How to Adjust Carburetors

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Idle speed and idle mixture are the two most common adjustments made to carburetors, and these adjustments start by warming up the vehicle to full engine-operating temperature. Find out how to perform a lean-drop adjustment with help from an ASE-certified master automobile technician in this free video on carburetors.

Part of the Video Series: Auto Repair & Maintenance
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Video Transcript

Carburation was the standard form of fuel induction for many years and there's a lot of different types of carburetors out there. But there's two adjustments of the many that there are they're common to almost all kinds of carburetors; being idle speed and idle mixture and so we're going to take a look at that today and how to properly adjust that on this car. This car has a two barrel carburetor and for our purposes, the important thing is we can see here at the bottom; there's an idle mixture screw, not easy to see in the seam but there's another one for the second barrel as well. So we're going to perform that adjustment twice. So the first thing we're going to do is we're going to start the vehicle up and we're going to warm it to full engine operating temperature. Now that the vehicle is at the full engine operating temperature, we need to first start by adjusting the idle speed to the specifications. The emissions label on this car specified that this vehicle should idle at 600 RPM in neutral. Currently we can see by the tachometer that we were idling in about 750 RPM; so we will adjust the idle mixture screw or the idle speed screw rather to bring that down to 600 RPM. Our next step is to perform what they refer to as a lean drop adjustment. We will start by adjusting one of the two idle mixture screws out 'till the engine run smooth and the engine speed up to seem really increase that in fact as I've turn it on it's, it's going down. So I will start to turn the screw in which would begin to wean out the air fuel mixture. You do that the RPM raises a little bit and we have to apply it when all of a sudden the engine speed rise. And we get there we'll actually back that idle mixture screw back out about a quarter of a turn and attach to the proper spot for this particular one. We'll now readjust the idle speed to bring it back closer to 600 RPM. We'll now repeat this for the other idle mixture screw. That completes the process for this carburetor. Once again, we would have to at the end of this readjust the idle speed as it will change sometimes as you adjust the idle mixture.


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