1974 Bronco Question



I have a 1974 bronco with the 302. I am trying to restore it. It seems the person i bought it off of put a manual choke on it. I have rebuilt the autolite 2100 carb and can not get my vacuum up to 20 inches of mercury. It stays around 12-15. When i put the vehicle in drive it stalls out. Also at idle i can not get the rpms below 1400 even with the idle screw turned all the way out. Can somebody please help me 



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Oct 18, 2005
Coronado, CA
Yo Jonathon,

Sorry, my wife has been in the hospital and others seem to

 have left this site due to the friggin pop up ads.

scrapman 06:50 PM 09-03-2014

First, make sure your engine is warm and the choke is fully opened. Hook your vacuum gauge up to manifold vacuum. Start by screwing both mixture screws all the way in(just until they bottom out), and then unscew them 4 full turns out each. Crank the engine and read the vacuum. Turn BOTH screws in a quarter turn at a time, blipping the throttle after every adjustment, and reading the vacuum.

You will see the vacuum get higher and higher and then start going down. As soon as you see it drop, back up to the adjustment that gave you the highest vacuum reading. This is your BEST LEAN IDLE.

*Both of your idle screw should be the exact same number of turns out.*

 Ivy66GT 10:45 AM 09-04-2014

I would in general agree with Allen's procedure.

I assume you have an Autolite 2100. Having just done this adjustment on a 2100 less than a month ago I would suggest your starting point for adjustment should be 2 full turns from bottom instead of 4. The final adjustment is slightly less than 2 turns for my carb/engine and I doubt my 289 would even idle if it were at 4. (Ford said to start from 1 1/2 turns out.) If its running halfway decent already you can also just start adjusting from the screw positions as they are now. Even though my screws ended up being backed out by the same amount Ford allowed as much as a half-turn difference between the two.

If you can, you should also make the adjustments with the air cleaner on the carb. Even with a clean, new filter element the AFR changes considerably depending upon whether or not the air cleaner is installed. Part of that is from air flow restrictions but part is due to the fuel bowl vents (those small tubes sticking up on top of the 2100). The vents dump fumes from the warm fuel into the main air flow which richens the mixture since the fumes are trapped inside the air cleaner. Since you are concerned about how the car will idle/run with the air cleaner in position, that is how you really should make the adjustments.

Also, don't kid yourself into thinking that this will in any way be a 'lean' idle setting. You are simply maximizing RPM and therefore vacuum and the AFR at that point is around 12-13 which is moderately rich. As RPMs increase the mixture gets steadily leaner which is normal since the starting point is so rich. Unlike some other carbs, an Autolite has no true idle mixture adjustment, you are only adjusting the volume of fuel delivered from an idle circuit with fixed jetting.

BTW, the most accurate method possible for adjusting those screws is with an AFR meter. 1/4 turn on those screws is nearly a full AFR number. However, wideband AFR meters start around $250 so either a tach, vacuum gauge or your ear is the tool used most often


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