TR4/4A SU carb Lean [SL] needle for TR3/4A?

What engine or carb conditions would require the use of the Lean needle in the H6?

High altitude? Living up in the mountains is the only thing I can think of don't carburetor cars usually run rich in the mountains if tuned for lower altitudes.

Or if the sockets that the needles fit into were worn out from use, and the owner or mechanic didn't want to rebuild the carbs, but did have a couple of fatter needles on hand.

Since altitude can be compensated for with a twist of the nut (I do it every summer) I don't see that as requiring a new needle.

Since replacing the jet is easier and about as cheap as replacing the needle I also don't see that as an explanation.

My impression is that the change is in the profile of the needle, causing a mixture change at some point in the range -- possibly due to specific engine mods such as a non-stock cam. But all this is way over my pay-grade.

Well it appears that I have enough new spares left over from other H SU's for a rebuild that I wouldn't have to order a kit. Except I don't have the standard needles but do have a new set of SL's. I wonder what would happen if I put the SL's in a worn Jet assembly for a stock engine? Would the fatter needle compensate for wear?Would I be able to adjust out any issues?Would there be a downside? And no I'm not that cheap that I can't afford a stock jet and needle. Just wondering?

I think you can certainly try them as the result should be somewhat measurable.

One typically sets the mixture at idle then in some fashion (plug color, Colortune) examines the mixture at speed to obtain final adjustment.

I believe using a "lean" needle should give you somewhat better fuel economy, at the expense of poorer low-range power. I don't have my SU reference material at hand, but as I recall the "SL" needles are only leaner through a few stations (like 3-5) and then as rich or richer than the "SM" needles at both idle & full throttle.

They're easy enough to swap, so try it out and let us know!

I'd try them myself except all my needles (even the new ones) are mixed together and I don't know which ones are which. Some fine day Real Soon Now I'll measure them and find out.

Oh wait, I do have an ancient DOS program with the SU needle database ... here is it's output:

Thanks for the info Randall. I'm rebuilding a set of SU's to keep busy during the winter months as I dont have access to a heated garage. In the Spring they will go on a 4A project car that has Strombergs on it. I'm more familiar with SU's and dont want to go into a whole new learning curve on Stroms. The SL needles should work good enough in the SU's for initial starting and engine tinkering as the car wont be on the road for sometime afterward.

Karl, RE learnign curve for Strombergs, for the ones you find on TR250s and TR6s and later MGs and Spitfires and such there is a learing curve, for the early CD series as found on the TR4a, if you understand SUs you can understand the early Strombergs, they are very simple and work on all the same principles. The Strombergs get a bad rap because the later ones are more complicated to adjust, but the early ones have an idle screw on the side and a mixture nut on the bottom and a float (under the carb body instead of to the side) and a choke wich is a little different but you can see how it all works just by looking at it, as the old commercials used to say, try it, you'll like it.....

If you understand SUs you can understand early strombergs by a quick inspection.