If you have a stock 1981 GPZ 550, you probably noticed the idle gets out of control during starts with the choke on. This is because the stock TK smooth-bore carbs use a choke plate with spring loaded choke flaps. This creates a hysteresis effect with the RPMs. In other words, if you rev the engine a little, the revs climb uncontrollably and if you let the revs drop, they drop too low. This is actually normal for this bike especially in cold weather. It is an inherent flaw in using spring loaded choke flaps.

The choke lever not only lowers the choke plate over the intake, it also opens the throttle to achieve a faster idle. When the engine first starts, the choke flaps are closed and the throttle is held slightly open. Since the flaps are closed, the engine doesn't get enough mixture to take off and the RPMs stay down even though the throttle is slightly open. If you open the throttle more, to rev the engine, the vacuum to the flaps increase and they open up. As they open, the engine gets more air/fuel and revs more, this in turn opens the flaps more and the engine revs uncontrollably. Closing the throttle does little since the fast-idle cam on the choke lever is already holding the throttle slide open enough to maintain the runaway idle. In this condition, the engine is getting plenty of air and is burning fuel which accumulated before the revs took off. The mixture eventually becomes too lean and consequently the RPMs will drop. As they drop, the flaps close and the mixture once again becomes rich preventing the engine from dying completely.

There is no cure for this behavior short of fabricating some sort of linkage to the flaps, controlled by the throttle. However, the behavior can be made far less volatile. The trick is to set the fast-idle screw very carefully. This will minimize the runaway idle behavior, but the setting is only valid for a given temperature. So when the weather gets colder, the bike will die more often (if it even starts), and when the weather is warmer the runaway idle gets worse.

What is really needed is a way to access the fast-idle screw without having to remove the gas tank. Then a simple turn of a knob can compensate for any change in the weather. I have done this and now I don't have to wait 10 minutes for the bike to warm up before I ride in colder weather. The choke lever still has to be adjusted once or twice as the bike warms up, but the idle is no longer crazy and I can ride the bike as the engine warms without much trouble.

The mod itself is simple. Just get an appropriate allen screw (they are preferrable since they are usually harder) about 2 inches (50mm) long. Then find a large nut and press it onto the head of the allen to act as a knob. The fast-idle stop screw will be replaced by the allen screw. As the throttle is opened and closed, the screw rotates with the throttle shaft and the tip hits a stop. Because of this, it is important to grind the tip into a rounded point to prevent the threads from being damaged. Also, a spring is needed to stop the screw from turning from vibration. A washer or two would also be a good idea.

Replace the fast-idle screw and locknut with the allen screw and spring as shown.

This is what you see from the side of the bike. The "knob" should just stick out above the carb tops. There should not be any interference when the throttle moves, but double check and adjust as necessary. If the screw is too long it may hit something.

Now you have a way of controlling the fast-idle independently from the choke lever.