SU HS6 on GT6

[NonMember]

3 hours ago, Colin Lindsay said:

The hole in the centre matches a Stromberg 125; too small for the 150. The mounting holes also line up with the Stromberg.

The other two line up with the mounting holes for a SU taken from a Morris Minor, which would make it an HS2 AUD13, also 1.25 inch. 

Which is as Peter said. It makes sense because the angle between the holes means it's intended to mate up a diagonal flange to a vertical one, and the HS2s are the ones with vertical flanges. So are they for fitting 125CD on an HS2 manifold or for fitting HS2s on a 125CD manifold?

[Colin Lindsay]

2 hours ago, NonMember said:

Which is as Peter said. It makes sense because the angle between the holes means it's intended to mate up a diagonal flange to a vertical one, and the HS2s are the ones with vertical flanges. So are they for fitting 125CD on an HS2 manifold or for fitting HS2s on a 125CD manifold?

Now THAT is the question! I had twin 125s on an Alexander manifold, but never thought when I had the twin-carb 948 of changing the SUs to Strombergs, as it ran fine on those.

EDIT:

You'll never guess what just came up on eBay, although the seller doesn't know what it came from. However, after many years of owning mine and never seeing another...

[Richard Heath]

Hi all

Thanks to both the advice and the parts found here I have acquired a GT6 manifold, opened out the inlets and mounted the carbs with adapters and spacers. My issue now: how to connect it all! Any advice on the best solution for brackets and linkages? The throttle controls I had previously look like those fitted to the Mk3, but I can't see a way of making the bracket etc work for the SUs. For throttle and choke linkages, is solid steel bar (7.5mm?) the best bet? Thanks.

[Clive]

I used 8mmbar, and filed a little off the ends. 

I used a bit of flat plate, big hole and used teh servo blanking plug to secure it. Not ideal, but it worked for 13 years for me.

I hadsome SU linkages, used on the new longer 8mm bar, in the middle I welded a bit of flat bar "on edge" to act as a lever. Length is crucial to get the pedal action. And I set it so full throttle te var was pointing up a little, at idle it was pointing down significantly. Better would be a "snail" as used on all modern cars, so you get a nice progressive throttle.

(towards the end ofthis article somebody has made one  http://www.fastgrandad.co.uk/documents/Throttle cam and roller boss.pdf   )

[Ian Foster]

Hi Richard

I have a spare late long branch manifold with HS6s and have attached a couple of photos of the linkages. 

The throttle attachment to the link rod appears to be a clamp type so could be fitted at the rear of the rear carb if you have sufficient spindle projection.

I have also included a photo of the arrangement on my GT6 showing how we have replaced the original rod linkage with the cable arrangement from a later Spitfire.

Hope this helps

Ian

 

[Nigel Clark]

If you take another look at the photo I posted of my GT6 HS6 installation on the previous page, you can see how I sorted the linkages.

The throttle cable sits on bracket made from steel tube. The ends have hammered flat, with two 1/4" holes for bolts to the manifold in existing tappings just ahead of the rear carb. The top of the bracket has been bent at a right angle,  with a hole and slot cut for the throttle cable. The rods between the carbs were cut from 8mm stainless bar and fitted with normal SU cranks at each end. The throttle cable attaches to a lever sourced from eBay.

There's an extra return spring with its top end attached to the throttle cable lever. The lower end (out of view) hooks onto homemade bracket down by the exhaust manifold. The top of this bracket fixes to the inlet manifold via the same pair of bolts as the throttle cable bracket.

The choke is easy. The cable runs through the guide on the side of one of the carbs then pulls on a standard SU crank, connected to the other carb by more stainless and a crank.

Tricky to describe in words, but not too difficult to make and it all works! 

Nigel

[Nigel Clark]

I've just found a close-up of the throttle linkage on my HS6 converted GT6, hope it helps. Ignore the knackered rubber carb mounts, they've been replaced!

Nigel

[Richard Heath]

Hi all

An update and further request for advice. Having opened out the manifold, acquired all the relevant parts, fashioned linkages etc I finally put it all together and turned the key. The engine burst into life, and once up to temperature I set about tuning up. However, shortly after that I noticed petrol pouring out of the front carb. I took off the float bowl lid, checked the valve was operating properly, reassembled and started up again..only for the same thing to happen, though this time on the rear carb. Whilst mopping up the spilled fuel, I realised there had also been leakage from the union at the fuel pump. I am obviously keen not to repeat this experience.

I am guessing this is an issue of excess fuel pressure - I gather SUs are more sensitive in that respect. Is fitting a spacer between pump and block the way to go?

Many thanks

[Nigel Clark]

SU carbs aren't unduly sensitive to fuel pressure, and the standard mechanical pump shouldn't produce enough pressure to overwhelm the needle valves in the carbs.

There should not normally be a spacer between the fuel pump and the block. Two questions come to mind:

- is the needle valve okay or does it need replacing?

- is the correct fuel pump fitted?

I would start by fitting new needle valves (Viton tipped aren't expensive) and checking float heights.

Nigel

[NonMember]

While Nigel is correct in what he says, there is something he omits, namely that a lot of reproduction fuel pumps generate far too much pressure. Have you fitted a new pump?

[Pete Lewis]

if you can fit a gauge  you need a max of around 2.5psi     as Rob says many repro push out  over 4psi and this overcomes the float valves

I have found on my HS4s  SU plastic floats a small wear line ( under a magnifyer)  in the valve contact pad  can jam the valve needle and this was small . and not easy stuff to fettle 

ended up with two new floats   and the repeat floods stopped , 

Pete