Unknown Adjustment on Carter AVS?

[plum70chrgr]

Can anyone tell me what this screw adjusts and how to adjust it?
Carb is a 4640 Carter AVS which I just purchased. I'm familiar with AVS carbs, but until now never knew there was an adjustment screw here that is typically capped off.

 

[HALIFAXHOPS]

@Woodruff Carbs

 

[Dragon Slayer]

That controls a bleed that leans the carb off idle. It was factory set and capped. I found this years ago but can't validate it. You can find a lot of tops with this bleed removed and plugged.

 

[fesser]

What D.Slayer said...

 

[Geoff 2]

Very interesting, thanks for that.

 

[plum70chrgr]

That's some good info, thanks guys. I measured mine and he top of the screw is .180" down from the surface of the carb.
Not sure where to go from here. Think I'll start off leaving it where it is and see how the carb performs.
Any thoughts as to what symptoms I might see if it's too far in or out?

I do have another AVS carb with it blocked off, but its the smaller cfm version, wonder if might be adjusted the same?

 

[Dragon Slayer]

Moving it in, will block the passage and less air goes into the idle/transistion slots when the spring and ball are exposed to vacuum. So that keeps mixture richer. If you move it out the needle exposes more port and more air can come in leaning the transition. I guess it would take an AF recorder to really appreciate what is happening if everything else on the carb is good.

 

[Woodruff Carbs]

I just responded to your pm. As said above it is for idle bypass air. Factory calibration is usually fine except in the case of a longer duration cam. In that case, I like to see the idle feed restrictors enlarged slightly also to bring your mixture screws back into range.

 

[Dragon Slayer]

Dave when you say IFR for a Carter AFB/AVS are you talking about the idle fuel bleed being enlarged, or do you mean the internal economizer size which controls/restricts idle and transition mixture?

 

[Woodruff Carbs]

When you pull the booster, 2 tubes are pressed in from underneath. The smallness is necked down to form the IFR. Typically between .030 and .039 in a Stock AVS

 

[Dragon Slayer]

Idle fuel jet. Got it. Thank you.

 

[RJRENTON]

That controls a bleed that leans the carb off idle. It was factory set and capped. I found this years ago but can't validate it. You can find a lot of tops with this bleed removed and plugged.

View attachment 1411419

The device was an adjustable air bleed that works on differential pressure in the off idle transition circuit. It was factory set during initial calibration and also functions during closed throttle deceleration to prevent rich mixture during coast down. It was part of an emission control system to prevent overly rich condition and high CO readings.....it also created an off idle stumble in some applications. Explained in Chrysler's service technician book #68-2.
BOB RENTON

 

[Woodruff Carbs]

Thanks Bob. I’d say if backing it out stops an off idle stumble on a stock motor, then it’s only because they run leaner on today’s fuel. Which is cured by a slight increase in the size of the IFR. Or there is accelerator pump issues. Assuming ignition timing is sufficient.

 

[Dragon Slayer]

I wouldn't use the term adjustable, since it was factory set and sealed. Variable may be a better word. Bottom line is that as vacuum was applied to the circuit when throttle blades opened off idle, the pressure difference pulled the ball against the spring and opened the port allowing more air into the transition/idle mixture. The needle plug setting determines maxim air allowed.