Yamaha’s XJ series of engines incorporated an intake balancing system consisting of a gallery located just below the intake passages connecting all four intakes together via small air passages (approx. 1/8″ ) the gallery itself is approx. 3/8″ in diameter and 16″ long on the 1100s, slightly less on the 750cc bikes.
To properly syncronize the carbs on a YICS-equipped Yamaha engine, the YICS must be deactivated so a true vacuum can be established.
A special tool is required to seal the YICS passages while this adjusting is going on. The tool incorporates three rubber hose sections to act as seals, they are located in such a way as to isolate the passages from each other.
You may purchase an official YICS tool from Yamaha, by special order from Yamaha, or search for one on eBay, or for a few bucks build one yourself:
Parts to build a YICS tool:
1 – 16′ long section of #6×32 threaded rod
7 – 5/16 washers with a 1/8″ hole
1 – 3/4″ fender washer with 1/8″ hole
1-1/2″ section of rubber gas line hose – approx. 3/8″ O.D. and 1/8″ I.D. cut to 1/2″ long (3)
1 – #6×32 wing nut
1 – #6×32 lock nut
12″ of 1/4″ I.D. metal tubing, no greater than 3/8″ OD
It will look something like this:
To use just remove the gallery plug, it’s located on the left side of the engine just below the carburetor inlets, should be 12mm. Slide the tool all the way in until the fender washer come in contact with the tube. Now while holding the bent part of the threaded rod tighten the wing nut till firm. Hook up your vacuum gauges or carb sticks. Start the engine and allow it to warm up. It is normal for the engine to idle slower than normal so you may have to increase the idle speed to 1000 – 1200 rpm. Now proceed to adjust the idle mixture on each carb for maximum rpm. should be approx. 1- 1/2 turns out. Sync. the carbs for even intake vacuum balance.
Once the carbs have been synced, shut down and remove the YICS block off tool, the carb sync tool, replace the gallery plug and the rubber caps from the carb intake boots, restart and set the idle to @1000 RPM.