Tickover revs will not reduce

[Nick Salmon]

The RP has a standard Zenith 26VA carb. The spindle has no excessive movement up down and sideways but does move in and out by about 2mm.

I cannot get the idle revs to come down to a sensible level.  I have removed the linkage, the choke arm is not in contact with the throttle arm, the idle screw is not touching the throttle arm. Yet it still is idling at around twice normal revs. It picks up cleanly.

Presumably the butterfly is not closing sufficiently?

Any suggestions?

[Alan]

Either that or you have an air leak elsewhere. I'd check that the butterfly isn't bent, damaged or otherwise modified. There shouldn't really be in/out movement.

[Colin Wilks]

Are you able to achieve a sensible idle by lifting the linkage by hand in order to close the throttle (i.e. pushing the throttle lever on the carb onto the idle stop)?  I had the same issue on mine and resolved it temporarily by moving the return spring nearer to the arm (the coiled spring had crept away from the arm's pivot point and so weakened the return pressure, leaving the mechanism a bit floppy).

[Reckless Rat]

Nick,
Have you checked that there isn't an air leak at the carb/manifold flange interface. The Zenith 26VA is made from very soft alloy and the flange face can often be distorted, causing air to be drawn in raising the tick-over speed. It's easily remedied by taking the carb off the car and rubbing the flange face on some emery paper on a firm flat surface.

Failing that a slightly stronger spring on the linkage might help. I backed mine up with a coil spring between the linkage arm and the fuel pipe to the pump (the same spring as used on the brake light actuator).

[Bob Culver]

If there is end movement fully closed the butterfly disc rim is likely worn, and/or cut into the body, or is positioned wrong on the spindle. Should block the passage very completely. The butterfly is bevelled and must be fitted right way round. All would be obvious with carb removed.  On other diecast carbs, if flange repeatedly distorted the body becomes oval and butterfly will not seat. Linkage should not force spindle one way or the other. Normally, the butterfly should close on the stop, not internally on the body. The lid must form a reasonable seal above the idle jet.  If casting is bubbled, bent or battered may not. Has the idle screw fallen out?If it has vacuum wiper is that bypassing somehow?

[Robert Leigh]

The RP has a standard Zenith 26VA carb. The spindle has no excessive movement up down and sideways but does move in and out by about 2mm.

I cannot get the idle revs to come down to a sensible level.  I have removed the linkage, the choke arm is not in contact with the throttle arm, the idle screw is not touching the throttle arm. Yet it still is idling at around twice normal revs. It picks up cleanly.

Presumably the butterfly is not closing sufficiently?

Any suggestions?

Assuming you have taken the carb off to follow up the other suggestions, you should be able to check whether the butterfly is seating properly by using a thin feeler gauge around the edge. If it appears to be off centre on the spindle it should be possible to re-position it by loosening the fixing screws and re-tightening with the butterfly held properly closed. If the problem persists you probably need a different body. I can probably help with this.

[Bob Culver]

If the screws chanced to fall out of the butterfly, would produce all the symptoms and, depending where the screws ended, possibly more. An unlikley occurence and hopefully not what has happened!