Engine Sizes and how similar CFM rated carburetors are used efficiently

Engines of dramatically different size and power (rpm) range can produce similar peak horsepower output, but require completely different fuel curves from carburetors of the same airflow rating and design.

The below examples help to understand how you can have two completely different engines using the same carburetor, with the same airflow ratings, but require completely different fuel calibrations.

Example #1:

Chevrolet Big Block 500cid, 9.2 compression, dual plane intake manifold, cast iron production cylinder heads with moderate porting performed, RV hydraulic camshaft designed for 5500 rpm peak output, Model #3310 HOLLEY® 750 CFM carburetor.

Rated Horsepower = 450 @ 5200 RPM

Example #2:

Ford Small Block 306cid, 12.5 compression, single plane intake aluminum, aftermarket cylinder heads, solid roller camshaft designed for 6500 rpm+ peak output, Model # 4779 HOLLEY® 750CFM carburetor.

Rated Horsepower = 450 @ 6700 RPM

In example one, a HOLLEY® 3310 750CFM vacuum secondary was used. On this mild cammed engine, the carburetor would utilize a standard 2 corner idle circuit system, a power valve with a vacuum rating between 6-9 inches, and a relatively standard tuning set-up.

In example two, a HOLLEY® 4779 750CFM mechanical secondary. On this radically cammed engine, the carburetor would not utilize any power valve (vacuum operated high load enrichment valve) because of the lack of vacuum produced by the large camshaft. Because of the high compression, and high rpm operating range, the metering blocks, air bleeds, and boosters would require extensive modifications.

Two engines can use the same size and style of carburetor, but the internal tuning of each carburetor is completely different to if you want to optimize the carburetor for each particular application.

When you purchase a commercially built new carburetor out of the box, what it is tuned for? Well, nothing really, these commercial carburetors are set up with a general fuel calibration and any tuning to optimize performance is left up to you the buyer.

There are seven (7) different basic circuits and literally hundreds of different combinations of how to tune any carburetor, and even more opportunities to get it wrong if you don’t know exactly what you are doing. When changing one area of a carburetor, that single change can effect one or all of the other circuits within the carburetor itself.

Quadrajet Tuning Instructions



IMPORTANT: Perform all the following steps before you put any fuel into the carburetor!


  • Inspect carburetor for accidental shipping damage. Call us immediately if something does not look right.


  • Inspect the work are on the engine to be sure that you have removed and/or disconnected everything necessary to install the new carburetor.


  • Make sure you have the correct length carburetor studs or bolts with proper nuts and washers.


  • Install the correct carburetor base gasket.


  • Set the carburetor on the manifold and install the bolts and/or nuts and washers.


  • Hand tighten the nuts or bolts in an alternating pattern from the corner to the opposite corner.


  • Tighten bolts to 12 foot pounds.


  • Operate the throttle linkage through its fill range and check for interference at the linkage.


  • Check for interference at the accelerator pump linkage.


  • Hook up the throttle linkage and adjust for full throttle with someone pressing the gas pedal.


  • Install a good throttle return spring. Make certain that it positively closes the throttle.


  • Have someone operate the gas pedal while you verify full throttle adjustment at the carburetor.


  • Hook up fuel lines and vacuum lines if used (refer to factory manual or under hood diagram).


  • Be alert for fuel leaks or flooding when you start the engine. This can be caused by dirt in the needle and seat, small particles of our fuel hoses, floats and needles that have settled during shipping or other debris in the furl system washed in when fuel is first applied. If flooding occurs, tap the area above the needle and seat with the screwdriver handle to dislodge any possible dirt particles. If flooding continues, stop the engine and refer to the trouble guide below.
  • If no flooding occurs, warm the engine up at a fast idle until the choke is fully open.
  • Now adjust each mixture screw for the highest vacuum reading on a gauge or for the highest rpm you can achieve without touching the idle speed screw. This may take a few tries for a precise setting.
  • You can now drive the car and begin to tuning process for optimum calibration.


Set the manufactures specifications or at the lowest rpm that you are able to achieve a smooth idle.

