72' mustang idling to fast



This last week I installed a new/reman. holley carb in my 1972 mustange coupe(302 eng., 2b carb). I have done everything by the book. i.e. checked for vac leaks, adjusted the timing, trimed down the curb idle speed(which didn’t change anything), ect… But is still idling a little fast. The only thing I have not done yet is adjust the idle mixture screws. According to every one I’ve talked to (friends and auto mechanics buddys) say that this should come preset from holley. So I’m a little hesitant to want to adjust these screws.

know that I have brought you up to speed (no pun intended) my questions are…

Will adjusting the idle mixture screws slow down the idle speed? or what else could i check for that would cause it to idle fast?


A vacuum leak (within reason of course) or a hanging fast idle cam on the carburetor can cause a fast idle.

Make sure the vacuum advance hoses are correctly routed. If you have manifold vacuum at the advance instead of ported vacuum this will also cause a fast idle.

As to the mixture screws there really is no “pre-set” on them. They may be placed at a good starting point for adjustment is all.
With the engine warmed up and idling you should run each mixture screw in one at a time until the engine stumbles a bit. Back it out until it smoothes out and continue to back it out until it stumbles again.
Keep track of the number of turns of the screw along the way. Go back in a bit more than halfway and this should put you close.

This is going to be touchy since there are so many variations, but with a Holley carb I would think about 1 and 1/2 turns out on each screw would be about right.


So first I’m going to try the fast idle screw. we’ll see if that helps.Ok asuming that I adjusted the fast idle and it didn’t solve the problem my next step would be to go ahead and turn the idle mixture screws all the way in and back out 1.5 turns(per Holleys instructions manual that I have)? I know this will make the car run richer or leaner, but will it cause it to idle faster or slower?

Thanks for the help



Changing the idle mixture to change idle speed is really the wrong approach. Are the throttle plates closing all the way when you release the accelerator pedal? I don’t know this carb. Do the plates open in syncrony or progressively. This could be as simple as a stuck or misadjusted throttle cable or a binding linkage. Excuse me if you have the knowledge to have checked these already. That is quite likely since you tackled a carb change, but I don’t have a feel for what your skill level is.


i wouldnt touch the screws until i knew everyting there was about carbs, it was a nightmare when i messed with my screws


Did you inspect/change the mounting gasket and tighten the mounting bolts in an X pattern? That seems like the most likely vacuum leak point if you just changed your carb.


The idle adjustments are not preset. Handling can cause them to be way off. The fast idle cam must be completely at the lowest setting when the choke is open. Make sure it isn’t touching the adjustment screw unless the choke is partly open.