I think this is a subject that has been way over-discussed in the past.
Crikey! it's a simple 2 cylinder engine...after all.
Warning: This post will take way longer to read than it actually takes to do the job (sigh!)
First off, there is a distinct difference between '
syncing your carbs' and '
adjusting your carbs'.
Before attempting either, it is important that your valves are OK, air filter clean, ignition system ok and timing dead on (S at idle AND F at high revs). Many people seem to think that F at high revs is all that counts, but unless your timing retards to S at idle, you will not get the idle smoothness and instant starting that a well tuned airhead should have.
If, and
only if, all the above are OK, you can now proceed to carb adjusting and syncing.
IMHO, you should throw out your manometers, vac gauges, fluid hoses, and all that stuff. They are totally unnecessary to achieve PERFECT carb operation on airheads. On second thought, don't throw them out, they may come in handy on many 3, 4 (or more) cylinder bikes or your buddy's 440-6pack 'Cuda. You can set up cooling fans if you want but the whole procedure should only take a minute or two.
I can (and you can too!) set up your carbs in less than one minute with only a flat blade screwdriver and a 10mm key!!! and they will pass the "manometer test' with flying colors if you decide to check them that way afterwards. Guaranteed.
Be sure your bike is warmed up to normal operating temperature...
Before attempting adjustments, make sure there is TONS of free play in both throttle cables (the butterflies must be held open only by the throttle stop screws, not the cables).
QUICK PRE-ADJUSTMENT TEST: Screw in the mixture screw on LEFT carb
until that cylinder stalls then back it out about 2 turns. Now do the same on the RIGHT cylinder. This will establish that both carb idle circuits are working and that your carbs are in fact
adjustable.
CARB ADJUSTMENT FIRST: ...with engine running....
1) TURN OFF LEFT CYLINDER Turn the mixture screw all the way in, just lightly seated, with very little pressure, or you will break off the tip and it could be a bitch to fix.
2) ADJUST RIGHT CYLINDER MIXTURE (screw in until it stumbles and then back it out, bit by bit in very small increments, until it won't go any faster. Now slightly back off the screw that holds the butterfly open and repeat the procedure again. Imagine you are tuning a single cylinder engine for now. Once you achieve a
slow but steady thump-thump-thump on only the RIGHT cylinder, you're half way there.
3) Get the LEFT cylinder running again and TURN OFF RIGHT CYLINDER by screwing its mixture screw all the way in, but this time note how many turns out it was and where its final position was as if on the hands of a clock; i.e. about 2 turns out, 7 o'clock. This can be easily done by counting in half turn increments as you screw it in.
4) Repeat step 2 on the LEFT cylinder, remember, slow and steady
thump-thump-thump.
5) Turn the RIGHT carb back on by returning the mixture screw to the previously noted position.
You should now have your engine
thump-thump-thumping (smooth idle) on both cylinders. Experiment with the butterfly stop screws until you have the idle speed you want (950-1100 rpm) keeping in mind that a too slow idle speed will contribute to backfiring and popping from the exhaust during deceleration on most airheads. To get your ear accustomed to 'tuning', experiment by cranking one carb up a 1/4 turn and back down, then the other. You will quickly HEAR the sweet spot where the engine is running perfectly even on both sides.
That sweet spot will be very obvious, even to an untrained ear.
HINT: On some bikes, taping the clutch lever to the handlebar eliminates transmission noise and makes the actual engine easier to hear.
NOW : THE SYNCHRONIZATION
Throttle cables must be in 'as new' condition. You can't sync cables that have bends in them (where the black outer sheathing meets the metal adjuster). Also, both throttle cables must be
floating. I.e. If you put one hand near the throttle end and one near the carb end, you should be able to slide the cable back and forth through the bike. It should not hook up anywhere around the steering head or under the gas tank or fairing.
To sync carbs, simply take out all but a smidgeon of slack in both cable adjusters. Then, as you take up the slack by VERY slightly twisting the throttle, both carbs should open evenly. Now zero in on that sweet spot, where the butterflies transition from throttle stop screw to cable tension. Experiment by pulling slightly on one cable by hand, then the other, and then both together. You'll see what I mean. When you get it right, that transition will be imperceptibly smooth. THERE! You're done.
NOTE: Trying to
bench sync your carbs at higher RPMs is TOTALLY pointless.
At cruising speeds, being a bit off on the cables makes absolutely no difference because, although 1mm of adjustment makes a huge difference right off idle when the butterfly might only be a few mm open, 1 or 2mm makes no perceptible difference once at partial throttle (20 or 30 mm open).
No matter how precisely you sync your carbs, once locked down, they may be off a bit at certain ambient or engine temperatures. Hence, my invention below. This my homemade adjuster, for toolless tweaking. It allows me to fine tune my synchronization without tools while stopped at a light. As far as free play in the cables, I like almost zero play. It makes the transition from closed throttle to open throttle a lot smoother in real world riding conditions.
Nuf said?
