Spitfire Mk3 1.3ltr choke sticking

[Rockape]

My engine starts pretty well on full choke and a couple of dabs on the throttle. 
Ease the choke in - so far so good.

But on pushing the choke knob fully home, the choke actually stays half open. Occasionally, pulling the knob out and then giving it a firm push home gets it to seat (but runs the risk of not being able to “catch it” with the throttle)

with the bonnet up, I can see the end of the throttle cable comes out of the socket - but there is no clamp mechanism. Which seems odd.

when operating the choke manually with the bonnet up, there is a definite “click” as the choke goes past approximately 25% open. But this is not repeatable - sometimes it closes smoothly.

i had thought the issue was that the bowden cable had come out of the socket (see photo) but I am not convinced. 

Any suggestions welcome…

 

Edited April 1, 2023 by Rockape
Typo

[Josef]

Well the Bowden cable there is _an _ issue. If it catches like in your photo then it will elongate the travel path of the inner cable and so keep the choke open. The Bowden cable there would be better off with a proper end fixing on which will then push fit into the little tube on the carb and stay put. 

[Pete Lewis]

the awful levers and links do get distorted and  you loose the sprung return of the jet so the  jet stays put ( down a bit) and the now spare cable rises out of its abutment 

i would have a careful look at how the linkage to drop the jet is aligned and working as best it can 

some cut a few mm off the jet fuel  tube as this is known to resist jet return if its a bit too long 

apart from some diy work there is no simple way to retain the outer case in the abutment like twist some wire around and anchor it round the carb casting 

there are no clips to help here 

Pete

[NonMember]

The bowden cable shouldn't need a retaining clip because it should (at least in theory) always be under tension. However, as Pete says, the jet linkage is well known for getting slightly bent and then sticking. This is what you hear with the "click" and it's also what causes all the other symptoms.

[Pete Lewis]

and a good squirt of grease on all the meccano links helps 

Pete

[Peter Truman]

4 hours ago, Pete Lewis said:

 

some cut a few mm off the jet fuel  tube as this is known to resist jet return if its a bit too long 

 

Pete

I had to do this on the HS6 Sprint carbs they seriously hung up with the pipe between the jet and float being to long & restricting the jet linkage, be careful the spring covered plastic/poly pipe is very thin and can be easily kinked.

[Stratton Jimmer]

The choke mechanism on the GT6 (HS4) has a return spring which I'm guessing the PO fitted. I took it off after reconditioning the carbs but put it back on as I was having the same issues as described. If needed, I will dig the car out of hibernation and take a photo. Let me know...

[Pete Lewis]

aha   the joys of SU i happen to have a 2000  with 2 HS4's  and have always had an avid hate of SU but they are working ok with no problems 

sorry Im a Stromberg Fan

take both types apart and lay out the component parts and there is an obvious big difference in their design and quality of components 

both can do a good job and after 50 years yes all can run into  problems  but the contrivance of levers on SU leave a lot to be desired 

Pete

[Rockape]

2 hours ago, Stratton Jimmer said:

The choke mechanism on the GT6 (HS4) has a return spring which I'm guessing the PO fitted. I took it off after reconditioning the carbs but put it back on as I was having the same issues as described. If needed, I will dig the car out of hibernation and take a photo. Let me know...

Thanks. If you can manage to do that it will give me something to compare with. I have a Haynes manual so I probably need to have a closer look at the diagram - it never occurred to me that there could be a missing spring. 
 

i will disconnect the choke cable and test the return free of that and see what happens. 

[Pete Lewis]

the usual  cheap  fix for sticking SU chokes is  

once warmed and off choke  open bonnet and with your best pinky give the base of the jet a prod upwards  

if it moves the mechanism is sticking and holding the jet down for super rich running 

Pete

[NonMember]

4 hours ago, Stratton Jimmer said:

The choke mechanism on the GT6 (HS4) has a return spring which I'm guessing the PO fitted.

You mean, an additional one over and above the return spring that was fitted as standard?

[1969Mk3Spitfire]

On 01/04/2023 at 17:52, Pete Lewis said:

the awful levers and links do get distorted and  you loose the sprung return of the jet so the  jet stays put ( down a bit) and the now spare cable rises out of its abutment 

i would have a careful look at how the linkage to drop the jet is aligned and working as best it can 

some cut a few mm off the jet fuel  tube as this is known to resist jet return if its a bit too long 

apart from some diy work there is no simple way to retain the outer case in the abutment like twist some wire around and anchor it round the carb casting 

there are no clips to help here 

Pete

I had the same problem of choke failing to fully return on my H2s.

I removed the link and increased the double bends to effectively reduce the length of its actuation. About 3mm iirc.

The link looks to be low grade steel and use over the years “pulls” the double bend to increase its length.

An easy fix that took me longer than it should have done to figure out.

[Stratton Jimmer]

[Stratton Jimmer]

15 hours ago, NonMember said:

You mean, an additional one over and above the return spring that was fitted as standard?

Yes - as shown in the two photographs in my post of a minute or so ago. One picture shows the carbs without filters and you can see the bracket to which the spring attaches. The second shows the spring in-situ with filters on.