SU return-spring arm positions ?

[rfs1957]

Couldn't find an answer to this with a search, nor in the Jag manual, can anyone guide me please ?

I've been trying to identify the source of what feels like excessive effort in the throttle-linkages on my 3.8

The arms (that carry the throttle return springs) can be pinch-bolted onto the butterfly spindles in apparently whatever position one chooses.

I've realised that in the 'shut' position, 2 of my carbs have the arms at about 90? to the spring - so with quite high tension and maximum mechanical advantage - and the 3rd has the arm almost aligned with the spring, so the tension is close to zero and lowest mechanical advantage.

Is there a "right" position or is it just a question of finding the lowest tug that will seat the butterflies correctly ?

[ChrisC]

by excessive effort do you mean you have a stiff throttle pedal?

Or do you mean you "think" there is excessive spring pressure? 90 degree angle is about right normally but the throttle pedal also has return springs (actually on the pedal itself)

[rfs1957]

The effort required of the quadrant and the linkages when operated by hand under the bonnet (and disconnected from the pedal and its spring) seems out of all proportion to the effort necessary to control the butterflies.

As in sledgehammer and nut.

Where is it written (anywhere ?) what the at-rest attitude should be between the arms and the springs ?

[ChrisC]

Its not written anywhere....

Operating the linkage by hand does take quite a lot of doing it has to be said its certainly not like a modern cable system and the butterflies in the carbs do "snap" shut as normal practice

Assuming you have the right springs fitted on your car and if you have the levers at 90 you are about right. It is of course possible that you have incorrect springs fitted... You should be able to take one off quite easily and drop us a picture or if you maybe to take a picture of the front carb without taking anything off at all...

The best thing to do of course is to look at another Etype. Where are you based? There is bound to be someone near you. If you get stuck I'll post a picture of my setup...

[38E]

It's more likely that your problem is elsewhere on the linkage system. The butterfly return springs are quite flimsy even x 3. The main return spring is attached to the servo.

Here's a pic of my throttle linkage which I'm pretty sure is about right since it''s not unduly stiff. It's quite easy to blip the throttles by hand by means of the arm on top of the manifold.

Regarding the angle of the spring levers, I'd think that an angle of 90 degrees to the spring ought to be about your mid-point position to ensure adequate reaction either side of that.

[Mark Gordon]

Beautiful, Clive. But where are the greasy oil deposits that Jaguar supplied as rust protection?

[38E]

Thanks Mark.
Actually, I took these pics just after I'd done some head work so I guess I'd already wiped most of the grease off by then. :)

[rfs1957]

My set-up is a stainless after-market one circa 1990 from previous restorer, no provision for the spring to the servo, but throttle return has never been an issue.

It appears there was a lot of lost movement in the drop-links, that I've since eliminated ; the linkage-arms were going excessively obtuse towards full throttle.

The spring-to-arm angle on the butterfly spindles would logically be 90? at mid-way, I agree, but what is the downside if you reduce the loading in the springs ? Throttles don't shut ?

I'll experiment once I've finished adjusting the clutch and brake pedals with the blue-tipped universal adjuster system, changed the clutch slave for a reduced-bore, made my adjustable throttle pedal, and got myself 2" of more leg-room ...................

[david muir]

Clive,
Very interested in your photo of the return spring fitted between the linkage and the servo. My March '64 3.8 does not have this spring arrangement and there is no hole in the linkage for the spring or mention of it in the parts book for a 3.8.
Was it only fitted to early cars?

David

[38E]

David,
You have me wondering myself now. You're correct, the parts list doesn't show a spring on the servo -only a smaller tension spring on the pedal itself (which I don't have).

So I honestly don't know if it's original or not. But being a very early car, it does have several odd-ball differences to later 3.8s. I've owned it since about 1975 and always assumed it was built that way -it's quite professionally done. And having restored the car over here, I never had much opportunity to compare it to other RHD 3.8s.

Anyhow, the set-up works fine and I've never experienced any trouble with it.

[1954Etype]

Don't think its standard but a great mod - particularly on LHD cars.

[Dave K]

My car a 1961 doesn't have the spring either it was fitted to later 3.8's as far as I know. Mine only has a spring in the pedal.

Dave