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Discussion Starter · #1 ·
Hi,

I'm going to be adjusting the idle air/fuel ratio on a newly rebuilt Holley 4150 750DP with mechanical secondaries and electric choke.
I'm going to hook up a vacuum gauge and adjust for highest vacuum. Correct?

Do I do primary driver side then primary pass side the same amount? Or adjust each one for the highest vacuum reading even if they are different?

After both the primary sides then I should do the secondary sides, correct?(Or is it primary driver, then secondary driver, then primary passenger, then secondary passenger?)

Thanks!
 

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4 corner idle circuits are a little trickier, first start with them all out around 1 1/2 turns from seat. Then go to driver's side primary , passenger side primary, passenger side secondary, then finally the drivers side secondary. Then go around it one more time. Hopefully when you are done all four screws should be out approx. the same number of turns. If you find that you can turn the seconday or primary screws in all the way and it makes very little difference the idle mixture screw on that same side is turned out too far. Hope that helps a little.

Dave
 

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phil..just remember that carb tuning follows ignition timing.Make sure your initial and total fall into normal ranges for your application..then finish off with the carb.Many a carb have been swapped out when the real issue lies beneath.
 

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Discussion Starter · #5 ·
Thanks for the tip George.
Yes, I'm trying to do them both together. Initial timing is set fine at 16* but mech advance is still too much so I have to work on that... wish I could hear if it pings or not over the loud exhaust... I'm aiming for 32 total but currently have wayyyy more.
 

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For a ballpark adjustment on a fresh carb, I will set the timing and get it warmed up. Then I get a buddy to get in the car and put it in gear (if it's a stick, I guess it doesn't matter so much), have him hold the brake, and I turn all the adjustment screws out about 2 turns. Next, turn all 4 of the screws in 1/8 turn at a time, waiting for the carb to adjust between adjustments. Keep doing this until your idle speed starts to drop, and back them off a 1/8 turn. This should be pretty close to where you want to be, but can get better. :)

For final idle mixture turning, I make sure I have the timing set and the engine warmed, like above, then I set the mixture screws by tapping on the accelerator pump to give the engine a quick shot of fuel, but without cracking the throttle open. If the engine speeds up, the mixture is lean, if it stumbles, the mixture is rich. If there is no change, you're pretty much there. I do this by adjusting the primary side first, adjusting the mixture screws equally every time by 1/8 turn, and allowing a few moments for the carb to adjust to the settings. Then I move to the secondary side, doing the same thing. Finally I do both at the same time, adjusting all 4 mixture screws at once, but with smaller adjustments. I've never had much luck with the vacuum gauge method myself.
 
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