Hello Brandon, I have HD8 carbs on my BJ7, so here is what I do. To adjust your slow run screw, which is basically your idle mixture control screw, your throttle plates must be "COMPLETELY" closed. This is different than any of the other SU carbs where-by the throttle plates do control idle. The Reason is that on the HD8 carbs the slow-run screw is adjusting a passage that flows a mixture of fuel and air directly off the main jet. So if your throttle plates are not completely closed you will be introducing more air flow and hence more volocity to suck fuel out of the main jet and you won't be able to get an idle. If your throttle plates are not closed, you may have to loosen the throttle shaft interconnection clip between the carbs. do so close both throttle plates and then retighten the clip.
With the plates closed, you may be able to turn completely down the slow-run screws and still have an idle speed at about normal rpms or even alittle higher. This happens on old carbs because they suck air from everywhere. Be that as it may, with the plates closed and the slow run screws turned in, then adjust the jet adjusting screws to get the mixture for each carb where you want it. Turn the jet adjusting screws clockwise to richen a carb and counterclockwise to lean it. Check the mixture by lifting the pin to raise the inner piston, if the engine momentarily increases in speed and returns to normal then the mixture is about right.(although that could be alittle rich for emission testing). If the engine stalls apon lifting the pin the mixture is too lean.