Carburetor Tuning Tips

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1.) How do I select the right size of carburetor?

Carburetor performance is largely based on the air-speed traveling through its venturis. If they are too big the air speed will be reduced, too small and vice-versa. Probably the best instruction on carburetor choice is found in the Demon Carburetor Selection guide. It is based on camshaft duration at 0.050in of valve lift, the type of intake manifold (dual plane or single plane) and the type of transmission (manual or automatic) and stall speed. Beyond this it is always best to discuss carburetor selection with a qualified technician.

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2.) What is one of the most commonly overlooked carburetor-tuning issues?

One of the greatest obstacles faced by amateur carburetor tuners is failing to understand the essentials of initial ignition timing. Ignition timing is affected by a host of elements including fuel type, mixture strength, combustion chamber shape, compression ratio, temperature and humidity. Bigger camshafts and larger cylinder heads with matched intake manifolds require increased ignition timing to promote better air velocity and maintain efficiency.

02-Timing-0875 Thus the carburetor is helpless in its plight if the initial timing is late or the timing advance curve is slow. These two potential timing impediments frequently exhibit poorer starting and drivability troubles. The ignition is always timed to fire the spark plug before the piston reaches Top Dead Center (TDC) in the cylinder. Firing before TDC is necessary because of the time it takes for the flame front to ignite the air-fuel mixture in the cylinder. Demon’s carburetor selection guide consists of helpful recommendations on ignition timing settings with consideration to things such as camshaft duration, valve lift, and other contributing factors to assist in the timing process.

When carburetor or drivability complications occur under 3,000 rpms, improper timing is usually contributing to issues such as rich idle, hesitation, or dying when switching gears and stopping.

3.) What carburetor tuning myths do tuners frequently fall victim to?

In carburetor mythology probably the most misleading dogma involves changing jets in order to cure a rich idle setting. Carburetor main jets are tuned solely to achieve one thing: the best performance at fully open throttle. Jets should never be changed to correct an idle impediment. Instead the idle circuit should be adjusted and the initial ignition timing re-checked. Setting the idle mixture to the highest vacuum can be achieved by using a vacuum gauge connected to the constant-vacuum port of the carburetor base plate. Slowly adjust the first idle-mixture screw. Make one adjustment only to the first screw. The adjustment should be no more than an eighth or quarter turn. Then leave sufficient time for the carburetor to respond and move to the next adjuster screw. Gradually work your way around the carburetor, making just one small, slow adjustment to each screw. Sloppy idle-speed adjustment is the carburetor’s number one tuning problem. It causes the transfer slot to be over exposed and creates mayhem.

03a-SetIdleMixture0202 Setting the idle mixture to the highest vacuum can be achieved using a vacuum gauge connected to the constant-vacuum port of the carburetor base plate to adjust each idle-mixture screw to its highest value. Keep moving around the carburetor, adjusting each screw an eighth to a quarter turn and waiting for the carburetor to respond.

img01 Over exposure of the transfer slot at idle and insufficient initial ignition timing are the two chief causes of carburetor troubles. In the idle condition the transfer slots should give the appearance of a small square when viewed from underneath the base plate, and ignition timing should be double checked prior to any carburetor adjustments.

4.) Clogged jets or sloppy idle mixtures are often accused for hesitation or flooding under driving conditions, or starting and idling issues, but are there other things that could contribute to these issues?

Float adjustments are often a disregarded step of carburetor tuning, even though hesitation or an excess of black smoke from the exhaust could be the result of an improperly adjusted float level. After the idle mixture and ignition timing have been properly set it is time to check the float levels.

04-FloatAdjust-DSC_0176 With the motor running the fuel level should be in the middle of the glass window of the fuel bowl. If it isn’t loosen the lock screw on top of the bowl and adjust the nut clockwise to lower the float level and counter-clockwise to raise the float level. Make the adjustment by one flat of the nut at a time and wait until the carburetor has had a chance to respond. The ‘dry setting’ is accomplished by removing the fuel bowl, and with it turned upside down, set the dimension between the inside top of the bowl and the top of the float at approximately 0.375in to 0.0400in.

Float levels are often too high; they should be placed in the middle of the fuel bowl sight glass. The initial float setting when the carburetor is being assembled at the Demon factory (the dry setting) is accomplished by removing the fuel bowl and, with it turned upside-down, the dimension between the inside top of the bowl and the top of the float is set at approximately 0.375in to 0.0400in

5.) What are the rules for mounting a fuel pressure regulator?