Using a vacuum gauge connected to a direct vacuum source or tachometer, turn the mixture screws in(clockwise) one at a time until the vacuum reading or rpm begins to drop. Then turn them out (counter clockwise) until you achieve the highest vacuum or rpm reading.

The idle mixture screws are located on the bottom from of the carburetor. On some model Quadrajet carburetors , THEXTON tool #350 must be used to adjust the recessed screws.

The idle mixture screws are located on the sides of the metering block(s). In some cases, there will be four: (4)2-primary, and 2-secondary.

TYPICAL SETTING: ( These are just references, not rules)
Q-JET- 3 to 5 turns from seated.
CARTER- 2 to 4 turns from seated.
HOLLEY-(2 screws)- 1 to 2 turns from seated.
HOLLEY-(4 screws)- 3/4 to 1-1/2 turns from seated.

If you encounter a bog or hesitation when the secondarys open, more than likely the secondary air doors need to be adjusted. Use the 3/32 allen wrench to loosen the lockdown screw and use the adjustment screw to loosen or tighten as necessary. It will take some experimentation to determine whether it needs to be loosened or tightened. A small adjustment can make a large change, so make small changes( 1/8 turn at a time). Try one direction first, if the bog get worse, move in the other direction.

The adjustable part throttle (APT) screw can be used to fine tune the cruise (steady throttle, freeway speed) mixture. Simply turn the screw DOWN 1/2 turn at a time until you experience a “lean surge”. (It will feel like someone is moving the throttle, when their not, or a hesitation on light throttle acceleration). Then turn the screw UP, until the lean surge goes away.

If you are experiencing an “off-idle stumble”, you can turn the screw up at � turn at a time until it is eliminated. But be sure to adjust your idle mixture screws properly first, as they can also cause an off idle stumble.

Use a 12 volt test light to find a 12 volt power source that is energized with the key in the “on” and “run” positions only. The best source for this is one of the unused accessory terminals at the fuse box. DO NOT!!! splice into any wires related to the ignition system!!!



  • Most metering changes can be accomplished without removing the carburetor from the manifold or disconnecting the fuel line.
  • The most common changes can be made quickly without removing the airhorn/bowl cover.
  • If the engine has a radical camshaft it will exhibit low idle vacuum and require excessive throttle opening at the idle.
  • To correct this you need as much ignition advance as possible for the best idle. If you have a mechanical distributor have it adjusted for a low limit with plenty of initial advance.
  • If idle vacuum is 6″ hg or less the metering rods are probably in the up or rich position. This will cause the nozzles to discharge fuel at idle. Use a weaker power set-up spring to keep the rods down at idle.
  • To adjust the primary cruise, remove the hex plug just in front of the choke tower. This offers access to the APT or adjustable part throttle. Use a small screwdriver to adjust leaner by turning it clockwise.

Check Your Fuel Pressure!!!

Carburetors love fuel volume, but hate pressure. Pressure creates inconsistent aeration of the fuel in the float bowl, which causes inconsistent metering. Picture in your mind a water nozzle spraying into a bucket; the more pressure used, the more froth and air bubbles are created. If you have proper volume, the optimal fuel pressure is 4PSI for modern 2 and 4 BBL carbs. High horsepower drag care may not be able to run this low because G force works against fuel attempting to travel from the fuel cell in rear to carburetors up front.

So now we have established that volume is critical. Let’s examine factors that effect low volume, and there are many.

1st the obvious:
Fuel Pump capacity
Fuel Line size
Fuel Pump style
Fuel Pump placement

Capacity: Bigger is better. But… rating methods vary; some are rated with no output pressure or restriction. Others are measured at specified output pressure (A.K.A. 110GPH at 7PSI) (These ratings DO NOT specify the size, length, etc. of inlet of outlet size used.)

Line Size: Because manufacturer ratings fail to specify inlet or outlet size, we must assume the maximum size possible was used. Therefore, any reduction in size, no matter where it occurs, will reduce volume.

