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.

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 90degree bend in the system reduces volume by 12%.

Example: Initial = 100GPH
1st 90degrees = 88GPH
Initial = 88GPH
2nd 90degrees = 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 90degree bend, plus 45degrees, 30degrees etc. More common you will find 180degrees, 135degrees, 90degrees, 45degrees, 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.

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 28degrees total and go up in 2degree 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 28degrees. Did this hurt or help? If it hurt go to 34 degrees. If that hurt go to 31degrees. Back to your best time? Try 38degrees. No help? Put it at 32degreees and leave it.

4) Don’t get discouraged!

5) Keep Trying New Stuff!

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, 4x6 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.

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. Keep the primary and secondary throttle blades adjusted equally. 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 ¾” 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.