13. ENGINE RUNS RICH AT ONE OR SEVERAL THROTTLE OPENINGS. If the engine shows symptoms of being rich at one or more throttle openings, but not all of them, refer to the part of Chapter 1 that pertains to the carburetor you are dealing with. Then, read the section(s) that describe the particular throttle opening(s) that are troublesome. These sections tell you how to vary the mixture at each individual throttle openings.

14. NOTE: When tuning more than one metering system of the carburetor, always start with the smallest throttle opening and finish with the largest opening. This is recommended because, as stated earlier, all metering systems have an effect on the ones following it.

As an example, suppose you had a 24 mm I R Z carburetor that was lean at 1/4 to 3/4 throttle and also lean at 3/4 to full throttle. If you changed the main jet first to a larger one to make the 3/4 to full throttle operation richer, and then changed the needle position to make the 1/4 to 3/4 throttle mixture richer, you possibly would find that the 3/4 to full throttle mixture was now slightly too rich, due to the needle being raised. It would now be necessary to change the main jet back to one slightly smaller in order to attain the proper mixture proportions. If the 1/4 to 3/4 throttle mixture had been altered first, then the 3/4 to full throttle mixture would have only needed to be changed once.
 

Section 3:

Fine Tuning

2. To learn whether or not the carburetion is exactly right, you will need to take a series of spark plug readings or checks at 1/8, 1/4, 1/2, 3/4 and full throttle.

3. To take a spark plug reading, ride the machine at least 1/4 mile at the throttle opening you wish to check. If the bike has an ignition switch, reach down and turn it off and immediately disengage the clutch. The important thing to remember is that the fewer number of times the engine turns after the ignition is shut off, the truer the plug reading will be. Therefore, the quicker you can pull the clutch after turning off the ignition, the clearer the reading. All this must be done while holding the throttle at the position you wish to check ' otherwise the reading is meaningless. Coast to a stop and remove the spark plug to check its color.

4. If the machine has no ignition switch, as in the case of the Stiletto, the idle speed screw must be set so that the engine will not idle. Then, while riding at the throttle position you wish to check, simultaneously pull in the clutch and turn off the throttle, which will allow the engine to stop running. Coast the machine to a stop and remove the spark plug to take the reading.