JoeMacgruder

does anyone know where to find a detailed explanation on how to adjust mixture on a pre-1975 q-jet?

sray454

I've been reading lately that the best way to set the idling mixture is by using a vacumn gauge, and adjust the needle valves 1/8 at a time, going from side to side - to get the highest vacumn reading.

I haven't triied it yet, but plan to this weekend because I gotta get smog checked real soon.

Hope this answers your question.

R. Bruno

Where is the vaccuum guage connected to do this?

Rockn-Roll

Connect the gauge to Manifold Vacuum.

Here's the process:

Initial settings are to GENTLY screw the idle mixture screws in until they lightly seat, then back them out three full turns. Next, start the engine and bring engine up to operating temperature so the choke opens full. You may need to adjust the idle speed to 700 RPM.

Initial adjustment is with the engine running and the gauge connected to manifold vacuum turn ONE of the screws in 1/4 turn and watch the vacuum gauge to see if it climbs...it may not move until you turn it in about 1-1/2 turns so wait about 10 seconds and turn the screw in another 1/4 turn...repeat until you get the vacuum to increase some...idle speed will also increase so adjust the idle speed screw to keep the RPMs down around 700 or else with higher RPMs the main metering will kick in and you won't be running on just the idle system. Now that the vacuum has increase only turn the screws 1/8 turn at a time and watch the gauge until it starts back down...when that happens back it out 1/4 turn, then do the same with the other idle mixture screw and then you are done.

Main metering mixture is accomplished by changing the jets and using an exhaust sniffer to detect a rich mixture (high HC) or lean mixture (presence of O2) and changing to different size jets as required.