1.) make sure your throttle linkages operate without binding. Check for wear at points of contact such as where the shafts are held in a steel bushing surface. Excessive wear here can make it impossible to set the idle speed.
2.) make sure you have the same diameter, approximate length and wire gage size of piston springs. If you have two different types of piston springs on the set, tuning will be impossible.
3.) make sure your jets and needles are undamaged and that you indeed have a matched set of jets and needles.
4.) center your jets to the needles to get the pistons to work undisturbed
5.) a throttle return spring on each carb ( get the British S.U. kind, it's higher quality)
6.) examine the piston and dome for any evidence of a rub. Make sure the precision ground surfaces on the piston rod and in the damper tube of the dome are not damaged in any way. Look for anything which might cause a hang.
7.) make sure your throttle plates are not binding against one side or the other of the throttle body.
8.) while there check your throttle spindles for slop against their bushings by grasping the throttle spindle at the side of the carb body and wiggling the shaft up and down (not side to side).

A - This sounds very much like you have an improperly centered piston/needle or a bent needle. The needle is probably rubbing the jet at idle, preventing the piston from falling to its natural position and properly metering the air at idle. When you pull the choke cable this lowers the jet enough to free the needle allowing it to run (somewhat). First idea is check to see if your needles are bent, remove the entire piston/dome assembly, hold it upside down and spin the piston and observe the needle to see if it "wobble" when piston spins. Replace the needles if bent or you might try bending to straiten them (very difficult, ususaly doesnt work).
SUs must have strait needles to work properly! Once you have strait needles, it is critical that you center the needle in the jet so that there is no friction. The piston must be able to fall all the way to the bridge without the needle hanging up. Raise the piston with your finger and let it drop. You must be able to hear a distinct metalic
"clink" as the piston hits the bridge, otherwize the needle is rubbing. Carefully adjust the position of the piston chamber (dome) so the needle is centered, slowly tighten down the 4 chamber retaining screws while checking that the needle is still centered. This can be very tedious, sometimes takes me 10 minutes to get the piston centered when the retaining screws are tightened. The flattop "tuna can" SUs are much easier to center, they have rubber nubs that help center the chamber onto the body.

Should you need to make major changes to center the needles, follow this procedure:

The jet is the tube that threads through the bottom of the carb body and encloses the needle. The idea is to loosen the hex lock nut on the jet so that you can lightly tap on the carb body with the soft end of your screw driver to slightly move the position of the jet relative to the needle. Raise and drop your piston again. Do this a few times, and the piston should be able to drop cleanly. Then retighten the lock nut and test again. If this doesn't work, I'd be looking at that needle to see if it's been bent. If it is, you have to replace the pair with a new set.

Part of my setup regimen for SU's involves jet centering of the piston/needle assy over the jet. My practice is to push the piston of the fully assembled carb with air filters off all the way up and then simply let it drop. What I look for is an undisturbed travel to a full stop which should be concluded with a pronounced click as
the piston hits bottom. If the needle is not centered, you won't get the loud click. I also try to observe that both pistons glide down at about the same rate.

Steve C.

Sounds like you should try raising the jet to lean the low RPM mixture and then sand down the high RPM tip to thinner deminsion. Doesnt really require a lot of precision measuring to closely match two needles. Go ahead and sand the first needle to desired profile. Chuck needle in dremmel tool (on low speed) or in a drill, while the needle is spinning apply pressure with a file to remove metal even fast for rough shaping, then clean up with sandpaper. Metal is removed really slowly with sandpaper so you can precisely "adjust" to the final deminsion. To match the second needle profile to the first, you will need a vernier caliper with adjustment that you can lock in position. To compair the same station on two needles, close the calipers on the station of the reference needle and lock the measurement gap of the calipers. Note the length along the reference needle where the calipers will not slide further toward the base. Then pull the reference needle out of the locked calipers and slide the needle you are trying to modify into the calipers. Slide the needle into the calipers (tip first) untill the taper of the needle contacts the locked caliper gap. Instead of trying to cafefully read the fine graduations of the calipers, just note how far the needle slides into the gap before it stops, the further it slides the thinner the needle is. You can quicky compair diameter fo the station of the two needles just by noting how far they slide into the locked calipers. Ive had good results using this technique- easier to tune this way than sorting through the haystack of needle possibilities and tryting to buy the perfect taper pre-made.