Beyond size, angles in fuel lines cause restrictions that reduce volume. Every 90° bend in the system reduces volume by 12%.

Example: Initial = 100GPH
1st 90° = 88GPH
Initial = 88GPH
2nd 90° = 77.44 GPH

Look at the fuel systems on most all vehicles, race, street, off road, etc. Almost without fail you will find at least 1 90° bend, plus 45°, 30° etc. More common you will find 180°, 135°, 90°, 45°, one after the other.

So your 100GPH, by the time it gets to the carburetor, could be as low as 20-30 GPH. This is why fuel pump manufacturers rate GPH vs HP at what seems a completely excessive ratio. They realize that they must consider a worst case scenario: to safeguard against the most unacceptable installation.

To look at this situation from a completely different standpoint, view the following; Cost of engine: $10,000 for 600HP, Cost of Minimum Required Fuel Pump: $200, Cost of Maximum Required Fuel Pump: $600, Differential = $400.

Possible cost of repairing damage to engine to lack of fuel volume is a minimum of $500 and a maximum of $5,000. By using the best possible component when designing your fuel system, you protect a much larger investment, give yourself peace of mind and invest in something that you can trust to grow with you.

Drag Tuning

1) Be prepared! Make a checklist before hand, and then bring everything else too, because you’ll need it.

2) Make a log! Record the details of every run and any changes you make.

3) Be consistent! Remember details of your run, the burnout, how you staged, how you launch, what RPM you shift. If you can’t repeat this data you’re wasting time and collecting inconsistent data.

Starting with traction: make sure you can get your car to hook consistently and make sure you can put down 3 consistent 60 ft. times before any other tuning! If the car will not hook repeatedly your data will be inconsistent.

Begin your tuning with timing: Generally a motor will like the same timing no matter how the carb is tuned. Start low, say 28° total and go up in 2° increments. If it doesn’t pick up after two increases, Stop. We’ll come back to this.

Timing curve: Generally a motor will like the total advance to come in as quickly as possible as long as it doesn’t cause detonation or pre-ignition. Generally this will be 2200-2800 RPM, depending on fuel.

60 Foot Times: Now that you’ve got traction let’s tune the accelerator pumps. Start by adjusting. There should be no slack and also no preload between the pump lever and the pump arm. As soon as you move the throttle linkage fuel should move in a hard “shot” from the pump nozzle. Cars with hi-stall converters may find that they have to adjust the pump arms to the point that they begin to have tension at the point where the throttle is set when the car is on the line ready to launch. This way it actually gets the complete pump shot when they release the brake and hammer the throttle.

Now you can tune the accelerator pump nozzles: Go up in size until 60ft. times increase. If they increase immediately, go down in size until the 60ft. times don’t continue to decrease.

Now it’s time to Jet: MPH is an excellent way to judge jetting. If a car is lean as you Jet-up you will see MPH increase. When MPH seems to stay the same but E.T. decreases, you’re too rich.
SIDE NOTE- If you Jet-up and get better E.T. and MPH, but your 60ft. times increase, start retuning your Acc. Pump circuit.

Remember Timing? Time to start retesting this too. Go back to 28°. Did this hurt or help? If it hurt go to 34°. If that hurt go to 31°. Back to your best time? Try 38°. No help? Put it at 32° and leave it.

4) Don’t get discouraged!

5) Keep Trying New Stuff!

How to send your carburetor to SMI for service?

Shipping your Carburetor

1) Review the list of services available located further down this page
Choose the one that matches your requirements. If the service you choose involves tuning/modifying your carburetor to match your Engine/Vehicle, then you will need to fill out the spec sheet. This is found on the Home page in the Upper Middle, Right Side (CLICK HERE TO SEND US YOUR ENGINE/VEHICLE SPECS). If possible print out that page so you can place it in the box along with the carburetor. If this is not an option go ahead and send it to us by clicking the (Submit Information) button on the bottom of the page.

