Simple Synchronization for LINKED Carbs (1976 through 1983) with DUAL Exhausts:
First, you need to understand the function of the idle stop screw and the sync screws. Screwing the idle stop screw in increases the air flow to both cylinders, which creates more vacuum, which sucks more gas in, which makes the engine run faster.
Screwing the SYNC screw in ONLY INCREASES the air flow to the right-side carb, so only the right cylinder gets an increased flow of gas and more power.
Now to syncing: Idle the bike at 1200 RPM. Place your hands a few inches from each exhaust. The exhaust pressure you feel should be equal. If the right exhaust feels weak, then screw the sync screw in -- this only increases the flow of gas to the right cylinder, remember? -- until the pressure is equal from both exhausts. Re-set the idle to 1200, check again, and do a final sync adjustment if necessary.
If the left side exhaust was the one that felt weak, you do the same thing, except you screw the sync screw OUT, decreasing the flow of the gas to the right-side cylinder until the exhaust pressure from both exhausts feels the same. Re-set the idle to 1200, check again, and do a final sync screw adjustment if necessary.
NOTES: This should take about 30 seconds once you've done it a couple of times. The XS650 is not a space ship, and this does just as good a job as a manometer for all practical purposes. Some people like to sync at a higher idle, such as 1400 or even 1600 RPM. That's fine by me, but there really shouldn't be much difference.
Simple Carb Air Mixture Screw Adjustment:
Start with the mix screw set to specs. Set idle to an RPM that is low but still steady -- about 1000 RPM if you have a smooth-running bike. Now go to the right-side carb and screw the mixture screw in in half-turn increments until the engine starts running slower. Now, turn the mix screw out in half turn increments. The engine should speed up again, and then maybe go faster, but eventually it will start running slower again. The correct mix screw setting is the place where the engine runs the fastest. When you think you are there, you can check and fine-tune by turning the screw in quarter-turn increments. Keep in mind that slightly rich (erring on the screw being more turned out) is better than slightly lean.
Repeat for the left side.
NOTES: This should take a bout 30 seconds once you've done it a couple of times. You are welcome to complicate the above procedure by pulling a plug wire (points only!) or by doing the dead-cylinder method, but the only reason other people suggest doing this is so that you can better hear the changes in engine RPM as you move the mix screws in and out. I can hear the RPM changes just fine with both cylinders firing.
FINAL NOTES: These are really simple procedures to do on a simple bike like the XS650. Don't-over-think this stuff or stress yourself out or be afraid of doing them. They ARE necessary, so be sure to spend the 60 seconds it takes to do them and then get on with your life.
