Idle adjustment screws

[Alan D.]

I have a 67 427/435 hp. At present it has a Holley 4bbl on it. To get a decent idle the idle adjustment screws are completely all the way in. What would cause this condition. Everything else is in good working order. Also, it appears to have a good idle when cold and slightly warmed up. The screws can be adjusted out some and the idle will be better. As the engine warms up the idle get worse and the screw must be turned in. What would the causes of this be. Thanks Al Dieterle

[Duke W.]

Re: Idle adjustment screws

At idle and low speed/load fuel is metered by manifold vacuum, not venturi vacuum. So carburetors have to have the idle/off idle circuits calibrated to each specific configuration to optimize performance and fuel efficiency.

For example, if you took a carb calibrated for a SB and installed it on a BB it would probably be lean at idle and off idle and might require the idle mixture screws to be turned full rich and still be too lean.

This is a common problem when a "one-size-fits-all" aftermarket carburetor is installed on an engine. The idle/off-idle calibration may be way off and the idle adjustment screws can only compensate over a fairly narrow range.

A similar problem can arise when installing a high-overlap cam on an engine with a carburetor calibrated for a low overlap cam.

The above could be a problem, but the symptoms you report also point to a possible intermittent vacuum leak.

Duke

[Wayne M.]

Re: Idle adjustment screws

Duke, Alan --- could this also be due to a ruptured or sticking power valve ? (when cold, the leak would act like the choke; when warm the leak would result in an over-rich mixture, requiring the idle circuit screws to be closed to avoid rich stumble or stall).

Just a thought.

[Clem Z.]

Re: Idle adjustment screws

check for the idle transfer slots being open too far. they should only be open .020 at idle. any more you are idling off of them and not the idle circuit. also check for warped metering body or carb main body between the the metering body or the throttle body

[Timothy B.]

Re: Idle adjustment screws

I agree with above, check proper position of primary throttle blades showing no more than small square of transfer slot, if that. You can open the secondary blades to achieve good idle after you get the primary set correct.

If for some reason you can't adjust smooth idle with emulsion screws you have other carburetor problems like Duke and Wayne stated.