Idling too fast

[morrisminor]

Any ideas why I can't get my car to idle slowly,  from cold it ticks over nicely, after about 10 miles its too fast,  idle screw and choke screw at minimum could it be air getting in somewhere ? (Zenith 26va)

[David Stepney]

I had exactly similar problems with my RP saloon. In fact I started a thread on this called 'Carburettor Problems'. The last post on that thread was 20th April this year, so a perusal of that thread may assist.

Ruairidh Dunford put me on the right track when he explained that there are two types of 26VA carb, and, if you get the wrong float chamber with the wrong carb body, then you can get an intractably fast tickover.

Once I had built a carb with matching body and float chamber, the problem was solved, and my car now ticks over at about 500 rpm hot.

[Bob Culver]

If anyone can trace out the circuits and explain the effect I would be curious.
The bowl needs be a close fit to the lid. Related Zenths had a gasket. The zinc can become bubbled giving a poor fit.

[John Mason]

I had a similar problem with my Ruby and the problem was the small jet underneath the body of the carb hidden from view by a brass screw. Care should be taken if removing it as it is easily damaged. I cleaned mine whilst it was still in the carb after removing the brass screw with a can of pressurised carburettor cleaner. The purpose of this jet which comes out in the Ventura tube is to allow air to bypass the throttle butterfly when closed thus allowing tick over.

John Mason

[David Stepney]

Bob.

There is a very good explanation of the Zenith carburettor to be found on the Cornwall Austin 7 Club website in an article called 'Carburettors Part 2'

[morrisminor]

Thank you David/Ruairidh for the idea , I found a matching float bowl but it hasn't worked, trying a new throttle spindle next

[David Stepney]

Perhaps I should have mentioned in my penultimate post that I overhauled the carburettor, including fitting a new throttle spindle before it went back on the car.

[Robert Leigh]

It can be that wear in the spindle holes allows too much air even with a new spindle. Ideally the holes need bushing when the the new spindle is fitted, and the spindle centre must be on the diameter of the choke tube, without any unintended offset. Steve Hodgson, SMH Services Ltd at Hove is a good man for doing this.
Robert Leigh

[Ian Williams]

Did you centre the disk in the spindle when you rebuilt the carb, if not the disk will be hitting the side of the body preventing full closure. Release the screws to allow the disk to move slightly, hold the spindle fully shut to centre the disk in the body, re tighten the screws and all should be well again.