Auto choke releasing too early

It could make a lot of difference. I have had a few different Jaguars with SU carbs fitted, and an auto choke facility implemented by a thing called an "AED" or Automatic Enrichment Device.

It's basically a small electric 3rd carb sitting between the 2 normal ones, and when the motor is cold a solenoid operates to open it up and let some extra rich juice in to the inlet manifold. As you may have guessed, the solenoid is dependent on the temperature sensor.

If your car is fitted with anything like this then messing around with the carbs is a dead end.

However, I have only ever heard of them shutting off too late rather than too early.

Without further information any speculation is a bit pointless though

If it is that sort of device, a short-term fix may be to wire a switch in parallel with the temperature sensor. That would, in effect, convert it to a manual choke - but only in the on/off sense rather than the progressive effect which you got in the old days with cable operated chokes.

How does the choke normally work? Does it have a heating element?

Just wondering if a voltage regulator might have gone awry and it?s receiving a full 12V rather than a regulated down 9V say? (All pure supposition but just trying to work out why it should be opening too fast).

Tim

This document may help:

It describes how the auto choke works on several different varieties of SU carburettor.

Thanks for the replies, guys. There's not much I can do other than post a few pages from the workshop manual, as the way this works is far from simple. I mean, the mixture is enriched largely by a butterfly valve in the main air intake - that much is simple - but how that valve is controlled is quite complicated. I'll try to put up the relevant pages in the morning if time permits, but I'm not sure they'll help much!

Yeah, why not - obviously the make and model of car is some sort of secret.

Completely irrelevant.

The mixture screw is connected to a bi-metal strip which alters the mixture for the choke. It should never require turning by more than a quarter of a turn in or out. If you try to richen the mixture by screwing it too far in, you will bend the bi-metal strip which will weaken the mixture. (and once bent you will never be able to set it correctly)

Hardly. Some SU models had their automatic choke adjusted by their ECU. Some had a bimetal spring.

Why be so awkward to someone who wants to help you?

Need more info. There are lots of ways to do an auto choke with SU carbs.

Early Rolls used a temperature controlled flap valve in the intake - bit like a conventional choke. Which also did fast idle. Jaguar and Rover used a separate starting carb. Rover SD1 a similar idea but called a FASD. (The SU starting carb didn't have a good reliability record - the FASD much better. Rover went back to manual choke after the SU one, until fitting the FASD)

Last versions of the SU had the mixture controlled by an ECU feeding stepper motors.

It is very relevant.

Because I don't want the thread to get de-railed (as will become apparent in due course no doubt.) I wanted to keep it general, otherwise some people who might otherwise have chipped in their hunches will think, "oh that's too exotic for me I wouldn't know nuthink about that."

Thanks, Roger. In the mean time, I've posted all I have on the subject from the workshop manual here:

But I very much doubt it will help!

Nope. Counter-productive as will become clear in due course....

It's Doom. Very secretive about everything. Everyone is out to get him.

Thinking on the Jags I've had. Carb ones had the SU starting carb -AED. They went injection after that.

Rover used the same unit. Either the Jag factory manual had full overhaul details or the Rover one. Can't remember - but the other said just fit a new one.

If I remember the AED from many years ago, it was a solenoid controlled by a thermostat. Bodge was to add a switch, bypassing the thermostat control.

IIRC, Burlen have re-manufactured them, using original tooling. Won't be cheap, though.

When do you think we'll find out? When is due course?

OK that isn't the one I had info on.

Junk the whole lot and fit fuel injection.

Well, he's shown us an extract from a workshop manual which shows two SU carburettors serving an engine with two banks of cylinders - possibly a V8. That may narrow it down a bit - but not a lot!

I spotted that after my post. It seems there is a solenoid and a standard bimetallic spring with a cam to set the idle speed.

The SU doesn't have a separate enrichment system to go wrong [1], so this suggests:

1) The bi-metal coil isn't as effective as it was. It has relaxed so relative positions has moved to indicate a hotter than actual temperature. 2) The fast idle cam is maladjusted 3) Some other effect that reduces engine efficiency, such as incorrect ignition or camshaft timing.

All these things could require additional throttle. However as the condition occurs after 30 seconds (assuming no over-enrichment to cause revs to drop through chugging/missing) it does look like (1).

[1] I recall a Ford VV carb that over-enriched during the warm-up period due to a damaged O-ring. Resulting in some chugging.

It would have been so much easier to have posted the type of carb. I thought ones of this type were fitted to Rollers.

A crappy manual can be found here:

the similarity with the pic supplied by CD!

Why was CD so worried we might find out what he drives?