Synchronizing the Carbs

By Walt ZRXOA 255.


Synchronizing the carbs on the ZRX should be done periodically (about once each year), after a valve adjustment or whenever idle and throttle response is noticeably poor. You can pay your friendly dealer $100 and hope it’s done right, or DIY! Here’s how: 

1.  Purchase a mercury “Carb Synchronizer”. Prices I’ve seen vary from $150 to $30. Mine was purchased from Donelson Cycles in St. Ann, MO for $29.95 plus $7.50 shipping & insurance. It was made by “Motion Pro”, does the job great, and has simple instructions on usage. Call 314-427-5523 to order. You should have it within a week. When assembling the synchronizer, do not use any kind of lubricant to ease putting on the hoses, such as water, soap or spit, as it will contaminate the mercury columns and screw up your readings! Caution: Do not get any mercury on your skin, as it is extremely toxic. 

2.  All work should be done outside or in a well-ventilated area. Remove the fuel tank from the bike. Set it on a card table, step ladder or whatever you happen to have handy to place it above the level of your carbs. If you don’t like the economy approach you can buy a nifty remote tank for about $50 from various places. I’ve seen them advertised by Holeshot and Motion Pro also has one. (Rag's note: You can also turn the fuel petcock to prime for a bit to supply the fuel to the float bowls, and then entirely remove the fuel tank. This will work if you work quick)  

3.  Purchase from the local auto parts store, 4’ of 3/8” fuel hose and one 3/8” barbed coupler. Connect one end of the hose to the fuel tank petcock and the other with the coupler to the fuel hose from the carbs. Cost was about $3.50. While at the auto parts, buy yourself a 16” #2 Phillips Screwdriver, if you don’t have one (for making the adjustment between the right and left pair of carbs). This should cost about $7.50.

4.  Borrow the wife’s electric fan and place it in front of your bike, set up so it’s blowing on the radiator.  

5.  Remove the vacuum hose from the top of each carb. Note which hose goes where for replacement later. 

6.  Hang your carb synchronizer on the handle bars and connect the hoses provided with it to the vacuum nipples on the carbs. 

7.  Start the bike and let it warm up.  Once warm, note the relative differences of the 4 mercury columns. Kawasaki says they should be within 2 cm of each other. This translates to approximately 0.78”. It so happens the graduations on the Motion Pro Carb Synchronizer are 0.75” apart. Neat! 

8. If your bike is in good shape, you should be able to do much better than this, so let’s get to it! There are linkages and three adjustment screws between each of the 4 carbs. The left screw synchronizes #1 & #2, the right screw synchronizes #3 & #4, and the middle screw synchronizes #2 & #3.  First adjust the left screw so #1 & #2’s mercury column heights are approximately the same.  A “normal” #2 Phillips screwdriver will work here.

 9.  Now adjust the right screw to the mercury column heights for #3 & #4 are approximately the same. A “normal” #2 Phillips screwdriver will work here. 

10. Adjust the middle screw so the mercury column heights for #2 and #3 are approximately the same.  You’ll need to use that 16” Phillips screwdriver to reach through and past all the wires, cables, frame and linkages.

11. Reset the idle speed if necessary.  

12. Slowly increase engine speed with the throttle to 3,000 RPM and hold it there while observing the 4 mercury columns.  They should be within one graduation of each other. If they are not, note which way they are off, and make adjustment at idle speed to compensate while still keeping the columns within one graduation of each other. Recheck your results at 3,000 RPM and repeat adjustments at idle if necessary. 

13. Reset the idle speed and you’re done! Remove the carb synchronizer and store it carefully. Reconnect the vacuum lines, reinstall the tank and take a test ride. Note how much smoother the low speed idle, throttle response and roll on is. Congratulate yourself on a job well done! Enjoy the ride, then park the bike and celebrate. It’s Miller Time!