05-FuelPressureReg Avoid mounting the fuel pressure regulator on a bulkhead or inner fender well. Instead, position it close to the carburetor. As demonstrated by any data acquisition system, the further distance the fuel pressure regulator is mounted from the carburetor, the slower its reaction time.

6.) After carburetor tuning what final steps should be made before hitting the road?

Always start the engine and check for leaks before closing the hood. Better to repair a leaking fuel line fitting or a malfunctioning float level than to witness the paint being burned off an immaculate hood.

Discussion
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10 Responses to “Carburetor Tuning Tips”
    • Customer Service Techs

      Hello there, thank you for your question. In order for us to assist you further, do you have a question in particular to tuning or just in general? If you have any further question, please contact us at 1-855-706-3534.

      Reply
  1. Don

    Did some minor work to my Holley #4110 carb,replaced both power valves and secondary diaphragm. When I went to start it now I have a small backfire out the exhaust. Any idea why ? Didn’t change anything else. Power valves were replaced whit the same that were in it.

    Reply
    • Customer Service Techs

      Thanks for the question, we shared it with our friends at Cottrell Racing Engines and here is the response we received:

      If it’s popping out the exhaust it’s rich unless he removed the distributor at the same and it didn’t get installed in the same spot. Retarded timing will do this. Otherwise he may have a leaking power valve more than likely. Look down the venturis while it’s running and see if fuel is dripping from the squirters etc… If he removed the spark plug wires to perform the work, make sure they are installed correctly on each corresponding cylinder. It could also just be a coincidence and have a fouled plug.

      You didn’t mention however when it was backfiring… at idle, accelerating, rapid decelerating… etc. as rapid deceleration may indicate a lean condition, a vacuum leak, distributer advance hooked to the wrong carb port, etc.

      Wrench Safe, Mark

      Reply
  2. Jim Kennedy

    I have a “stumble” in my 55 chevy, I have a 600cfm Holley 4 bbl.carb,But I Think the accelator pump adjustment may be my issue. What is the proper way of adjusting the accelator pump? It is a nut/bolt setup with a spring mounted on it.. I don’t think my floats are the issue nor the mixture screws…
    Your advice would be greatly appreciaited..
    Thanks,eh
    Jim

    Reply
    • Customer Service

      Hello Jim,

      I think you’re on the right track, assuming it’s a accelerator pump issue, or at least a good place to start.

      Holley does have a great video on just how to check and adjust the accelerator pump.
      Here is a link: https://www.youtube.com/watch?v=k-Nx5HEzvlY

      From my experience, I always start by checking the accelerator pump diaphragm itself, as they can become torn, especially if the adjustment was too far off on the pump.

      Wrench Safe,
      Mark CCRC Video Membership

      Reply
  3. Hal

    I have a bored 350 chevy engine with a 671 GMC blower on it. I’m using 2 500 cfm Holley blower carbs.
    I need a starting point for the jet sizes. I am in Denver at 5280 feet above sea level. I have also installed O2 sensors in the collector portion of the headers. What should I be reading on the O2 gauge if everything is right? The engine is not in the car yet, but is on a test stand.

    Reply
    • Customer Service

      Hello Hal,

      When setting up and tuning twin carbs on a blown motor there are more items to adjust than simply the jets. You’ll need to also consider accelerator pump cams, high and low speed air bleeds, pump discharge nozzle, squirter size, accelerator pump diaphragm size, and power valves. Having once owned a 350 SBC with 2 650cfm Holley carbs, I recall settling in on a jet size of #76. Will that jet size work for you? Maybe? There are simply too many variables to make an accurate determination, and given your altitude and smaller carb size your engine may be happier with a smaller size main jet.

      You’ve invested a lot into your engine, and a test stand will help you get it timed and running, but is no substitute for running it under load on an engine dyno. A good dyno operator can easily assist you in deciphering all the data and quickly steer you in the right direction when making changes to your setup. Yes it costs a little money to go on dyno, but the frustration it can save will pay for itself.

      The O2 sensor works like a miniature generator and produces its own voltage when it gets hot. Inside the vented cover on the end of the sensor that screws into the exhaust manifold is a zirconium ceramic bulb. The bulb is coated on the outside with a porous layer of platinum. Inside the bulb are two strips of platinum that serve as electrodes or contacts.

      An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen in the exhaust. When the mixture is lean, the sensor’s output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts. The oxygen sensors will need to warm up before delivering an accurate reading.

      Wrench Safe,
      Mark CCRC Video Membership

      Reply
  4. stuart522

    To be honest, most all of these articles and tips are above my experience level. …adjusting floats in a carb would mean me buying a new carb and making another mortgage payment for the local mechanic…

    Reply