Tuning Tips: Do Bigger Carbs Mean More Gulps of Fuel?

origin_847053639

Photos: Courtesy of Holley

Drag Race 101: Tuning Tips for the Drag Strip Part II 

This final installment of tuning for the drag strip is going to deal with getting more fuel to the engine now that we have more air going and the ignition installed (see Tuning Tips: Letting the Engine Breathe and Tuning Tips: Let’s Get Sparky.) The more air and fuel that we can get into the engine, the more power our engine can produce. So, let’s see what we can do to increase the amount of fuel being fed to it.

Install a Bigger Carburetor

Carburetor specifications are based upon the amount of air that the carburetor can flow, and are measured in cubic feet per minute of air, or CFM. The higher the number, the better. A typical stock carburetor will flow in the area of 500 to 600 CFM. That’s enough to make a fairly powerful four cylinder engine, but is only enough to create a docile kitten of a V8. To get us to our dream of sub-ten second passes, we’re going to need to double that.

A Holley Gen 3 1050 CFM Dominator Carburetor
A Holley Gen 3 1050 CFM Dominator Carburetor

Being old school, my favorite is Holley. I’ve just had great results with them over the years, but my friends and competitors have also had good results from Edelbrock and others. The choice is yours. Two things to remember, to make max power, you want a minimum of 1050CFM, like the Gen 3 Ultra Dominator.

Now, Get with the Jet Set

The carburetor mentioned above is rated at 1050CFM of air flowing through it. This means the carburetor is capable of properly mixing the correct amount of fuel with that volume of air flowing. However, with our scoop and max flow air filters, we can actually flow more air than that. However, without modification, the carburetor can’t flow enough fuel to keep up with the increased air flow.

Stock carburetor jets have an orifice diameter of about .060 inches. These Holley performance jets are .084 in diameter.
Stock carburetor jets have an orifice diameter of about .060 inches. These Holley performance jets are .084 in diameter.

This is why performance carburetors are equipped with parts known as jets that can be changed. The jet is the internal carburetor component that does most of the metering of the fuel. This means that if we install larger than stock jets, we can flow more air and fuel and make more power.

The exact procedure for changing jets varies by make and model of carburetor, but it’s a pretty straightforward removal and replacement. The process shouldn’t take much more than five to ten minutes for each (There’s two.). With Holley carbs, you have to remove the front float bowl to replace the jets.

Here are the steps to follow:

  1. Remove one of the lower screws securing the float bowl to the main carb body and place a cup or paint can cap under the screw hole to catch the fuel as it drains.
  2. Remove the fuel inlet(s). (You BETTER have two lines! If not, replace the carb with a true double-pumper.)
  3. Remove the other three screws.
  4. Carefully separate the float bowl and metering block from the main carb body.
  5. Carefully, repeat carefully remove the old jets with a plain blade screwdriver.
  6. Carefully thread the new, larger jets into the front metering block and snug them down CAREFULLY with the screwdriver.
  7. If your carb isn’t equipped with the blue, non-stick gaskets, you’ll need to replace the two gaskets on either side of the metering block.
  8. Taking care to face the metering block the correct way, reassemble the metering block and float bowl to the carb body and snug the bolts down. Don’t over-tighten them. You may need to push down on the accelerator pump linkage to get the metering block and float bowl back on.
  9. Re-attach the fuel lines without over-tightening them.
  10. While I’m at it, while the carb is partly disassembled, now would be a good time to upgrade the power pump, also. A larger one will provide a bigger squirt of fuel upon initial acceleration.

Set the Curb Idle Mixture

For this part, I could get all technical with you and tell you to install an oxygen sensor in both banks of the exhaust so we can get the idle mixture just right. However, when we’re tuning to go fast, “just right” isn’t what we’re looking for. We’re actually looking for “a little too much.”

Adjustments

Take a look at the image above. In it you will see four different points of adjustment on the carburetor. From the front they are the accelerator pump adjustment, the front idle mixture adjustment screw, the idle speed adjuster, and the rear mixture adjuster. Let’s focus on the idle speed first. Hook up a tachometer first, we need to be able to get within about 50 RPM of our target and that’s pretty hard to do by ear.

  1. With a medium plane blade, turn the screw to get desired engine RPM, approximately 800.
  2. Next, adjust the idle air mixture screws by tightening them carefully. Don’t use much force or you can deform the needle seat at the end.
  3. Tighten the adjuster screw until the engine starts to stumble.
  4. Back off half to ¾ of a turn (For stock engines we would only back off ¼ turn.).
  5. Repeat on the rear adjuster.
  6. Check and rest the idle speed.

We can adjust the accelerator pump (the linkage arm leading to the front of the carburetor.) to give us a bigger squirt of fuel when we launch. You want to lengthen the screw pressing down on the lever that actuates the accelerator pump so that it pushes the pump diaphragm farther and faster. Which way to turn the screw and not will vary, so you’ll have to eyeball it.

Now We Need More Fuel!

Now that we’ve probably come close to doubling the amount of fuel that our carburetor can flow, we need to make sure that the supply can keep up with the demand. This means we need to either replace the stock fuel pump, or insert a pump between the tank and the primary pump, unless our primary pump is already in the tank, in which case, the secondary will go after the main pump. I’m going to go on the presumption that you’ve got a stock cam-operated pump attached to the side of the block.

 

12-327-13

 

Here again, for me, Holley is an old friend. The image above is of a Holley chromed aftermarket performance mechanical fuel pump that is capable of delivering more than 130 gallons of gas an hour to the carburetor. This pump will be perfect for most amateur class drag applications. If you’re looking for a pump that will flow enough for a blown engine, go for the 170 GPH+ version. The image below is of a Holley electric fuel pump for the strip and street that flows more than 110 gallons per hour.

 

One Final Carburetor Adjustment

Float-Bowl

We’re almost done. The final step is to make sure that the new fuel pump doesn’t cause the carburetor to overflow and start a fire under the hood.  Take a look at the picture above. You’ll notice a screw/bolt with a flathead screwdriver top with a nut under it. That’s the float bowl adjuster. It controls the fuel level inside the float bowl/chamber. You’ll also notice a round opening in the side. That’s the chamber sight glass that lets you visualize the fuel level.

With the engine running, loosen the float adjuster screw and let the float bowl fill up. Tighten the adjuster and watch the fuel level through the sight glass. On some carbs, this is actually a plug that needs to be removed. Ideally, the level will hover just at the bottom of the glass, but low enough that you can see the top of the fuel. With a sight plug, you want to adjust the float bowl to where fuel just dribbles out of the sight hole. Once you have that balance reached, tighten the lock nut. Perform the adjustment on the rear bowl and you’re ready to go.

About Mike Aguilar 200 Articles
Mike's love of cars began in the early 1970's when his father started taking him to his Chevron service station. He's done pretty much everything in the automotive aftermarket from gas station island attendant, parts counter, mechanic, and new and used sales. Mike also has experience in the amateur ranks of many of racing's sanctioning bodies.
Copyright © 2005-2017 RacingJunk.com All Rights Reserved.

Designated trademarks and brands are the property of their respective owners. Use of this Web site constitutes acceptance of the RacingJunk.com
Terms of Use, Classifieds Disclaimer, Privacy Policy, and Cookie Policy