Carburetor Synchronisation
I do my very best to tell the correct things, but of course I can give you no warranty!
Tools you will need:
- SW 10 and 12 wrench
- Philips screw driver
- long-shanked screw driver (at least 20cm long)
- vacuum gauge
For best performance both carburetors must supply it's cylinder with the same amount
of gas-air-mixture. If not, one cylinder has to drag the other along with it, resulting
in a lack of performance and unusual vibrations. From time to time you have to synchronize
the carbs, at least when having adjusted the valve gap clearance. To do this, you will
need vacuum gauges and a long-shanked screwdriver. You have to dismount the gastank (for 
just testing the synchronisation, you don't need to dismount anything). In order to supply
the carbs with fuel, fill some fuel into a little plastic bottle, hang it up the handlebar
and connect it with the fuel tube. One of the vacuum gauges you connect with the rear
cylinder head, while pulling the vacuum hose off the cylinder head (ok, it's already done, since the
tank is dismounted... ;-) ) and replacing it withe the hose of the gauge. The other gauge
you connect with the front cylinder head, after removing the little screw you will find
there. You will need an adaptor (should be shipped with your gauges) to twist in the hole
instead of the screw. Start the engine (it must be warm) and let it run with idle speed.
Now you can see the gauges' pointers quiver less or more. If they are quivering too much,
adjust them, so that they are quivering between two graduation marks. If both gauges show
the same values, you're lucky. If not, you have to hit
this screw
with your screwdriver through the hole in the airfilter dome and turn it, watching,
what the pointers will do, and until both
gauges will show the same value. Well, that was it. Honda recommends a maximum difference
between both carbs of 40 mm Hg, when using mercury gauges. What that may be on your gauges, I don't know, it depends on
several things. Calculate it yourself. The difference is IMHO rather big, so if you synchronize
the carbs as exactly as you can, it should be ok.
Pictures scanned from "Reparaturanleitung Honda XL600V" by Bucheli Verlag, Zug/CH