2) Preparing your carburetor for Shipping
If carburetor has fuel inside turn upside down over a proper container, tilting and tipping the carburetor slightly, back and forth to remove as much fuel as possible. Also open the throttle blades until no fuel is coming out of the accelerator pump discharge nozzle. Leave the carburetor in the upside down position overnight. BE SURE TO DO THIS IN A SAFE, WELL VENTALATED AREA AWAY FROM ANY POSSIBLE IGNITION SOURCE. WEAR APPROPRIATE PROTECTIVE SAFETY EQUIPMENT. Next, place the carburetor in a plastic bag of appropriate thickness to avoid being ripped or torn. Seal the bag as tightly as possible and repeat this process with a second bag.

3) Choosing the proper packaging materials
Place in a box of appropriate size.
-4BBL Carburetors should be in a minimum size box of 14″x14″x14″
-2BBL Carburetors should be in a minimum size box of 12″x12″x12″
Fill the remaining space with packaging material (bubble wrap, newspaper, packing peanuts, etc.) Dispensing the material evenly between the sides bottom and top. Pack the contents very tight to minimize the possibility of the carburetor moving inside the box. THIS IS VERY IMPORTANT!!!
As an example UPS requires that a package must protect content sufficiently and that a 20ft. free fall to a solid surface will not damage the contents in any way. Even if you insure the package, If they determine that you did not provide sufficient protection they will not pay the claim.

4) What to include in the box
Written information and be as detailed as possible:
-Name, Address, Phone #, Email
-Description of the work to be done to the Carburetor
-Engine/Vehicle Spec form if Necessary
Use a large piece of paper it is not recommended that you use: post its, 4″x6″ cards, bus cards. Once all of the information is collected, place it in an envelope or in a bag and secure it to one of the flaps of the box using tape.

5) Shipping your carburetor
Take your package to any carrier (UPS, FEDEX, DHL, AIR BOURNE, USPS, ETC..) We recommend insuring the package. While most carriers provide a tracking number to check the status of your shipment the USPS does not so you will have to ask the postal employee for that service.

Holley 2300/4150 Tuning Tips

Float Level- The fuel level in the bowls should be set at the bottom of the sight hole so that you have to jostle the car to get the fuel to come out the sight hole when running. We set the floats in the shop to a particular setting; but fuel pressure dictates fuel level. Therefore you must set this when you first install the carburetor. Finally, after setting the floats, always give the carburetor time to burn off enough fuel to reopen the needle/seat so that your new setting is really what you are seeing. Many times people lower the floats too much because they haven?t waited a sufficient amount of time for the fuel to burn off and the new setting to take.

Throttle Blade Adjustment- The throttle blades should be set at an rpm that is as low as possible to keep the transfer slots covered at closed throttle position. If you should lose your setting just back it completely off, put 1 round in the primary and 1 round in the secondary and start there.

Idle Mixture Screws- Always adjust the 4 idle mixture screws evenly. The settings on all 4 should be the same. Our original setting is 1 1/2 turns out. If for some reason your engine absolutely needs a different setting on one side or corner you can just about bet something is wrong with the engine causing it to have a different signal/vacuum on that side/corner.

Fuel Pressure- 6.0 to 6.5 pounds. You should absolutely know what your fuel pressure is! Many racers have no clue what their fuel pressure is, and they chase engine problems for weeks when a simple fuel pressure gauge would have indicated the problem straight away.

Vent Tube Clearance- You must have at least �” clearance above the vent tubes. It is ok to lower the vent tubes if absolutely necessary, but you will begin to run the risk of fuel spilling over in the turns if you are not careful.

Pump Circuit Tuning- Because of the differences in track conditions and driving styles you may need to adjust the pump circuit, i.e. pump cam/pump arm, to correct “off corner” stumbling issues. If an engine stumbles two or three times after the driver steps into the throttle this usually indicates too much fuel and can be corrected by adjusting slack in the pump arm or installing a smaller pump cam. If an engine has a “dead hesitation” and then picks right up and goes, this usually indicates not enough fuel on the pump circuit. First check that the pump arm has no slack in the adjustment then proceed to increase pump shot with either a larger pump cam or a larger pump nozzle.