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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24 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
    • Arnie

      Owning a classic car requires that you have some knowledge to fix simple things…like float levels. Do a Google search on how to do it, then try it. If that is a no go, get a friend to show you how.

      Just my $0.02….

      Reply
  5. Jim

    My Holley 600cfm carb is mounted a a 300 plus HP 327 cu.in engine.. I have a “stumble” upon acceleration ( when passing or coming off the traffic lights) & I can not figure the issue out, Any suggestions ???

    Reply
    • Arnie

      Sounds like accelerator pump. Someone answered this question up above with a Holley link…go there for a how-to.

      Reply
    • Mike

      Thee are 3 common adjustments that can cause this. The accelerator pump is coming in late, or is weak; the idle mixture is lean; or the float is low. The first two are easily diagnosed. With the engine running, let it idle for several minutes and shut it off. Look at the discharge nozzles in the throat of the carb and gently and slowly advance the accelerator linkage (be sure there is no play in the linkage). You should see gas dribble from the discharge nozzles immediately when the linkage advances, or if there is play in the linkage, when the throttle plates move off idle. If not, check the adjustment of the accelerator pump, correct if necessary, run the engine for a minute and repeat the test. If gas flows immediately, quickly advance the linkage to wide open and you should see a strong steady stream of gas flow from the discharge nozzles from the beginning to wide open. If there is not a strong steady flow, check the accelerator pump’s condition, the ball valve under the pump which allows gas to enter the pump chamber when the throttle is returning to it’s start position and to seal the chamber when the pump is activated to divert gas to the discharge nozzle, and check the discharge nozzles for dirt clogging the discharge holes.
      If the accelerator pump is not the problem, the IDLE MIXTURE SCREWS may be set too lean. Turn the IDLE MIXTURE screws in all the way and stop turning as soon as the screws bottom out. DO NOT TRY TO SEAT THE SCREWS FIRMLY, it will damage the screws and/or the seats. Count the number of turns so you can return to the beginning point. Next, turn the screws out to factory specs and start the engine. If the factory specs call for 1 1/2 or more turns than you counted by seating the screws, you have probably already fixed the problem. Adjust the idle speed to set proper idling. Since the factory setting is only a starting place, you still have to set the correct mixture for proper idling. With the engine idling turn the IDLE MIXTURE SCREWS out a half turn. If the idle speed increases, reduce the speed by turning the IDLE SPEED SCREW out to return to proper idle speed. Continue adjusting the idle mixture screws 1/2 turn at a time until the engine either stumbles or the idle speed either remains the same or decreases. Turn the idle mixture screws in 1/4 turn and road test the car. You may have to turn the mixture screws in another 1/4 turn for best performance. If the idle speed decreases when you turn the screws out, return to factory setting and turn the screws in, 1/2 turn at a time and continue adjusting in that manner. The best setting is usually about 1/2 way between the richest limit and the leanest limit setting. Don’t be surprised if one screw is set richer or leaner than the other by a full turn.
      If this does not solve the problem, check the floats per factory specs and adjust if necessary. If the problem still exists, set the timing 2 degrees more advanced and test. Repeat until the car pings on acceleration, then back off 2 degrees from pinging setting.

      Reply
    • Charlie

      Jim, I have the same issue with a 355SBC, Edelbrock dual plane intake and a Holley 670 Street Avenger carb. Hesitation just off idle starts around 1200-1600 rpms. Have adjusted the float level, the idle mixture settlings, the accel pump adjustments and tried different cams, no real change. Beginning to think its the dizzy.

      Reply
  6. STEPHEN KEPLEY

    Could use some advice. I have an early style Holley 4150 with the LeMans style center pivot bowls. It is a 715 CFM as used on the early GT 350’s. Used atop a mildly modified 289 HiPo, a little more cam than stock, manual trans. The problem is getting the engine to idle below 900. No amount of adjustment seems to cure it, and have checked for vacuum leaks. Also so mid RPM hesitation. Anything I have missed?

    Reply
    • Customer Service

      Hello Stephen,

      Thank you for contacting us. Great question! The ‘Ask an Expert’ section is currently for members of our online community. By becoming a member, you will have access to our expert’s knowledge in Car Restoration. With your membership you will also receive discounts on products and hours of Premium video content.

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      Sincerely,

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      Classic Car Restoration Club Video Membership

      Reply
  7. Richard

    I have a Sunbeam Alpine with a 1592 engine. The carb has been converted to a Webber 32/36 manual choke.
    The previous owner installed a fast road cam. Inlet opens 32 BTDC vs stock 14. Closes 68 ABDC vs stock 52. Exhaust opens 68BBDC vs 56 BBDC. Ex closes 32 ATDC vs 10. The car starts and runs well. The idle screw is in as far as it can go and you have to keep your foot on the gas until fully warm or it stalls. The choke doesnt provide any fast idle. It is properly adjusted. The car has very little power under 3000 rpm, above that it’s a lot of fun to drive. Any suggestions? Thanks.

    Reply
    • Customer Service

      You’ll have to forgive me as I feel the need to forward your question to a few of my associates whom are more familiar with this great little sports car. It may take a few days to hear back from them, so please bear with me as I await their response.
      Wrench Safe, Mark

      Reply
    • Customer Service

      Hello Richard,

      Thank you for your patience while I got the answer from the expert, here is the reply:

      My apologies for the long delay in response the engine expert we referred your question too experienced a sudden death in his family and has been called out of town to help settle the estate.

      We referred your question to others in his absence, and here is the feedback.
      Camshafts are measured in three ways: Lift, Duration and Degrees of Lobe Separation.
      Using only the duration specifications you’ve listed, while helpful it fails to give us the complete story as to how your camshaft should perform.
      The failure of your engine to idle without additional throttle input suggests it needs additional tuning perhaps even larger primary and secondary idle jets.
      We suspect the lack of power below 3000 rpm may be cam related, but don’t have enough information to say that definitively.

      Wrench Safe,

      Mark
      Classic Car Restoration Club

      Reply
  8. G. JEROME

    Hello,
    I need help switching from my 2 Barrel Rochester Carb set up to a 4 Barrel intake manifold and Carburetor. I have a 67 Olds Cutlass v8 330 bored .30 over with 9:1 compression. My question, what intake manifold do you recommend and what size/brand Carburetor for a daily driver? Any help is greatly appreciated.

    Best,

    Jerome

    Reply
    • Customer Service

      Hello Jerome,

      Here’s what our expert had to say regarding your question:

      Hi Jerome,
      A nice daily driver combination would be a Edelbrock Performer #2711 intake and a Edelbrock Performer 500cfm or 600cfm carburetor.
      Running the numbers, ideally you should have around 550cfm carb. of course nobody makes a 550cfm carb.
      Most guys opt up to 600cfm, although it will run fine with a 500cfm carb and have better vacuum.

      Sincerely,
      Julia
      Classic Car Restoration Club Video Membership

      Reply