• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Opinions on idle circuit carb adjustments

hunt2elk

FBBO Gold Member
FBBO Gold Member
Local time
2:52 PM
Joined
Jul 15, 2009
Messages
10,540
Reaction score
11,692
Location
WI
For you guys running a 4 corner idle mixture adjustable mechanical secondary carbs. Do you back all 4 screws out evenly, or open the fronts up more than the rears? And also to go along with that question, do you adjust the primary idle speed screw in more than the rear? I have a Quick Fuel Q750 with everything adjusted evenly front to back, but have recently heard of people running theirs with the primary side set open 30% more than the rear. Supposed to get a crisper takeoff. I have tried both ways, and really can't tell a difference.
 
Maybe I'm wrong, but I adjust them by turning in the screw until idle drops, back it out until idle comes up. Then turn out 1/4 turn. I start at the primaries, then the secondaries. I do it a couple times to be sure they are set to highest RPM, then reset idle speed. Usually I don't have to make any big changes, unless something is wrong. I'm not an expert but my car runs well,so I don't mess with it.
 
Maybe I'm wrong, but I adjust them by turning in the screw until idle drops, back it out until idle comes up. Then turn out 1/4 turn. I start at the primaries, then the secondaries. I do it a couple times to be sure they are set to highest RPM, then reset idle speed. Usually I don't have to make any big changes, unless something is wrong. I'm not an expert but my car runs well,so I don't mess with it.

This is a good approach in my opinion. I'm now adjusting at "6 corners" and this is what I'm doing until I can get an oxygen sensor(s) installed. I've been turning my idle down really low (600rpm) so I can better hear idle changes. I go corner-by-corner adjusting the idle mixture screw until my rpm's/vacuum are at their highest. Then I turn the main idle screw back to 600rpm or so (down) and go on to the next "corner" and repeat. Once you've optimized the rpm/vacuum at all 4 corners, set the idle speed screw where you like it and you're done.
 
This is a good approach in my opinion. I'm now adjusting at "6 corners" and this is what I'm doing until I can get an oxygen sensor(s) installed. I've been turning my idle down really low (600rpm) so I can better hear idle changes. I go corner-by-corner adjusting the idle mixture screw until my rpm's/vacuum are at their highest. Then I turn the main idle screw back to 600rpm or so (down) and go on to the next "corner" and repeat. Once you've optimized the rpm/vacuum at all 4 corners, set the idle speed screw where you like it and you're done.
So you don't really worry about having all 4 corners backed out the same? The reason I am asking is because I have read the Holley books and they all state the 4 screws should be out an even amount.
 
Maybe I'm wrong, but I adjust them by turning in the screw until idle drops, back it out until idle comes up. Then turn out 1/4 turn. I start at the primaries, then the secondaries. I do it a couple times to be sure they are set to highest RPM, then reset idle speed. Usually I don't have to make any big changes, unless something is wrong. I'm not an expert but my car runs well,so I don't mess with it.
I will try it this way and see how they end up.
 
I basically set my rear throttle blades to set just where the small amount of the tarnsfer slot shows and then I leave the sceondary alone. Most carbs should come with the secondary throttle blades set with the right amount of transfer slot showing. I then also set the primary throttle blades the same. Then I turn all the mixture screws in all the way and back them out 1-1/4 turn. Then I put the carb on the car and after warming it up and setting the timing I then adjust each mixture screw one at a time for highest vacum. Then I go back and tweak each one for good vacum and idle and then last set the idle speed and hope it dont have to be moved much and that it keeps the throttle blades where the right amount of transfer slot shows. Sometimes it takes alot of work and tweaking to get the right idle speed and mixture with the throttle blades at the right angle. Many newer carbs have an air adjusdment in the middle below the air cleaner stud so you can add or take away some idle air and change the idle speed some so you can get the throttle baldes at the correct angle. Ron
 
Many newer carbs have an air adjusdment in the middle below the air cleaner stud so you can add or take away some idle air and change the idle speed some so you can get the throttle baldes at the correct angle. Ron

Is this adjuster accessed from the top of the carb? I really like this carb, but the only complaint I have is there were no instructions at all included. I even called Quick Fuel and was told they don't provide any because it is a race carb. So because I am not a racer, I am learning as I go.
 
Is this adjuster accessed from the top of the carb? I really like this carb, but the only complaint I have is there were no instructions at all included. I even called Quick Fuel and was told they don't provide any because it is a race carb. So because I am not a racer, I am learning as I go.


On the ones that have worked on yes you can get to it from the top of the carb. I dont remember which all models have it but I believe my buddies Demon did. You just remove the air cleaner stud and then down in that hole is an air adjustment screw. Many racers would drill holes in the throttle plates to give the eng more air so they could lower the throttle blades to be in the proper posistion with the transfer slot. Holleys even come with 1/8" holes in some of their throttle blades. Now when a carb has the air adjustment screw below the air cleaner stud you can give the eng more air to raise the idle speed and not have to turn the throttle blade idle speed screw. That way if the idle needs to be higher or lower once you have the throttle blades set right in relation to the transfer slots you can leave the throttle blades right where they need to be and tweak the idle speed if needs be by the idle air screw under the air cleaner stud. I believe Demon carbs have the air speed screw and some others but I am not sure which ones have it. Ron
 
Auto Transport Service
Back
Top