SU choke cable

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Willem
Posts: 43
Joined: Wed Jun 30, 2021 11:32 am

SU choke cable

Post by Willem » Thu Aug 11, 2022 2:36 pm

Hi All, I have a small problem, the choke slowly comes out of the dash, pushing it back in again results in the engine running much less rich, not really a big deal but it does seem odd that it has started doing it since I oiled the cable. In my old Mini the choke could be locked by turning it but there is no such function on the P1. The carb is an SU
Any suggestions on how to solve this are welcome.
Will
Rover 12 Tourer, 1936
Range rover L322, 2009

SHyslop
Posts: 51
Joined: Thu Dec 30, 2021 4:17 pm

Re: SU choke cable

Post by SHyslop » Sat Aug 13, 2022 3:57 pm

Hello Will, regarding your choke cable. I'm presuming from your vehicle list that your car has two control knobs, one marked "Mixture" and one marked "Throttle". On your Mini, the functions of these are combined into one "Choke" knob. Through a set of interconnected levers, the Choke knob both pulls the jet on the SU carburettor down to a rich position to enable starting and also increases the throttle opening to give a fast tickover until the engine is warm enough to allow both a reduction in tickover and weakening to normal of the mixture. Both controls were spring loaded to return to their normal running positions.

Austins in the 1930s had the same on their Zenith carburettors and you usually found that an Austin of that period came with the handy accessory of a clothes peg because although the control had a midway notch it could be set to, this wasn't always suitable and the clothes peg enabled the driver to set the choke knob to whatever setting they wanted until the engine was warm, which could take a long time on an Austin. The locking knob on BMC cars came in the middle late 60s and they were good while they worked but after a time the spring loaded segment which held the cable could wear and the clothes peg came back out again.

The Rover is different. The two separate knobs have no interconnection. The Mixture control has no spring return and simply pulls or pushes the jet in or out. A rich mixture alone doesn't may start the car but it won't keep running due to the lack of oxygen to burn the rich mixture so the hand throttle is pulled out too, easier worked usually by applying the accelerator pedal and then pulling the throttle knob out. This increases the air supply through the carburettor .Once the car has started with the Mixture knob well out, it should be possible to return it to about a half way position and not too long thereafter, to push the Mixture control right in but a slightly higher tickover speed may still be required until the engine is properly warm when both knobs can be pushed right in.

If the Mixture knob is actually being pushed back out of the dashboard then about the only possibility would be the needle actually pushing the jet due to it not being centralised but I wouldn't have thought your car would be running very well if that was the case. However, you do seem to be saying that is what you're seeing so I am trying to think of what the cause could be.
Some other makes had spring returned Mixture controls but if your car has the same jet lever as mine, there's no provision for this, it's a straight in and out job.

Just for completeness, the cars with Solex carburettors operate in a quite different way having a separate starter device in the carburettor which is usually fitted with three positions controlled by the one knob : Off, Half and Full. There is one Solex design which has a progressive setting but so far I've only come across one of them but I'm sure they are all out there somewhere.

Do let me know how you get on as it will be interesting to know.

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