SU Carbs, Whats the oil for?



Hi Guys,

I just started reading up on SU carbs and I'm stumped by the 20 weight oil that the carb uses.
I've never seen a carb that you check the oil in :LOL:

Are there any good websites for doing a little reading on the SU's?

These old carbs are rare down here. I don't think I've ever seen one in a salvage yard.
What are it's advantages and disadvantages?

The oil acts as a damper for the venturi piston. You can adjust the rate at which it opens by the weight of oil you run in the carbs. Cool!

There's a little more to it than that.

The SU doesn't have an accelerator pump like most fixed venturi carbs. With no modification, if you whack open the throttle, the mixture will go way lean and you just sit there for a while.

The combination of the vacuum piston, spring, and dampener oil form a virtual accelerator pump in the carb. Now you whack open the blades, and the oil and spring hold the vacuum piston in the throat of the carb. Since you now have a higher vacuum signal at the jet and needle, more fuel flows, richening the mixture. With the blades open, the engine starts to accelerate, increasing the vacuum signal through the venturi, raising the vacuum piston (or slide, if you remember old motorbike Mikuni's) increasing airflow.

The key is the oil and spring resist the efforts of the vacuum piston to rise, and the resultant fattening the mixture. There are (I think) three spring colors (blue, yellow, and red?) with different preload and rate; and the dampener oil viscosity variation that have an impact on how fast the vacuum piston rises.

Before I know much (OK, anything) about these carbs, I didn't inderstand why when I blipped the throttle there was a momentary stumble before the rpm picked up. Filled the dashpots with oil, and the hesitation went away.

How much oil do you put in? Well, if you overfill and blip the throttle at idle, you'll see oil squirting out the vents in the top of the dampener pistons. I could get it to shoot about 4 feet when I really overfilled the carbs.

These are really simple mechanically, with five adjustments available-
1) Jet
2) Needle
3) Needle position (adjusted by a screw at the base of the needle)
4) Spring
5) dashpot oil.

The only thing that really wears is the jet and needle, and these are easy replacements. All parts are available still (Burlen took over the rights to SU, and Curato on the east coast can provide almost any part.).

They are kind of forgiving from a venturi standpoint- they only meter as much air as needed, and the needle design and jet height set the mixture strength. There are about 350 needles for .090 jets and about 250 for .100 jets. Needles can be easily modified at home. Don't know much about the .125" jets.

Factory sizing was about 1.5" throttle for 900cc (usually feeding two cylinders of 450cc each) and 1.75 throttles for 550cc to 750 cc cylider displacement. 2" throttle are not common. Carbs are sized by number of 1/8" OVER 1"- HS-4 is (4 x 1/8) + 1 = 1 1/2" Latter carbs are Metric sized (HIF-44--> 44mm = 1.75in = HIF-6)

Carbs are commonly available in HS and HIF type. HIF have mixture compensation for hot feul, but are considered less adjustable by many. There are other types such as H, HD, but thee are rare in the US
More info. Forgot to put it in my earlier post.

Look under google for SU carbs. There's tons of stuff under Scions of Lucas, Vintage Triumph, and I think the Morgan guys have some pretty top notch material (this is from memory). is my personal favorite.

Haynes published a sidedraft carb book- weber, dellorto, SU, and ZS- somewhat comprehensive.

Carbs came on basically on all the English stuff before 1975, when they all pretty much went to ZS (considered by some to be an inferior carb, bit it's just different and has different defect and failure modes.) Early 70's Volvos, and, interestingly, the early 240Z and 260Z Have Hitatchi SU's built under license (Round dome type- avoid the square 'tuna can' versions at pretty much all costs). I personally believe the Japanese carbs are a little better thought out (banjo fittings rather than hose and worm clamps- that kind of thing- the typical Japanese attention the detail of the period.) Z Therapy in Salem Oregon does some interesting stuff with roller throttle shaft bearings in lieu of the standard bushings.

One final item of interest- The Z-car carbs are really not copies of HS-6's- they're really about an HS-6 1/2, if I scaled the throttle bore correctly.

On size- Most vintage racers use HS-6's when feeding 2 450cc cylinders, SCCA racers are constrained to the HS-4 carbs and bore the bridge out for flow. Makes better WFO power, but the bridge lets the car idle and run better at low RPM.

Advantages- Simple to set, simple to tune, you can probably get the car running with almost anuy needle & jet combination (although you'd be wise to contact someone who sets these up and give them some displacement information.) Carbs are almost too simple.

Disadvantages- These carbs are set up to open at the same time. You need something like a unisync tool (about $40) to sync multiple carbs up. Biased needle types wear the needle and jet out in about 30,000 miles so mixture goes out of kilter- less of a problem w/ fixed needles. Rebushing is a PITA. but not a big deal.

If I was to think about putting these on a log head, I'd get two or three off a 240Z. Jetting would be close enough w/ a 2 carb system to use as is- would probably require to be fattened w/ a three carb setup. I'b mill a flat in the side of a log, bolt an aluminum plate fabbes to accept the SU similar to the Offy deal.