In addition to the comments above...

The HD6s also require the throttles to be completely closed and use the idle screw. They are essentially the same as HD8s but smaller.

Once the car is tuned, it's useful to pull the domes and use a dial caliper to check the jet drop on each carb and equalize them. For instance if one is down .055 and the other is .045, change both to .050. Barely noticable turns of the mixture screw can make big differences in the mixture. We're not talking quarter turns here, but the thickness of the screwdriver blade. Clockwise is richer.

I used to use two colortunes to get in the ballpark on the mixture but Healeys don't seem to run right unless "colortuned" on the rich end of the scale. Now I use two K&N oxygen sensors, but that's another story.

Make sure the choke linkage returns the jets fully home. On the older cars, the choke link-rod between the two jets can have the effect of keeping one of the jets down when the other is fully seated.

The acceleration system is a combination of the weight of the damper oil with the tension of the big spring inside the dome. My friend's HD6 car had the weakest spring and we tried first the mid-range spring, then the stiffest, then settled back on the middle one. The difference in driving with the middle spring is noticable over the light one. These 3 springs are listed in Moss.

If you have to crack the throttle, it usually means you have either a leaky bushing or a bad O ring on the slow idle screw. Both these are easily fixed on an HD8.

My guess is actually the bad O-ring - these are bad all the time on HD8. VERY easy fix.

Yes you are right, but on the Mk III, if the levers have been adjusted previously, they need not be touched again to get a clean idle, unlike other dual/triple SU carb set ups, which need to be adjusted every time.

Keoke, you are a guru on this list and I defer to you 95% of the time - but cracking the throttles to get idle on a car with HD6s or HD8s means something is wrong with the carb. Usually it is just a bad o ring on the slow idle screw, other times it is worn throttle bushings and/or shafts.

Healeynut, You are Quite Right.

I adjust my HD8s so that the throttle stop adjuster screws just touch the throttle shaft stops, then another quarter turn. I'd rather have the iron (steel?) stops stop the throttle plates than the soft aluminium bores of the carburettors.

From _SU Carburettors Tuning Tips & Techniques_ (a Brooklands reprint of the original G. R. Wade book), p47:

"Note that whenever throttle adjusting screws are fitted it is they, and not the slow-running valves, which must be used to adjust the idling speed.

Screw down the slow-running screws (which must remain closed) and set the throttle adjusting screws 1 1/2 turns open to give the required idling speed."


Other books I've read say to use the slow-running screws. I've never read any reasoning for using either the throttle adjusting screws or slow-running valves exclusively, so I split the difference. The slow-running valves are a better fine adjustment, so I open them up a half-turn or so at high altitude when the idles slows, then close them back when I get back to sea level.

But cracking the throttles on a car with HD Carbs to get a good idle means: It won't idle correctly unless you do.-----Keoke--

I can close the throttles and get a good idle on the slow-run valves if I choose--in fact, I used to do it this way until I read at least 3 books on SU tuning and thought about it quite a bit--but I choose not to as explained above.

The throttles and slow-run valves are both simply ways to get a minimal amount of mixture to the engine at idle; I don't see a particular advantage to either, except the slow-runs are a finer adjustment and handy to tweak the idle depending on air density altitude.

I think this thread started with adjusting the carbs for idle speed/mixture strength and whether or not you choose to to adjust the carb opposite of the one you want to adjust? Crack the throtlle if you want to or not, but I think you need to "shut off" the carb you are not adjusting. MOst cars have worn bushings so crack the throttle plates a "skosh" and you'll be fine. This sets the idle only. The mixture for modified Healeys in another thing altogether. As was explained to me by a mechanic that used to tune Healeys at Hollywood Sport Cars, he said that after warming up the engine they would find a straight stretch of road and accellerate full throttle from about 25mph to 55mph where they would push in the clutch and turn off the ignition. The #2 & #5 plugs would be pulled and examined for mixture strength deposits. If they didn't have needles that would work with them they would file them down with "sandpaper" of they wanted to richen the mixture.
Patrick

Well'p I certainly think we gave Bob plenty of ideas to point him in the right direction to get his carbs set up.
Bob, if you still have questions, you might be afraid to ask but don't be bashful.

Well Vwtte I know one thing I sm glad I do not own a 65BJ*---Keoke--

Ok, All -

As I enjoy flogging a dead horse here... just finished rebuilding the HD6 carbs on my '59 Jag Mk IX (3.8 Liters).

Using the slow idle speed screw alone, with throttle butterflies completely shut, I can get her easily up to about 1,000 rpms.

Since the Jag is a much bigger motor, there should be no reason to crack the fast idle on a Mk I or Mk III with HD6s or HD8s to get her to idle correctly.

I think the main thing is just making sure the O ring on the slow idle speed screw is sealing TIGHT. If you have that, it should idle properly....

Yep you just floggin a dead hoss.--Keoke--

Did you get them to hissssss?
Cracking the throttle probably compensates for some wear. Who wants to rebuild carbs all the time?
Patrick

Patrick!, you got it zactly right!.--Keoke--

.Bob ran into a dist. problem and it was sent off for refurbishment. The Chinese cap places the carbon center wiper too low when you have the added rotor height with the magnetic sender on electronic ignition. My original Lucas was a 1/16" shorter in the center. Wiped out the after-market version in about 5min. That aside, I have purchased a suction gauge and I think I will start with the butterflies slightly open with slow run seated. I will make the crabs the same and test spray the shaft bushings. If all is ok then, I will find the sweet spot in each slow run by ear and correlate with a suction check.

The thing is I've never really had throttle body wear on HD8s or HD6s... but of course I only use original spring-rate throttle return springs.