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The fuel mixture screws control fuel. They are used in conjunction with all of the carbs other jets to allow a means to compensate for differences in seasonal temperature, and geographical locations.

Like a general rule turning the screws in restricts fuel giving learner mixtures at idle and in the cruising mode. This can be useful when operating in hot weather or high altitudes.

Turning the screws out increases fuel flow giving richer mixtures which can improve cold weather operation.

You must do these adjustment procedure with your carbs previously synchronized

Well, first of all, where are they?... in our FJ they are at the top of the carbs body. See the photo. Each screw must be covered by a little plug, but it could be a lot of FJ without them (like mine). In FJ European models you can take the plugs out with a little screwdriver, but in Usa FJ types you must use a drill very carefully to extract them.

Basic Yamaha base settings: FJ 1100 & 1200 to 91: 2 turns out from fully closed

                                            FJ 1200 from 91: 3 turns out from fully closed

1.- Start taking the fuel tank out in you FJ 1100 model and plug the vacuum line that is attached to the tank and use a alternative fuel tank or fit longer fuel lines to position your tank in a better place to get access to the screws. 88 on models only turn 180º the fuel tank like at the pic. Previous models need an auxiliary fuel tank.

2.- Check that the pilot screw are positioned at their base settings.

CAUTION The fuel mixture screws are easily damaged by careless adjustment. If you turn the screws in and do not stop when it seats lightly, you can break the metering tip off. Sometimes the screws may be tight in the threads and if you are careless by the time you find it out the screw slot is rubbed off.

The best procedure is to get a screw driver with good sharp edges that fit the slot closely. Then turn the screws in first about 1/2 of a turn. Now, blow out any metal shavings or crude. Next put in a few drops of penetrating or light oil. Then turn the screws out a couple of turns and back in, to make sure they move free. Now turn the screws in until they seat *lightly*  then turn them out the number of turns given as the base setting

3.- Warm the engine up to normal operating temperature

At this point, previously I have synchronized the carbs. But don't  leave the motor gets too hot.

4.- Set the idle down to approximately 500 rpm. The best method for setting the mixture screws is using the “Blip the Throttle” test.  Start with the mixture screws set at 2-1/2 turns out and blip the throttle.  Pay attention to how the engine RPM returns to the initial idle speed.  If the mixture is rich, the engine speed will “hang up” and slowly return to idle speed, and if the mixture is lean, the idle will drop quickly, then slowly recover to the correct speed

5.- Adjust the mixture screws within 1-1/2 turns either way of the base setting to attain the smoothest and fastest idle. If the idle speed increases after setting any screw, reset the idle to around 500 rpm before going to the next carb. The full mixture screws do not have to be at the same settings as long as each setting is the best running setting for that carb.

6.- Go carb by carb: adjust 1/2 turn out in one carb. If the idle raises, reset the idle again to around 500 rpm and go 1/2 turn out. Repeat the procedure until the idle drops. Then go back at the setting before this one. leave this carb at the setting obtained and go for the next.

7.- Reset idle to factory specifications.

8.- Switch the engine off and replace the fuel tank.

I think this is not a difficult task and cheap too. Other ways is to use a gas analyzer attached to the original collectors  fittings (very expensive) or use a Colour-tune. It replace the plug and show if the mixture is lean or rich changing its colour. I have not tested, but I have read great reviews. Take a look at Gunson limited

Good luck and enjoy