Ruby lacks power

[Dave Mann]

I have a friend with a late Ruby with a standard engine with late head and a 11/8" SU carburettor which has a GG needle, Is this the best needle for this engine? It starts and runs OK with a high idle speed but on the road my RN will see it off.

[Hugh Barnes]

GG or AN needle are the two I have used with success, though I have a 1" OM fitted, so a slightly different carb. There are other things that would affect performance, of course (though I hardly dare suggest that to you, Dave) but is timing right and, most importantly, does the throttle open to its fullest extent? The late, great Ray Walker would always do such a test. Standing by an open bonnet, he would ask the driver to put their foot on the accelerator as far as it woud go, then he would grab the throttle on the carb and see if it had any more travel.. Though I am sure you have thought of such thkngs...

[Tiger]

Fairly sure the Ruby is carrying a lot more weight
So I would expect the difference in performance

[Chris KC]

I'm pretty sure I have an AN in my Ulster with 1 1/4" SU - in the colonies at present so can't readily confirm. It is a little lean at high revs (or rich at low revs) I think but is certainly serviceable. Some day I will try alternatives...

[bob46320]

Clogged silencer ??

[Ruairidh Dunford]

Fairly sure the Ruby is carrying a lot more weight
So I would expect the difference in performance

What he says…

[Alan]

I’d kind of expect your RN to see off a Ruby, but a GG seems quite a weak needle. My Chumm works well on an AN and a 1 1/8” SU, but really you should get it properly set up on a rolling road if you are in doubt. There are so many variables that it’s safe to say that which works on my car may not on yours. A half hour on a rolling road with someone who knows SU carbs will sort it.

[Hugh Barnes]

If you chart GG v AN on Minty Lamb, one is just a tad weaker than the other, but they have almost exactly the same 'profile' over the entire range. I used GG quite happily for a number of years until I was encouraged by Chris Parkhurst encouraged to try an AN. But I do agree, someone who knows their way round an SU on a rolling road is a good approach. On a Riley 9 I owned once, an hour's work on a rolling road got 31bhp at the rear wheels up to 41bhp with just careful set up...

[Alan]

SU carbs are wonderful things. They will work tolerably well when miles out, but to get them running properly you really have to understand them.

[Colin Morgan]

So, the secret is that the specific profile of the needle allows the correct of amount of fuel to be pulled through at the appropriate vacuum, as the needle is raised and lowered?

I know of late Rubies that struggle to reach 45, and others that can do 60 - but perhaps they're not quite as nippy in terms of acceleration as an RN - because of the extra weight - the price for all that additional luxury?