRST, what exactly is the problem? No idle?, Bog?, or topend/total stall? Or a combination of these? SECOND, Did it start all of a sudden, or gradually get worse? Or, has it always been this way? You need to think about this!! If it started all of a sudden, most likely you have a mechanical change caused by part failure or contanimation somewhere in the fuel delivery circuit. A gradual change is commonly caused part wear, and/or gradual contamination or corrosion. In the "always" been this way category... most often is caused by carb(s) which were changed by someone (not knowing what they were doing), carb(s) too big for the engine application, and lastly... were never set-up for your particular application! This, and carbs too big is the single most touublesome to work with! Too much carb is too much carb! And all you can do is replace them with smaller ones. All the re-jetting and tuning in the world WILL NOT change your VE rating!! VE is "Voulme Efficiency", that is... CFM vs. CUBIC INCHES over RPM ranges. Yes, you can mess around and maybe get a little more out, but it will NEVER be correct! It is much much much  better to be under carburatorated than over carburatorated! So if you fall into the OVER carburatored category... you are on your own...

Let's talk about ALL basic carb functions for a moment! All carburators are vacuum dependant, and all have three basic functions IDLE circuit, TRANSITION circuit, and BOOSTER circuit. And, they all attempt to deliver approximately 14.7 pounds of air per 1 pound of fuel at sea level. No matter what brand of carbs you have...  each carb manufacturer has their own way of handling the IDLE/TRANSITION/BOOSTER circuits. But they ALL do EXACTLY the same job!

Let's look at these circuits...

IDLE circuit: All it does is idle your engine.
This is the MOST complex circuit, and the most vulnerable to blockage of air/fuel passages. It is also very sensitive to air leaks from gaskets and worn out parts.The Idle circuit also gets tampered  by people, more than any other circuit!  If your engine is idling correct, leave it alone!

TRANSITION circuit: This is yor accellerator pump assembly & related components

It is the most mis-adjusted, and mis-understood circuit of all. And, it is made worse by "want-to-be-carb techs". On this circiut, you need to do the OPPOSITE of what you may "think" you need. I will explain this later in great detail!

BOOSTER circuit: It is your off idle and full power air/fuel mixture to your engine.
These are found in the center of each barrel, located at the peak of each venturi. The fuel flow to these come from both "main" jets and your power (booster) valves. These are also heavily tampered with by  "want-to-be-carb techs". And, really screw up a potentially good running carb. 

So, if you do not have a thourogh understanding of how a carburator functions... it is very likely the carb(s) will never operate as you would like, especially if you start tampering with it or them. 

Look at the illustration for a moment...

You will notice that the accellerator pump overlaps both the IDLE and BOOSTER circuits for a brief period! This is extremely critical for a proper idle to booster transition. Let's think about what's goung on here. While idling you are pulling good vacuum, while crusing your are pulling good vacuum, but when accellerating... your vacuum drops dramatically! In some cases, the vacuum will drop to absolute ZERO! Now remember... I told you that a carb is vacuum dependant! So in essence... your carbs are NOT functioning!!! That's right.. no vacuum... no carb!!! So it is the job of the accellerator pump(Transition Circuit) to keep fuel flowing long enough for the engine to develope vacuum to activate the boosters. Yes, it's that simple! However, in the real world it's tough to get these circuits to work together like they are supposed to!

Let's talk symptoms now!

If an engine idles OK and it bogs when you attempt to accellerate, but does eventually accellerate... what is causing the BOG?   Look at the chart, my guess is... the accellerator pump is either not shooting long enough, or perhaps shooting TOO long or possibly too much! You can tell by observing the exhaust while accellerating. If it is blowing black smoke you have either a too long shot or too much fuel! On the other hand, if there is no black smoke the accellerator shot is most likely too short and/or too lean. You could also have a linkage mis-match or cam mis-match on the accellerator pump. Another cause of bogging is TOO MUCH CARB!!! Here is what happens:  your engine cubic inches does not have enough VOULME to restore the vacuum signal at the carb(s), thus your boosters take forever(if ever) to kick in. Now, if you are slightly over-carbed you can compensate with either a longer accellerator pump shot and/or more fuel shot. If you are grossly over-carbed you will NEVER get the needed vacuum signal to the boosters... end of story(unless you are cranking 10,000 RPM)! So again, go easy on your carb selection. Make your carb selection to have a VE value as close to 100% as possible for the type of driving you do. In the REAL world, you can expect a VE of about 80% for street driven vehicles! And, a VE of 95% or better for drag racing. SEE: Technical Page "Two Fours and Tunnel Rams" for more information.

