SU H4 Choke Linkage

[NickD]

I’ve recently fitted a revised choke linkage to my twin H4 SUs, which might be of interest to others who still have these carbs.

Users of twin H4s will be familiar with the compromise imposed by the choke linkage, where a choice has to made between using one or both choke levers, as explained by Mark in an earlier post at: https://mk1-forum.net/viewtopic.php?f=3&t=31377

"It is always better to use both choke levers, but the pull on the cable can be VERY HEAVY! My preference nowadays is to use both levers with a "T" handle choke cable. But if you want to retain the standard choke cable then you are probably better off using a choke on one carb. BUT IT WILL BE HARDER TO START."

I wanted to produce a linkage with a higher mechanical advantage/leverage ratio so that both choke levers could be operated with a standard-looking “Choke - Lock” cable. In the end I came up with an alternative which doesn’t look too different to the usual setup but which halves the cable load when compared to the original.

All that was required was to replace the wire stirrup (used on the standard linkage to connect the two choke levers to the cable) with connecting rods from each of the levers to the central spindle of a pulley. The outer sleeve of the cable was left attached to the C-AHT1 bracket but the inner cable was rerouted to run around the pulley and then back up to a new anchor point on the bracket, so requiring twice the original cable travel to move the levers a given amount.

Some pictures should make this clearer

Pulley.jpg

Linkage.jpg

The result is a dual-lever linkage that feels as easy to operate as the standard twin HS2 setup and which, when choke is needed, allows easy starting of the engine from inside the car rather than (on my car) having to operate the linkage from under the bonnet.

A downside is that the choke cable needs modifying as the standard inner cable is too short to reach around the new pulley.

[Spider]

Very nice work !

[AndyPen]

Ingenious, is that because the levers don't move enough with a conventional set up Nick?

[mk1]

What a wonderfully elegant solution.

I'm most impressed.

[65MK1S]

Thats some really, really nice work

[NickD]

Thanks for all the favourable comments – much appreciated.

Andy, it’s not that the levers don’t move enough with the standard system, but that the force required to move them both together is so much that the standard choke cable is very difficult to operate. On my car, the load was so high that the inner cable pulled out of the crimping on the knob assembly after only a few uses, so it’s not just the driver being weak!

[Reynard]

Nick ,just seen you post ,as everyone else a great solution and one I would like to try out ,did you arrive at the arm length and wheel dia by calculation or experiment ! Have the same issue with some H4 on my 64 S could you help please

[NickD]

Hi Reynard,

Not much calculation required. The 2:1 mechanical advantage is delivered by the principle of running a cable around a pulley to give more travel with a reduced load and gives the same 2:1 ratio regardless of pulley diameter.

The only experimentation needed was to establish what minimum diameter of pulley the inner choke cable would curve around as the inner is too rigid to follow a small diameter. For the cable I had, this diameter was 1.5". To allow a bit more freedom, and because I had some aluminium bar of this diameter, I used 1.75" (45mm). Anything more than about 2" starts to become too large to fit in the space available

The two brass links were sized to allow full travel of the choke arms without the pulley hitting the C-AHT1 bracket, and ended up at at a similar length to the original wire stirrup, at 2⅜" (60mm) c/c.

Finally, the parts were assembled using standard SU clevis pins and clips