The SU has got to be arguably the simplest carb yet designed this side of the
pond, with the possible exception of the Wick Carb designed by George
Lanchester. From what I've been reading on SU threads over the last few weeks,
seems that some people are getting in a hopeless tangle tuning them. Plenty's
been written on how to go about it and checking the butterfly closes fully is
the very first step in preparing to retune. If it doesn't shut completely, its
the throttle stop screw holding it partially open (as you mentioned) or the
butterfly spindle has worn. I've come across very
few cases of the latter, though no doubt there have been exceptions of which
I remain ignorant. At the risk of being repetitive, the starting point for
setting up an SU is a simple six fold check.
1. Ensure the fuel needle is not bent. If it is, don't try to straighten it. Get a new one. 2. When refitting the needle, the base of the shoulder must be parallel with the top of the chamfered hole in the piston, NOT parallel with the bottom of the chamfer." 3. Ensure the sliding clamp pin operating into the yoke on the throttle spindle is entirely clear of the of the yoke. 4. Ensure the butterfly is fully closed by unscrewing the adjusting screw so it no longer bears on the spindle. 5. Ensure the jet is exactly parallel with the jet bridge. 6. The piston damper is filled with normal engine oil and falls with a soft metallic 'clok' when lifted.
That's the set-up. Initial procedures prior to fine tuning for mixture and idle speed are:
7. Turn down the jet adjusting nut by 8 flats 8. Screw down the idle screw by one point five turns to open the butterfly.
Each carb has to be adjusted individually for mixture with the other one totally shut off. Ain't no way you can do two at the same time. When mixture is the same for both, air intake is regulated using a Unisyn - or, a piece of rubber pipe. Alternatively, you can do it "in stereo" with an old stethoscope. It's only AFTER all this other work has been done, that the interconnector pins are put back into the throttle yokes and tightened. People who fail to do this and think they can adjust their carbs with two locked yokes will NEVER get it right.
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