Hello Bob, HD8 carbs were the standard model on the BJ8 Healey so that is what I believe you have. I have HD8 carbs on my BJ7. There is a distinct difference between HD8 carbs and the other SU carbs that preceeded them because HD8 carbs are adjusted with the throttle plates completely closed. You see the slow run speed screw is the same as an idle screw on an American carb in that it passes the full idle mixture around the throttle plates and at idle the engine runs on the mixture passing around the throttle plates via the passage past the tip of the slow run screw.
As it says in the book, close the throttle plates completely, loosen the interlink between the carb throttle shafts and allow the throttles to close all the way. Then retighten the interlink so the throttle shafts move in unison. Set the large slow run screws to 2-1/4 to 2-1/2 turns out. Set them both the same. now start the engine and let it warm to operating temp. It should idle alittle high. if too slow to idle turn the slow run screw each in alittle just to keep the engine idleing. If you can't keep it running on just the slow run screws then your jet adjustment which is your mixture is way out of the ball park or maybe your spark plugs are severly carboned from previous attempts to adjust carb. I don't remember what the initial setting is to the jet adjusting screw but hopfully the engine will run and idle till it reaches operating temp. If not, back out the jet adjusting screw till it clears it's contact on the jet adjusting arm. Then turn it down till contact and then another 1/2 turn. This should get the mixture in the ball park to idle the engine to operating temp.
Now with throttle plates closed and interconnection of the shafts tightened and the slow run screws at 2-1/2 turns, lift the piston lifting pin to see if the engine will stall, or increase speed or increase just momentarily. If when lifting the pin the engine stalls or slows down the jet on that carb is lean. you must then turn the jet adjusting screw on that carb in (clockwise) to richen the mixture for that carb. Obviously, if the when lifting the pin the engine speeds up and stays up then that carb is rich and you must back out the jet adjusting screw (counter clockwise) to lean that carb. If when lifting the pin the engine speeds up jus t momentarily then the jet on that carb is set just fine. Do the same technique to both carbs. Make sure your choke cable or anything else isn't hanging anything up and make sure you throttle plates stay closed when adjusting and testing mixture/jet.
On the model SU carbs before the the HD8s, idle speed is set by setting the position of the throttle plates to allow the fuel/air mixture to flow around the plates to get to the engine. That is why on the earlier models you had to balance the carbs to maintain near the same air flow. But with the HD8 carbs the throttles are kept completely closed to adjust. That's a significant difference and makes HD8 carb one of the easiest to set up. If I haven't gotten the explaination quite clearly enough, I hope that what I have said and another reread of the book will bring it all together for you. Good Luck. Dave C.

Hello Patrick, I believe you are right about the book saying to basically block up the piston on the carb you are not TESTing to get a good read on the one you are lifting with the pin. But also, I do not believe the book will tell you to loosen the connecting linkage again, after you have made sure the throttles are closed. Although it might direct you to leave the throttles separated until you are finished with each carb adjustment. But you see if you blip the throttle to see how the engine is responding to your work the throttles must be tied together. Blocking up the piston on the carb that you are not adjusting is illregardless of the fact that you must ensure that the throttle plates are closed or you will be feeding too much fuel to the engine via the carb that you are not adjusting and you won't be able tell what is going on. I and my fellow British car nuts around here do not bother to block up the other piston (on HD8 carbs) and have had good results for about 30 years. But that is why I told Bob to reread the book again in hopes that with what I said will help to make the methodlogy alittle clearer.
As a last point, you will see that with the HD8 carbs, the syncronizing the air flow with a tube or hose or unisyc is moot since the throttle plates must stay closed. That is why HD8 carbs are so much easier to adjust that previous SUs. With the previous SUs, you must separate the linkages and set the air volume thru each carb to match by adjusting the position of the throttles and listening or measuring the air flow. Then with balanced air flow you go ahead and adjust the fuel richness by adjusting the position of the jets. With HD8 carbs, if they are not flowing the same amount of air you would have to have a major physical malady with one carb or the other. With the throttle plates closed they have to be flowing the same amount of air. If you look at the carb schematics you will see that the passage that flows FUEL/AIR MIXTURE around (bypassing) the throttle plates IS the "MIXTURE", it is drawn off part of the carb structure after the main jet so it is really a "MIXTURE". All the slow run screw does is reduces its volume or increases it. With the previous carbs, there is no bypass idle circuit (slow run) around the throttle plates. On previous SUs, you would set the throttle plate open alittle, thus allowing air into the engine and then with the main jet situated in the main vortex of the carb you adjust the jet to give it fuel. Anyway i am not an engineer so my symantics may be confusing. I'm sorry for that. But it works for me.

Patrick -

On the HD8, the idle speed is controlled by a needle & hole rather than the throttle valve, so disconnecting the cross shaft is not required.

All HD carbs are like this.

Alan

I use the butterflies cracked a slosh technique also. Some cars will not idle correctly unless you do. Alan regarding adjustment of the cross shafts it is necessary that the throttles be in synch which requires good positioning of the levers.--Keoke

Hi Alan,
Speed controlled by needle & hole...this is true, but they seem to work better with the throttles cracked open a sckosh(very very slight cracked open throttle). Don't ask me why, but mine aways have worked better this way. Could be bushing fit, etc.
Patrick

I use the butterflies cracked a slosh technique also. Some cars will not idle correctly unless you do. Alan regarding adjustment of the cross shafts it is necessary that the throttles be in synch which requires good positioning of the levers.--Keoke

Can someone point me to a clear procedure for makeing the run mixture correct? I think I have the slow run procedure but perhaps not.

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....