How to Set Your Carburetor

How to Set Your Carburetor

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The world of EFI and aftermarket tuning has evolved to the point where now 1000+ horsepower street applications are not that unrealistic anymore. Despite the advancements in the tuning era, and more standalone computers being produced everyday, we still see the traditional hot rodder with a good old fashioned carbureted race car to keep things simple.

And why not? Carburetors are still an easy way to produce power at the cost of efficiency and convenience, but how easy is it? Today we’ll be showing you just how to set your carburetor for the engine of your choice.

idle_mixture_screw_closeup

 

Many customers seem to think that the mixture screws on a carburetor are just dials to be turned all the way in and backed out a set amount of turns to achieve maximum output. People who own a carb vehicle and want to know how to set your carburetor can find a lot of useful information in our guide.

Weber Redline's DCOE Carb - Image courtesy Weber
Weber Redline’s DCOE Carb – Image courtesy Weber

In reality however, nothing could be further from the truth as many different factors weigh into how you should be tuning your carbureted setup. The setting for your idle mixture screws are directly affected by the engine’s displacement, the cam profile, idle vacuum, timing, and much more.

Today we’ll be showing you how to set your carburetor for maximum performance as well as achieve a ‘lean best idle’ adjustment for progressive carburetors.

How to Set Your Carburetor Idle

  • Start the engine and let it warm up.
  • Turn your idle screw so that the engine is idling at a normal RPM.
  • Now slowly turn one bleeder screw on one side of the carb inward until the idle either drops or rises. If it drops, you are going the wrong way, so turn it the other way until the you hear the engine’s idle rise back up again. You will want to turn it up until the RPMs go up and peak, you will want to achieve the highest idle setting here before the dropoff.
  • Now go back and re-adjust the actual idle speed screw to re-adjust the idle back down to the normal RPM again, rinse and repeat on the other side of your carb.
  • When you are done with that side, and you have found the highest possible idle, now go to your idle screw and adjust your base idle to where it should be

idle_mixture_screw_closeup

TIP : Remember that these screws are your idle bleed screws only, and will not lean or enrichen your vehicle after your throttle plate has opened. To truly tune or adjust your fuel mixture, you will need a performance carb with adjustments available on the outside of the unit, or open the carb and swap out the main jets to enrichen or lean out your fuel curve.

How to Set Up Carburetor tips

  • Remove your carburetor and write down your factory settings. While they are useful to jot down for future reference, you should never just assume the factory delivered settings will work fine for your car.
  • Warm up or disengage your choke, if you own an automatic choke open the choke butterfly by hand and insert something to wedge the flap open while the linkage is cycled. The choke cam will need to be cleared during this process, and you will hear a slight metallic click when the cam is released.
  • After this is done, check your fast idle screw under the choke assembly to confirm that you are not hitting the fast idle cam.
  • Set your idle stop screw by turning the screw all the way out until it is not touching the throttle stop lever. Now cycle the linkage until it is barely touching the stop lever, make sure that the linkage does not bind or become crooked.
  • Now set your mixture screw by screwing it all the way in until the screw bottoms out. Now back out the screw 2 full turns and begin your adjustments.

 

How to Tune your Carburetor

  • Start up your engine and allow it to warm up to operating temperatures. It may run rough to begin with, if you have not set the idle, scroll up and complete the “How to Set Your Carburetor Idle” section.
  • Make sure to set the idle mixture screw to achieve the ‘lean best idle’ setting. Turn in this mixture screw until the engine dies or is close to dying, and back out the screw slowly at quarter turn intervals.
  • Once your engine picks up speed and begins to smooth out. Back out ½ turn more, or until the screw does nothing or runs worse then turn back to the point where it ran its best.
  • Now that the mixture screw is at its best running location, you can adjust the Idle speed the screw. The screw will be sensitive and should only take ¼  to ½ turns to achieve the idle speed you like
  • Check and double check your idle, and then move onward to put the car into gear and apply a very slight load. If your vehicle is equipped with air conditioning, turn it on and try a small test drive. Set the idle now and make sure not to set it too high, as it may cause premature clutch and brake wear.
  • Recheck timing and vacuum hook ups. Recheck mixture screw to lean best idle again, if your idle remains smooth and ideal, then confirm and note the final settings.
  • To confirm settings with the engine running. Start by screwing  in the mixture screw and count the number of turns it takes to bottom out and note if the engine dies. If Idle Mixture screws are with in ½ turn of base line setting then all is well and have fun.
  • Also check the speed screw and note how many total turns from initial contact. If the settings are other than described then you may want to recalibrate the Idle circuit (low speed circuit) to your engines needs.

This does it for our How to Set Your Carburetor article, please let us know if you have any questions or comments below!

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