How To Adjust Idle Mixture Screws

How To Adjust Idle Mixture Screws


When you are building a hot rod knowing how to affect your air/fuel ratio is a vital part of tuning and racing your vehicle. For those preferring the simplicity of carbureted fuel delivery, the act of adjusting idle mixture screws is one of those jobs that’s easy in theory, but difficult to master. Today we’ll be showing you how to adjust idle mixture screws to get the most out of your hot rod, and keep your engine purring.

Before you begin our How To Adjust Idle Mixture Screws DIY guide, make sure that you have your idle speed set correctly on your carburetor. If you need a guide on How To Set Idle Speed, check out our handy carbureted guide here.


Before adjusting your idle mixture screws, make sure you locate them and don’t confuse them for another part of your carburetor. If your vehicle was built after 1981 and still sports the factory carb, chances are these screws are covered with caps or limiters. These were installed due to a Federal mandate, making it more difficult for you to squeeze power from your carbureted engine.


These mandates and restrictions were put on factory carburetors to prevent people from fattening up their fuel mixture and increasing emissions. If your factory carburetor is still stock and has these limiter caps in place, it’s a good idea to leave them alone unless you are an experienced carburetor technician.

If your carburetor has these plastic caps on them, and you need help adjusting your mixture screws, it’s best to take it to a experienced mechanic.

What if I have a single barrel carburetor?


Unfortunately you won’t have the ease of adjustability that typical carburetors do, it’s a good idea to find out where your mixture screw is by reading your manual. If you still cannot adjust your idle mixture screw, try looking for tags or a special instructions panel somewhere on your hood or engine bay.



Check both on the hood and engine, as well as the air cleaner and intake manifold. Once you’ve located the screw, you can adjust your idle mixture by following these steps.

  1. Start car
  2. Bring engine to the correct idle speed
  3. Locate idle mixture screw
  4. Turn screw in until the engine begins to stumble.
  5. Slowly back out adjustment mixture screw until idle evens out.
  6. If your engine starts to run too rich (too much fuel) turn back in until your idle is even.

Still having problems setting your idle? Try using a dwell meter which will allow you to accurately read the engine speed, eliminating the “tune by ear” factor of this DIY guide.

How To Adjust Idle Speed


Q : I’ve set my idle correctly but my car won’t start in the morning.

A : If you are still experiencing problems with your engine not starting when cold, or in the morning your choke may be the problem. You can easily test this the next time you go to start your car in the same circumstances. Before starting your car the next time, remove your air cleaner and look down the barrel to inspect the butterfly valve.

If it’s not closed try depressing the accelerator or manually opening the throttle arm to see if the butterfly valve closes. If your choke is manual try activating it now to see if the valve closes. Do not attempt to start your engine during this testing period, as fire or ignited fuel could travel upwards at you.

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Butterfly valve seem stuck open or doesn’t want to move? Try spraying some choke cleaner at the valve and mechanism to see if the action improves. If you are still having a problem, head on over to our How To Adjust your Choke for more information.

You now know how to adjust idle mixture screws using our DIY guide, if you have any questions or comments please leave them for us below.