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Old 06-18-2002, 11:17 PM
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Synchronizing the Carburetors

Synchronizing the Carburetors
by Rob Brotherston and David Hansen

Synchronizing the carburetors is easier than you think and very effective at improving the performance, efficiency and ride of the XS11.

You can buy a simple mercury synchronizer (2-foot tall 4-tube "stick" filled with mercury) for around $40 - about what your local moto-shop will charge you for a single synch visit.

Synchronizing the carbs should be the last thing you do as a part of a tuneup.

Here's the process for synchronizing your carbs. The following directions are for a '79 Special - other models have slightly different fuel line and vacuum hose configurations, but the basic process is the same.

Before you begin, make sure the bike is warmed up first to normal
operating temperature. Shut off the bike and proceed as follows:

  1. Pull the bike onto the centre stand.

  2. Remove the seat.

  3. Turn the fuel petcocks to the OFF position. Remove the gas tank and all neccesary hoses and turn it around so the rear of the tank faces the front of the bike. Position the tank far enough to the rear of the bike to allow access to the throttle adjusting screws for the two inside carbs. Use some extra long fuel lines as nessesary to reconnect the vacuum and fuel intake lines to the opposite sides from the original connections. Alternatively, you can just leave the gas tank in place, remove the bolt securing the tank to the frame and prop the rear of the tank up (e.g., with a block of wood). However, there is not much slack in the fuel lines and getting your hand and a screwdriver to the adjusting screws can be a knuckle-buster.

  4. Remove the vacuum line from the #2 carb on the manifold side of the carb, this is the one from the vacuum control splitter, not the vacuum advance line. BTW, cylinders are counted from left to right from the rear of the bike.

  5. Remove the rubber caps from #1, #3, and #4 carbs.

  6. Connect the mercury sync tool to carb #1-#4 where the caps/vacuum hose was disconnected.

  7. Open the vent hose or plug on the syncronizer.

  8. Turn the fuel shutoff valve to prime (PRI). (Very important)

  9. Start the bike and let it run for a bit until it reaches normal
    operating temp again. Only run the bike at idle while warming up.
    *DO NOT REV THE MOTOR* you may suck the mercury from your synchronizer back into the engine.

  10. If the carbs are synched at this point (all columns of Mercury are even) then you are done. Otherwise use the following proceedure to even out the columns:

    1. First sync carb #1 to #2 from above using the silver, slotted screw on the throttle linkage, halfway between the manifold for the two carbs (carbs #1 & #2). Turn the screw until the mercury columns for carbs #1 & #2 are even (within 1 cm). The adjusting screw will change the reading in one of the two columns - the goal isn't to get the columns to some specific height, it's to get them all even.

    2. Next sync carb #4 to #3 using the screw
      between the carbs #3 & #4.

    3. Finally, use the screw between carbs #2 and #3 to
      bring the two pairs of carbs (#1-#2 and #3-#4) into sync.
      NOTE: Carb #3 cannot be changed, all other carbs must be synced
      to it.

  11. Once all mercury columns are even, the carbs are syncronized. Re-adjust your idle (if necessary) and make sure that the carbs are stilled synchronized.

  12. Remove the hoses from the carb stick and re-install the vacuum line
    to carb #2 and the rubber caps to #1, #3, and #4.

  13. Turn your fuel petcocks to off and re-install your tank the right way around, re-install all fuel and vacuum line routing - make sure you reconnect them properly. One is for fuel, the other is for vacuum. Turn fuel petcocks back to the ON position.

  14. Reinstall the seat.

  15. Ride

NOTE: XJ owners will need a tool to plug the YICS path - see Norm Kokes' page for information on easily building this tool for about $5 instead of paying the $95 that Yamaha will charge you for theirs.


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