Mixture Screws
Junior User
Joined: Jan 2005
Posts: 75
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From: Lexington, Ky
I was in Bowling Green Ky couple years ago where Holley carbs are made ran into one of their techs he told me to do this and it worked for me. You will need a vacuum gauge.
How To Adjust The Idle Mixture On Holley Carbs - Blogs - Holley Performance Products Forums
How To Adjust The Idle Mixture On Holley Carbs - Blogs - Holley Performance Products Forums
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Junior User
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Typical idle vacuum should read somewhere around 18". As the engine is running and you're looking at the vac. gauge - turn/adjust the screw(s) and watch the gauge. If it starts to fall (like to 17) stop and turn the screw the other way, and the reading should start going back to 18. Keep turning until you see if fall again. Notice where you got the highest reading (say 18.5) and adjust the screw back to where you got that 18.5 / highest reading... Good luck!
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Joined: Apr 2002
Posts: 19,311
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From: Waterloo, Iowa
It helps if the engine is fully warmed up. Idle speed at spec. Make sure you're done setting timing, float level set correctly. If possible set mixture with air cleaner installed. Rev throttle to clear fuel out of the idle circuits between adjustments. Then reset idle speed. The idea is best smooth idle just about as lean as possible, but slightly on the rich side.
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Thanks everybody. I appreciate all the help. I'm still learning about all this. My truck is in pretty bad shape. It was givin to me from my girlfriends brother and I had to get it running. It had been sitting for a few years. Been through who knows how many owners. So idk. Im pretty new to mechanics. So now its my job to get this thing up and running with the least amount of money possible. Next step is to install manual choke and get fuel filter and new pump. Possibly fix the heater. Its blowing pretty good but is getting lost somewhere in the truck. No air is coming through the vent next to the headlight and windshield wiper controls.
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Joined: Apr 2002
Posts: 19,311
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From: Waterloo, Iowa
The important thing is to remember idle mixture is one of the very last things to be done, and when certain other fuel related maintenance is done it needs to be reset. Lots of things interact.
If the float level is too low, or too high, maybe someone tried to compensate for that in the past by mistakenly adjusting idle mixture (after they replaced the fuel pump, etc.)
Then uncle Fred changed the timing, etc. With carbs always start a baseline using a known good fuel pump having proper volume and pressure. Doesn't have to be new, tho make sure it's within 4-7 psi and get the float level dead on. It will never run right otherwise because messing with other settings can't compensate for that.
If the float level is too low, or too high, maybe someone tried to compensate for that in the past by mistakenly adjusting idle mixture (after they replaced the fuel pump, etc.)
Then uncle Fred changed the timing, etc. With carbs always start a baseline using a known good fuel pump having proper volume and pressure. Doesn't have to be new, tho make sure it's within 4-7 psi and get the float level dead on. It will never run right otherwise because messing with other settings can't compensate for that.
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