Its simple to tune twin SU's - Balance the butterflys and set the idle mixtures, its that easy.
Step 1. Balance the butterflys at idle.
Loosen the butterfly linkage adjustment between both carbs and start the engine without the air cleaner cover.
Using a short piece of hose or flexible tubing, put one end to your ear and put the other end in each carb whilst the engine is idling.
Adjust the butterfly adjustment (idle) screws on each carb until their suction sounds match (butterflys are balanced).
Screw in or out each idle adjustment screw in equal amounts to get the engine idling at 750rpm.
Recheck the suction sound. "Fine tune" each idle screw to get them sounding (sucking) the same.
Rebalance the idle screws if rpm is changed.
When you are happy that both butterfly's suction sounds the same at the idle speed of 750rpm approx, turn off the engine.
Step 2 Set the idle speed fuel mixture.
Locate the mixture adjusters. They are a large, round, knurled, and horizontally positioned under the carby base below each needle. Later type SUs have a position locking arm that needs to be held away from the mixture adjustment nut as you turn it.
Screw each mixture adjuster until each is positioned fully "up". The adjuster wont turn any further. This lifts a brass tube inside the adjuster surrounding the mixture needle to its highest position, which is its leanest setting. Now, screw each mixture adjuster down two full turns. This richens the mixture setting by lowering the mixture tube down the needle.
Start the engine
At this point you may have to go back to step 1 and reset the idle speed and rebalance the butterflys (using the hose suction tester), as the new mixture settings could be leaner or richer than they were before.
With the engine back at 750 RPM, locate the small piston lifter shaft protruding under the base of each carby bell chamber. Push each upward in turn, not together. This pushes up the piston in the bell chamber and the mixture needle attached. If the mixture setting is right in this carb, the engine speed will increase momentarily, then return to normal idle speed. If the speed stays up, the mixture is too rich. Release the lift shaft and then adjust the mixture screw underneath the carb up by a quarter of a turn. Repeat the piston lifting process for this carb. If the engine speed stays up after the next lift, repeat the mixture setting adjustment by taking it up a further quarter turn. Repeat until the engine speed just increases then settles back to its idle speed.
If the engine dies when the piston is lifted it's too lean and the mixture screw needs to be lowered (anticlockwise turn) by similar quarter turns until the desired engine response is achieved. If you find you are lowering the mixture tube by more than three turns and getting no change, you probably have an air leak downstream (see "problems" below)
When you are satisfied each carb's idle mixture setting is correct, you may have to go back to step 1 and reset the butterflys to get the correct idle speed as the engine is now getting the right fuel mix at idle.
Step 3 Reassemble
Adjust the butterfly linkage screw between the two carbs, so both carb's butterflys start to open exactly together. This is important to ensure all cylinders are getting the same mixture for maximum power.
Check the oil levels in the piston dampers are covering the dampers, to ensure the pistons rise at the correct rate under throttle load (High suction).
Reconnect the accelerator linkage and fit the aircleaner.
Job done.
My son has twin SU's on his Datsun 1600 (L18) daily driver. He learnt to tune them in one session 10 years ago. Its done a squllion k's on two engines and the only SU carbs problem - two sunk floats due to cause noted above. It always starts easily cold or hot, idles smoothly at 700rpm and it pulls hard to 6000rpm.
_________________ RichardC
SSS016
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