Carb Sync for CV carbs
Blaine Hoopes 7/21/99
Occasionally I'll find a carb bank that when sync'd at idle are far from sync'd at partial throttle openings. I have found that when reassembling a carb after cleaning, the butterfly valve is easy to get off center so it doesn't close perfectly. If one carb has a butterfly that isn't centered, that carb will seem to have a higher idle speed setting than the other carbs and if adjusted to sync at idle will be off at partial open throttle.
To get the butterfly centered I loosen up (but don't remove) both screws that hold the butterfly so the plate can slide around freely and adjust the idle speed screw loose enough to let the butterfly close all the way. If you hold the carb up to the light you can open the butterfly a crack and watch the light coming around both sides and verify that it closes centered and all the way. While holding a little tension on the throttle lever to keep it closed, carefully tighten the two screws that hold the butterfly and verify that it is still centered.
I also like to visually adjust the sync while the carbs are out by holding the carbs up to the light and adjusting so they all have the same thin sliver of light under the butterfly when the idle speed is adjusted to barely open them. This is just a starting place, but if you set them visually you should be very close to proper sync if there are no other problems with the carbs. If you set them visually and then have to adjust them a lot to get proper sync with engine running there is probably something else misadjusted in one or more carbs.
Obviously, you should also make sure that the mechanism between the carbs is clean and works smoothly since any hanging up here would prevent the carbs from opening and closing in unison.
Here is a handy method of sync'ing carbs from the dynojet
instruction sheet:
WARNING! This was written
for 4 cylinder engines. The 650 twin may not idle at 500 and
doing so for any length of time may starve the cams for oil.
Carb Sync and Throttle Plate Relationship Check
The current method of carburetor synchronization [non-Dynojet??]
is a carry-over from the method used to sync the fixed slide
style carburetors. This method is not the most acceptable
method to use with Constant Velocity (CV) carburetors. The
Dynojet Method of Carb Sync insures that the throttle plates are
doing what the manometer (sync tool) says they are doing.
Remove the fuel tank and connect the carbs up to a remote fuel
source. Connect your regular carb sync tool to the
carburetors.
Start the engine with the motorcycle on it's center stand.
Allow the engine to warm up to operating temperature so it will
function with the chokes off. Using the idle adjustment
screw, you should slow the engine down to 500 rpm. Sync the
carbs at this time. Once the carbs are in sync, adjust the
fuel mixture [pilot circuit] screws to the smoothest running lean
side setting. (Example: if the engine runs smooth when
mixture screws are between 2 turns out and 4-1/2 turns out, set
the screws to 2 turns out.) Once you have set the mixture
screws recheck the carb sync. If it has changed, adjust the
sync screw[s] again until the sync is correct, then adjust the
fuel mixture screws again [to get, or maintain, the smoothest
running lean side setting.] After the second time,
you should find that the sync no longer changes. Carb
synchronization is now complete. Remove the sync tool and
raise the engine idle up to 1000 rpm. (This is the
recommended idle setting for motorcycles using Dynojet
Kits.)
NOTE: There is a very good chance that your engine will not
idle at 500 rpm. Failure to run at 500 rpm indicates that
your carbs were functioning in a false idle condition. This
condition exists even when the carb sync tool shows perfect sync
using the old method. To fix this problem you idle the
engine at the slowest speed it will idle at (example 800
rpm). Perform the Dynojet Method of carb sync [using the
mixture screws] at this speed. When complete, you will find
that the engine will now idle slower than it did before.
Slow the engine down to the slowest idle or 500 rpm which ever
comes first and repeat the sync process. Repeat the sync
process 2 or 3 times until you are able to perform it at 500 rpm.