Seems like stiffer springs would have the effect of "delaying" the maximum rise of the piston/needle. This might possibly be good for smooth driveability- has sort of same effect as usign a smaller carburetor. I found that the piston/needle was fully raised on my L20B around 4000 RPM (w/ stock hitachi springs). I dont think it is neccesarry to delay the piston max rise all the way to peak HP PRM or redline. Maybe match the full piston rise closer to the torque peak RPM? The SUs just go into "non constant-velocity mode" and function more like a traditional e-tube carb once the piston fully raises. Using stiffer springs will have the effect of raising the venturi velocity (and vacuum) through the rest of the low/mid RPM range before the piston reaches full lift. Stronger vacuum is produced to counter the stiffer spring downforce and increased vacuum will also work to ritchen the low/midrange RPM air/fuel mixture (just like thicker dampener oil ritchens the mixture under acceleration). So- sounds like your choice of the stiffer red springs may in part be contributing to your ritch-at-idle/midrange problem. Before customizing you needle profile to work with the stiffer springs, try using the soft hitachi springs again and see if your mixture ballance at low/high RPM is improved.

Hitachi SUs came with a bewildering assortment of different fuel inlet valves and related parts. The two thread sizes used for the valve are 6X1mm 10X1mm. However, beware that the fuel inlet valves came in several different lengths and you will need the correct length to match with the float bowl top. The float bowl top (besides having the two different thread types), can also vary the depth that the valve seat boss is countersunk into the casting and also can vary the height of the fulcrum holes for the pin that retain the float lever. The early dome toped 1600SSS SUs used brass tipped fuel inlet valves with hardened steel seats while the later flattop SUs used noeprene rubber tiped valves. The two types of valve-tips are non-interchangeable. The entire valve for the flattop SUs is supposably non-available in USA but you can "rebuild" the valve with the neoprene tip from a 240Z fuel inlet valve- (just the tip, not the entire 240Z valve interchanges). Some later SUs including the '72 Z car used different height valves on each carb so that the fuel height would be different in front and rear carb (emmisions thing mostly I think- the '70-'71 Z used same valve and same fuel height). If you wind up with fuel inlet valves that are the wrong height for the bowl top, you will most likely be unable to adequately set the fuel height because the adjustment range from bending the float tab is minimal. Because the availability of parts for the 38mm carb float bowls is poor and confusing, if you are buying new float tops I would suggest that you instead order floatbowl tops, inlet valves and floats for '70-'71 240Z. These floatbowl tops are interchangeable onto the 38mm carb floatbowls and availability of correct height inlet valve replacement parts will be better than for the 38mm SUs. Another possibility for replacement of the valves is to use "Grose-jet" brand valves. These are available in a wide variety of deminsions, just be sure to match valve height and threading type for the floatbowl top. See http://www.ztherapy.com/GroseJets.html

The vaccum advance port on the SU carburetor gets it's signal from an area just upstream (atmosphere side) of the throttle plate. At idle, when manifold vacuum is high and the throttle plate is closed, the vaccum advance port receives no signal and so the distributor sees no advance from it. Crack the throttle open a bit like when you begin to accelerate (part throttle) and the vacuum port sees lots of manifold vacuum and gives you a bunch of advance at the distributor. This is also true while cruising down the freeway at part throttle. Not only do you have your mechanical advance in (30-38 degrees total or so) but the vacuum advance will give you another 10 degrees or so on top of that (40-48 degrees or so) which helps extract the maximum amount of energy out of partially filled cylinders due to part throttle running. Now stomp on the gas and manifold vacuum drops to a very low level. Vacuum advance drops off to keep the motor from pinging since the cylinders are now being filled fully, or somewhere near that. A motor would detonate in a heartbeat if there was 48 degrees advance and you were at full throttle. Here the motor only sees the mechanical advance which shouldn't be too much to cause pinging if it's set correctly.
Now if the vacuum advance was attached to the manifold instead of the carb, the distributor will see full vacuum advance at idle since there is no throttle plate to seal it off when the throttle is closed. Probably won't cause detonation at full throttle since the manifold and the carb vaccum port will see essentially the same signal at that point. Most likely you will see a much higher idle with the advance hooked to the manifold since you are getting 10 degrees or so more advance at idle versus hooking it tho the carb port. Some cars take their vacuum advance from the manifold but if you are running SU's, it's best to use the carb port.