AFB idle screws.
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Joined: Apr 2001
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AFB idle screws.
How many turns out do most of you guys with 750 AFB style carbs run on the idle mixture screws?
I'm perplexed that mine runs best at 1/2 turn out, and you can't make it miss even with both all the way in.
I'm perplexed that mine runs best at 1/2 turn out, and you can't make it miss even with both all the way in.
Post Fiend
Joined: Feb 2000
Posts: 5,130
Likes: 8
From: Brea CA
AFB idle screws.
Hi Bill, I only have a 600 Carter on my fairly stock 390 and the idle screws are 2 ½ turns out. It wants to run richer, say about 3 ½ turns out but I’m trying to squeeze more mileage out of it rather than power at least for now.
Don’t you have a fairly radical camshaft in yours? What’s your vacuum at idle? If it’s below 7” the metering rods may be in the up (rich) position. This combined with a high idle air rate may cause the nozzles to discharge fuel at idle. You can try changing the step-up spring to keep the rods down at idle.
Also, maybe you have an excessive amount of throttle opening to get it to idle. With a long duration camshaft, try running as much spark advance as possible at idle. Connect your vacuum advance directly to manifold vacuum. That way you don’t have to run a ton of initial advance and burn out your starter.
Once you are sure the rods are down and the throttle is not too far open, the idle mixture screws should came back into play.
Barry
Don’t you have a fairly radical camshaft in yours? What’s your vacuum at idle? If it’s below 7” the metering rods may be in the up (rich) position. This combined with a high idle air rate may cause the nozzles to discharge fuel at idle. You can try changing the step-up spring to keep the rods down at idle.
Also, maybe you have an excessive amount of throttle opening to get it to idle. With a long duration camshaft, try running as much spark advance as possible at idle. Connect your vacuum advance directly to manifold vacuum. That way you don’t have to run a ton of initial advance and burn out your starter.
Once you are sure the rods are down and the throttle is not too far open, the idle mixture screws should came back into play.
Barry
Post Fiend
Joined: Sep 2000
Posts: 8,238
Likes: 3
From: Oregon
AFB idle screws.
I wonder if you put in leaner jets and metering rods if the case would be the same. I'm not sure how beefy your 390 is put perhaps the lean screws are compensating for the rest of the system being richer than necessary? Finally, are you using a vacuum gauge and watching your readings as you adjust the screws?
If your knuckles ain't bleeding you did something wrong.
'72 F-250 "Hi-Boy" 4x4, Dana 60/HD44, FE428 @ 400 ponies , 4-speed, custom suspension w lift, mud on black.
If your knuckles ain't bleeding you did something wrong.
'72 F-250 "Hi-Boy" 4x4, Dana 60/HD44, FE428 @ 400 ponies , 4-speed, custom suspension w lift, mud on black.
Post Fiend
Joined: Feb 2000
Posts: 5,130
Likes: 8
From: Brea CA
AFB idle screws.
Just another thought, when I put a big cam in my CJ it would not idle unless the throttle was opened more than usual. This would uncover the transfer slot and reduce the adjustability of the mixture screws. On the Holley 735 you could adjust the secondary throttle stop and crack it open just enough to let in the extra air needed. Then you could back off the primary throttle and adjust the mixture. Unfortunately, I don’t see a way to adjust the secondary throttle stop on the AFB. Another way to get extra air at idle is to drill a small hole in each primary throttle blade. I know this works on the Holley but again have never done it on an AFB or if they even recommend it.
Barry
Barry
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