SU carbs question


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Is there any difference in function or versatility between HIF6 and HS6 carbs? Could you conceivably run a mixed set, if it helped with space (clearance) problems?

Thanks, Adam.
Technically, yes you could. On a triple SU, the needles have to run the same base jet, and the same needle profile from root to tip, and the same spring rate in the dash pot. The drop of the trottle side must be the same when you raise it 1/8 th of an inch. The HIF usually came with a bimetalic strip, submerged in the float bowl, to correct for temperature induced densitychanges in fuel, to run an enrichening device. If you used, say, an HIF 44 to clear the spring tower brace in a six, then you could leave the strip in it. The two end carbs could be HS6 1.75". But only the swinging needle types, because the earlier fixed needle type had a few differences in the needle and jet.All HIF's are swinging needle, with a spring tha grounds the needle against the jet. This is why later carbs wore more than the earlier ones with fixed needles. The thing is Skinner Union found that service on the earlier types was awful, so they incorporated this change in the emissions era to gain some consistancy. The 90 thou jet is best, and you often find these on P76's , 2262 Marinas, Kimberly/Tasmins. The smaller jets are too small, but the 125 thou jet is down-right rare! (I've got two SU HD8's from a Rolls Royce 4 liter engine which was stamped AH for the planed Austin Healey 4 ran a 125 thou jet. Perfect for a 250, but too big for a 200. A worked 221 just might handle one!)

The other thing is that often the back carby runs rich, while the front runs lean. If you just grabbed the three carbs, you could use a syncro test to even the idle vaccum, then a special glass spark plug to test the richness at various rpm levels. Then invest in drilling you extractors to fit a 1/4 BSP fitting to hold an Exhast Gas Temperature gauge. This is a platinum resistance thermometer a little more advanced than a wide angle O2sensor. Cost is often 200 smakers, but you may need a Fluke meter or voltmeter to calculate the resistance. Use this to check the temperature at operating conditions. You don't have to buy six of them, just grab a sensor and over the same road do some wide open throttle runs and have a passenger log the peak EGT temperatures.Then swap the sensor over, and do another bunch of runs. Often the front runs lean, so once you've found the troublesome cylinder, you can adjust the profile of the needle until you've ended up with something that won't hole a piston. You can then tune each carb to suit produce the same EGT temp about 1250-1300 deg F is ideal.

David Vizard, British mechanical engineer, thinks that individual maximising of cylinder performance is the ideal situation, even if there is a variance in compression ratio, fuel delivery, or even cam profile, overlap, lobe centre between cylinders. As long as they are on the same team, Adam, its going to work out fine!
Aha! I haven't acquired ANY SUs yet, just trying to sidestep my usual knack of buying the least useful parts... :(

I thought that the later HS6 carbs were temp compensated, too. I know the theory about balancing at idle, heard of the Gunston's plugs, and had a poke around the SU/Midel site to see what they currently offer.

Has the HIF44 in fact simply become the HIF6?

As for the exhaust temp, that's a good idea, but a platinum thermocouple? :shock: Hope it looks good on a chain around my neck when I'm done, LOL.

Typically, for a driven car, how much/often does the bias spring needle setup require retuning to allow for needle wear?

See! More questions! :D

Cheers, Adam.
Yeah, the thermocuple has apeared on Car Craft and Hot Rod articles. We used to use them a t the laboratory to control and calibrate oil bath temperatures for Bitumen testing. Thet can just hook up to a voltmeter, and the platinum tip will cahnge resistance as temperature changes. That's all an O2 sensor is anyway. Problem is , a stock O2 sensor won't tell you how rich or lean your are, and will expire in the first 10 minutes of use that close to the exhast valve because of the intense heat.

Yes, HIF6 is the more common than the metric HIF 44. The last of the Austin Metros had them. After 1986, most SU HIF carbs got electronic controls and more emission tweaks to hold up the cataylist regs introduced by Germany in 1986. These are best avoided, but distance from England has solved you of that problem!

