automatic choke

[Glyn Ruck]

Yes ~ I accept I was generalising but I know of no SU Carb with a second choking butterfly which is what I mean by conventional. I accept that if you throttled the inlet of an SU it would increase depression over the jet & enrich mixture. No provision is made to do this. SU rather believing in moving the main jet in relation to the needle to achieve controlled enrichment where the aux carb is not present.

My Otter switch operated AED works just fine. Just stocked up on a spare Otter switch.

American cars of the era used coiled Invar springs operating a secondary butterfly for their "Automatic Chokes". They were not trouble free.

[jonesdl]

As a matter of interest Glyn is your otter switch the original that's working ok or a replacement fitted during your rebuild. Also are they still available to buy new ? in the same form or have they been changed in how they work.
Dave J.

[Glyn Ruck]

My original red Otter switch still works. I have one on my spare carbs/manifold that doesn't.

Barratts sell two replacement switches & I don't know what the innards look like. On one of the many forums out there I read of someone opening a new Otter switch of unknown source & it had modern guts. They are no longer made by Otter.

Otter switch1.JPG (58.2 KiB) Viewed 2579 times

Otter Switch2.JPG (49.24 KiB) Viewed 2579 times

[JCS]

Glyn

“……..but I know of no SU Carb with a second choking butterfly which is what I mean by conventional…..”

You might then be interested in the following………………..

From 1952 until approximately 1985 every 4.5ltr, 4.8ltr, 6.25 ltr and 6.75 ltr Rolls Royce and Bentley chassis engine fitted with S.U carburettors had an automatic choke system with a single choke butterfly controlling two carburettors. These carburettors were either thermo H type or diaphragm HD operation. The earlier (inlet manifold heat controlled) R-R auto choke system was designed by R-R but generally produced by S.U.

The system has a Zenith patent number on the thermo spring unit, but this patent was from the early 30’s era and had expired well before it was used by R-R. The stamping of the patent number was done by R-R purely to acknowledge Zenith as the original designers of the thermo spring unit.

With my Jaguar hat on, I must agree with David, the Jaguar auto choke does the job, albeit a bit crudely, my car starts on the first compression and runs evenly. It could benefit from a fast idle control allowing weakening of the solenoid control, and it will do so in due course.

To complete the picture, the Jaguar S Type auto choke system was first fitted experimentally to the 1938/39 Bentley MK V engine. I believe shortages of materials initially prevented this system being introduced post war and by the early 50’s R-R was pushing the diaphragm design purely to reduce the incidents of jet leakage from the H and H Thermo types. A lot of R-R design time was given over to the S.U HD, and SU were allowed to supply to other makers such as Jaguar and Rover.

As automatic gearboxes became available on the European production lines it became necessary to introduce automatic chokes with no driver control. A number of makers took up the solenoid / otter switch as an economic option, but it lacked some finesse in certain climates. You might notice in all the Jaguar threads no mention of the tickler pin in the lid of the front carburetter, or the reason for it being there, or for that matter the two different idling speed requirements.

On the other hand R-R designed a “compensated” system with butterfly, three fast idle positions, kick off control, temperature control, HD carburetters, magnetic hold, oil pressure control and acceleration demand. That system was the best auto choke of its time, but obviously lacked humidity control. I know that system particularly well as I was teaching it 50 years ago at Derby Technical Collage to technicians.

The American systems of that time lacked all the compensation controls that R-R (expensively) built into their design. Unfortunately they went backwards when they changed the sensing over from inlet manifold water temperature control on the six cylinders to exhaust gas control on the R-R V8 engines.

[Glyn Ruck]

Thank you very much for that. Very interesting. I knew it had to be an add on if it existed & you have explained exactly that. Choking butterflies are not inherent in any SU carburettor or design.

I guess the point is that there is no easy way to put a manual, variable cable operated choke on an S Type with HD6 Thermo's. Switch is the alternative ~ on or off.

US systems were cheap & cheerful. Rochester & the likes.

Thanks again.

[cass3958]

Very interesting article, thank you. My mate has a 1956 Bentley S1 and I know he has the SU carbs on his with an Auto choke so I will have to go and have a look at it now. He also has a 1953 Alvis which has SU carbs but a manual choke. Again I will have to have a look at how it works.

[jonesdl]

What an interesting post this has been, this forum becomes very additive with all the knowledge from all it's respective contributions, keep them coming guys brilliant.
Regards
Dave J.

[Orlando St.R]

Yes, thanks very much for that, Norman. Most informative and interesting.

[NigelW]

This is the mech I made for my XJ to get rid of the abysmal AED, it pulls down the jet levers giving a variable choke the same as the E Type.

[johngosnell]

I am not sure that I understood all that has been said, But What I now think is that I may have an issue . When I turn the switch to operate the choke I can hear the valve operate and after the pump stops its normal click The car starts first time without any throttle but after that if I try to rev the engine she cuts out but always restarts . once warm and the choke switched off she ticks over and revs as normal. I have to sit there just ticking over for a couple of minutes until she starts to warm up.


john

1B1257BW