17084226 APT adjustment

[65malibu]

I've got a the above carb on my car and having a lean surge while crusing around 1200-1500 rpm.
Currently I've got 74 jets and Cliff's 50c rods in it with the APT out 4 3/4 turns. I started with the APT out 3 1/2 turns and it's an improvement over that. My question is, how far can I adjust the screw out before going up a jet size? The engine runs fantastic except in the 1200-1500 range when I get a slight surge.

[quadrajam]

Did you mean 50M rods?
I have several of those 80's truck carbs and they all run great with
73 jets and Cliffs 50M rods. I put the APT @ 3-1/2 turns.

Perhaps the hanger arms are bent down just a tad...??

Run a small screwdriver down through the vent and see if the
power piston is free to move up and down and not stuck down.

But in answer to your question I would go up now on the jet size.

Good luck

[Kenth]

If lean surge at 1200-1500 rpms i would rather look at the idle tubes, idle tube restrictions, lower idle air bleeds and idle needle holes than main jetting.

[65malibu]

Yes, they are 50M rods.
More info on carb/car: lower IABs-.063
                                 upper IABs-.070
                                 idle tubes -.038
                                 down channel restrictions- .055
                                 idle bypass air -.120
                                 idle mixture holes - .089
                                 mixture screws out 4 turns one side, 5 the other
                                 float height -5/16
 
engine info: SBC 406, 10.2 CR, AFR 195 heads, HR cam 229/229 @.050, .565/.565 lift, 109* lobe centers. 1 5/8" headers, HEI distributor, timing is 14* at idle/ 36* total, all in by 2500. 14.5" vacuum @ 900 rpm idle. Richmond 5 speed w/ 3.08 rear. The car is very responsive, just an occasional surge probably more in the 1200-1300 rpm. You can't hear it in the engine, just feel it in the car
I did make sure the APT arms were level with body and equal height, but never measured the height. When down, how far should the fat part of the needle be in the small part of the jet?

[quadrajam]

Sounds like your hanger arms are OK.
Like Kenth says maybe its in the idle/off idle circuit.

[77cruiser]

Does it help at all by opening the mixture screws more?

[65malibu]

Does it help at all by opening the mixture screws more?

I haven't tried that because where they are set now is the best idle.

[65malibu]

If lean surge at 1200-1500 rpms i would rather look at the idle tubes, idle tube restrictions, lower idle air bleeds and idle needle holes than main jetting.

Looking at the specs of my carb and engine, any recommendations on changes to the above areas?

[quadrajam]

Try .059ish on the DCRs. stock was .055
Might even try .040ish on the tubes.stock was .035 but first
see if anyone else will chime in with some other
ideas.

[77cruiser]

Or make the idle bleeds smaller.

[Kenth]

I would use .070" lower idle air bleeds, .095"-.100" idle bypass air and .095" idle needle holes.
Also, the total timing is in too early, i would delay to 3500-4000 rpm´s, using 12-16° vacuum advance.

HTH