What RPM to set Idle Mixture

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69Chrgr

Charger/Cuda
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O.k., another question for my buddies. At what RPM should I be checking my vacuum for setting the 4 corner idle on my QF 750 or 950? I have been doing it a 1000. It has a large roller cam as well, 238, 242 @.050. .555 @.585 lift. Trying to get the thing close before I head to the track.
 
This is the way that I do it, I get the engine running at a acceptable idle. Then use a vacuum guage and connect it to a good source of intake vacuum be it at the carb or the intake manifold. I turn the idle mixture screws in til I get the highest reading then go to the other side of the metering block and do the same. This will put the carb at the leanest point at idle, if your going to race the carb then screw the mixture screws out til the vacuum falls and then screw them back in til the vacuum it at it's highest point. There will be a area that the screws seem to have no effect, between the richest and leanest points, but the vacuum will fall in either case. And in either case bring the vacuum back to the highest point depending if your racing or street driving.

This is usually about 1 1/2 to 2 1/2 turns out for racing and 1/2 to 1 1/4 out for street. This will also be dependant on the size of air bleed that you have on the idle and power circuits. This should get you close to take it to the track.
 
I find a vacuum gauge invaluable. I have one in my Pro Street car so that when I'm driving I can economize by cruising at a speed that shows the highest reasonable vacuum.
Although I have used 4-corner idle carbs, I have never really found them necessary. This is how I adjust them: These carbs have an idle speed screw on the outside of the secondary throttle shaft on the choke side. Shut down the secondary throttle blades completely and close the curb idle screws in the jet block on the secondary side. With only the primaries functional, do what the man says with the vacuum gauge. Make a note of where the idle mixture screws are (how many turns). Remove the carb and drain the fuel so it doesn't leak on you and burn your scrotum! I use letter drills to gauge the opening of the throttle blades. Once you have those two pieces of info you are ready to cut them in half. Those final figures are then applied to the secondary side of the carb. The front and rear are matched. Put it back on and fire that mother up. Check the total vacuum reading. I have found that there is little left to do. If you make any idle mixture screw adjustment, try to do it on the secondary side and don't compromise the primary. Why? Because the primary side determins the "driveability" you will have. The secondaries don't operate until the mechanical slip link pulls them open. This leaves 30 degrees of primary throttle opening that makes the car driveable off the idle circuit. Again, I have never really seen a need for 4-corner idle. It came about due to deficiencies in intake manifolds with poor fuel distribution and it helped idle quality. You just don't find this problem in today's manifolds. I hope this all makes sense and helps you.
Pat
 
Now with all this info you should be a tuning guru to your buddies and later you can charge for your knowledge. LOL

Good points Pat, never thought of doing it that way or with letter drills because every engine is different. The way that a engine seals and makes vacuum will require a different setup, but then this is where you really need to have a good machinist, because he will either make or break your build. But your way will give a very good base line setting and then they can go from there. Kudo's Pat.
 
You have my respect too and I really like your Mopar! I learned a lot from Holley engineer/guru, Chuck Gulledge, a dear friend that I lost to cancer. My old friend, Barry Grant, (BG Fuel Systems) and I used to do carbs in St. Petersburg, FL before he quit FloridaPower Corp and moved to Georgia. Could you email me some pictures of your car? I'd like to put it on my office wall.
Pat
[email protected]
 
Sure which car do you want? Go to my web site and take a look and the let me know. I have a 3 drawer dresser full of pictures from race cars that I've driven or own. 30 years gives out alot of pictures.
 
Thanks BJRracing. I like the picture with your avatar.
Pat
 
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