Your ability to find an HIF6 will come from me finding the last O-series 1700 cc Marinas the Kiwis assembled here, and sending it to you by mail. Sing out if you want one. They're a dime a dozen over here. But HS6's are what you'll find in Oz.

The bias needle wears out enough to hurt performance after 50 000 miles acording to David Vizard. The brass wears the jet first, then the needle, and this needs jet replacement at this point. But there is less variance between any two cars set up with a fixed needle than a spring loaded needle, and that's why SU went on that road. The dash pots are also in need of attention. Use the correct grade oil, and police the free drop every 3 months. Screw up on the littlest detail here, and the performance suffers.

Zenith Strombergs are similar, but you can't tilt them as much as HS6's, and the CD175's are often difficult to get parts for. The Rover 3500 and Range Rover used them, as well as 202 XU1 GTR Toranas. They are less burdend by dash pot problems, but the diaphrams wear out often.
re Xecutes comments on jet size-

Most HS6's (at least in the states) come w/ a 100 thousanths jet, and many high rpm HS4's (usually mandated by racing sanctioning bodies) are convert to 100 from 90 thousanths. While the needle selection is not quite as good with the 100's, I was always running lean w/ a #5 or 6 needle in a 90 jet in an HS4 feeding 918cc w/ a big cam. I've since switched to 100 jets, and while I haven't got it mixture right yet, I feel much more comfortable filing the needles designed for 100's down as they start .01" thicker in diameter, and are a bunch stiffer.

Not much use of the .125 jet- I'm only aware of a Jaguar application in an HD(?)8. You'd probably be severly undercarbed to need that in a HS6 carb body
Some of the Astons ran 125 thou jets, along with the Rolls Royce engined Princess 4-Litre Vanden Plas. ;)

On the siamese port A-series, fine atomised fuel is of little benefit because of the raft of pulses the ports impart on the intake charge. I think this is why bigger jets, allowing a greater anulus or raw fuel at each point in the fuel delivery, results in better power. Yonks ago, David Vizard noted this.

Today, stock port Holden Toranas and other six cylinder vehicles with comparitively poor intake ports like a bigger jet. But they are hard to find for CD's and rather easy for HS6's. Conversely, the Aussie 265 Chrysler Hemi I6 justs loves 125 jets on HD8's, apparently, and its got awesome ports. I've never had the pleasure... :(

I love SU carbs. They are like a metal version of an EFI fuel map. There are more permutations and combinations of fuel air ratios than the most advanced EFI system, and run off an MAP -style sensor...the dashpot! They have an accelerator pump (the taper of the needle and the spring weight), and when the fuel leans out at certain points you just pass a needle file over the point on the needle. Oils now are much better, and the dashpot drop is easier to control now than in the old days. The bias needle makes centreing the jet less of an issue.

Once people start hooking potentiometers to these ancient carbs so they can trace on the needle exactly were the engine runs lean or rich, and treating them more like an EFI system, proper needles can be customised for the Ford I6.

But I'm lazy. My Impco propane carb has no needle as such, and is adjusted by only 2 screws, plus another few in the convertor. Who needs electronics???? :eek: :eek: :eek: :LOL: :LOL: :LOL:
I like most of the constant depression carbs for their adaptability. If you start with a reasonable carb size it's nearly impossible to overcarb an engine since the slide only allows as much air in as the engine can handle.

Years ago I knew a Jag tuning guru (he passed away several years ago) who would listen to an engine, drive it a bit, then chuck the needles into a jewelers lathe to finish tuning the carbs. Man, he was good.
MustangSix":2rks4uz9 said:
Years ago I knew a Jag tuning guru (he passed away several years ago) who would listen to an engine, drive it a bit, then chuck the needles into a jewelers lathe to finish tuning the carbs. Man, he was good.

Showoff :p A drill clamped in the vise and some crocus cloth works well too ;)