OK let's talk Boosters & MAIN JETS...

I do not recommend novices messing with main jetting. 99.9% of the time, the factory jets are a perfect match for the CFM of the carb. Changing these without a thorough understanding of the application and function will result in all kinds of problems! Mostly, a terrible running engine. Leave factory jets in until it has been determined (by thorough anaylsis)that  re-jetting is needed for optimum performance.

Idle Time!

Your carb(s) contain an elaborate labrinth of air and fuel ports plus some re-directing circuits for air flow. Most carbs will allow adjustment for either AIR or FUEL but not both! You need to know which one your carb allows and adjust it accordingly. A vacuum gauge attached to one of the vacuum ports on the base of the carb is needed for setting proper idle mixture. You will tune each side if the idle circuit for maximum vacuum, then adjust the idle speed with the idle stop screw. If adjusters (usually a screw) have little or no effect on the vacuum then it's time for a carb overhaul, or possibly adjust the stop on the SECONDARY barrels(on Holley carbs).

FIRST things FIRST. To check out your carbs!

1) Fuel pressure: most carbs need 5 to 7 pounds pressure

2) Float level on both primary and secondary barrels

3) Accellerator pump linkage, pump assembly, cam, and shooters

4) Air leaks, fuel leaks

OK, now for you dual carb folks having fits with them.

You will find it to be to your advantage to tune EACH carb separately. You do this be disconnecting the throttle linkage to one of the carbs and closing the idle stop 100% Now you can test, tune, and try it out without having to guess which carb is messing up. And trust me... your hotrod will run great with a carb shut off! In fact... a lot of people just leave the second carb shut down permently and take advantage of the single carb reliability and keep the looks of the dual carb setup! This is something you may want to consider...

There are several ways to setup multiple carbs. The easiest way is the have ONE carb control the idle and the others only boost when needed. This is typical on progressive linkage three deuces and dual quads. The other option is 1 to 1 linkage with each carb having an idle circuit functioning. This setup takes a lot of work to synchronize as the carbs will tend to work against each other. Remember, these are vacuum dependant and all the carbs will want to "hog" the vacuum signal. If you can setup your carbs  in a progressive style, I would highly recommend it. This will eliminate a lot of your headaches!

Another thing to consider with multiple carbs is how much cam you're running. Too much cam will drop your vacuum so low that the carbs will not get their needed vacuum signal except at higher RPM. Generally, a single carb will tollerate a lower vacuum signal, but multiple carbs need at least 10 inches of mercury just to keep them operating, let alone hotrodding.

PROGRESSIVE Linkage advantages:

Progressive linkage keeps a higher "average" vacuum signal. It also (typically) utilizes one (1) carb to provide the idle circuit making tune up a lot simpler.

PROGRESSIVE Linkage disadvantages:

There really isn't any... however, in "instant off the line power" progressive is slightly slower.

1 to 1 linkage advantages:

They are a "no brainer" to connect up. They also provide instantanous "off idle" responce(assuming the carbs aren't too big).

1 to 1 Linkage disadvantages:

You generally have two (2) idle circuits to synchronize, and setting proper idle is a lot of trail & error. 

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Here, you are viewing my tunnel ram with two Holley 450CFM carbs setup in 1 to 1 linkage WITH both a vacuum and fuel pressure gauges permently mounted. My VE rating is about 78%,  so I am a little over carbed. I had compensated the low VE by increasing the accellerator pump duration and increasing the fuel shot size to 37cc's. Using the vacuum gauge, I set both idle circuits to pull maximum vacuum then adjusted the idle RPM to around 1000. NOTE: below 1000RPM my vacuum signal was about 7.5Hg. At 1000RPM I was getting about 9.5Hg. therefore I needed to change the power valves to Holley #85 or, 8.5Hg. release. The only other modification needed was to speed up the accellerator pump rate to compensate for the mechanical secondaries. I did this by moving the pump cam to its #2 postion and re-setting the pump spring.

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In conclusion, don't bother attempting to tune up carbs that need over-hauling or replacing, make sure your engine is in perfect condition before messing with the carbs, and, lastly don't arbitarily change parts on your carbs, especially jets, pump nozzles, pump cams, and power valves! Use your vacuum gauge to determine what needs to be done...

     CBandit

      National T Bucket Association