Introduction

This comment is a bump for those who come across this question.

Although I highly recommend using a vacuum gauge I'll tell you guys both ways. I also 100% recommend you contact a professional in your area, especially elder ones. They know hundreds of amazing ways of adjusting a carburetor cheaply since they did it for decades. I've only been doing auto-mechanic stuff for 4 years.

Before I continue. If it's a single plane intake you adjust both screws at the same time while in a double plane intake you adjust each side independently. You may need to look up/remove your carburetor to check.

Adjusting Idle Mixture by Ear

Start off by keeping the motor off, go to the carburetor and find the idle mixture screws. Just fingertip them all the way in with a screwdriver.

🛑⚠️ DO NOT TIGHTEN/TORQUE THEM DOWN. You will dig in to the body of the carburetor and it will become impossible to adjust and you'll have to rebuild it. This is a warning, it's so common with people who've never adjusted carburetors.

After they're the way in. Adjust them both 1-1/2 turns back out. This is a base mixture (and some engines might even like this amount as perfect, depending on the carburetor). Do not center them, idle mixture screws can be misaligned with each other but it does not change anything but looks.

Start the motor back up, and let her heat up to operating temperature. If she sounds like she's dying or unstable, raise the idle speed up.

After she's hot. Lower the idle RPMs where it doesn't die like you lower it and lower it and she starts to sputter or diesel. Raise her up until she stops. Get it as low as it will go without running rough 😎. Then adjust one side one at a time (or both if single plane), half turns where she gets the highest RPM. Turn the idle RPMs down to compensate the speed increase. If you turn one side and it doesn't act like the RPM raises lower it back down. After you get them where you feel comfortable, raise the RPM back up to what you like (for example, I like raising my idle RPM until you can't hear the piston slap from worn cylinder walls lolololol)

🛑⚠️This isn't recommended but go sniff your exhaust. If it still smells like rich gas, turn both mixtures quarter turns down and wait 30 seconds then do it again and repeat. This isn't proven to be accurate and isn't normally a thing people do but it works for me. If it never smells better, it might be time for a new catalytic converter.

Adjusting Idle Mixture by Gauge

This is easy. It's exactly like Adjusting by ear but instead of listening to the raise in RPM you watch the gauge for the highest vacuum possible. I still recommend lowering the idle to see the fine adjustments but it's not really required.

There is no set number base vacuum. It can depend on the cam in the motor and it's condition, the conditions of the valves, piston rings/walls, etc. Just get it to the highest number that the engine can achieve and then raise the Idle RPM's back up to what you want.

I followed these instructions. It doesn't run rich when in Idle, but it runs rich when driving.

If you have something like edelbrock, rochester, etc that don't have all out mixture screws you will need to buy a tune kit and put smaller jet's.

Holley's have Mixture screws that have to be carefully adjusted. These can cause annoyances like Dieseling, Hard Starts, Hard to start, Roughness, etc if not adjusted perfectly. Throw it out and buy a carburetor that requires a tune kit they are much better. PERSONAL OPINION.