Idle control is done with different methods. It could be external idle valve or stepper motor or it could be done with partial throttle motor or full drive by wire throttle. Due to the delay in mechanical control to stabilise idle, you need to tune the ECU and idle valve together. The reason is that the ECU can react faster than a mechanical air restrictor valve. And most of the times hunting is a result of incorrect engine tuning. If idle drops then the valve opens to correct. That adds more air resulting in a worst effect and it comes with a delay. Then the ECU will add fuel due to more air resulting in high rpm which in turn close the idle valve and this repeats out of control. Also after blip the idle valve must capture the engine rpm before it stalls. Again here the ECU can react faster by adding timing and fuel to assist in this process. This section describe what needs to be done on the ECU side to help smooth idle control. See the different types of idle control and how to adjust their settings in the idle control setup section.

Setting the Idle Jet for Time & Fuel
When an engine idles in neutral and is at working temperature, the idle timing should be around 15 to 18 degrees depending on altitude. This may vary from engines but this is a basic guideline. Increasing idling degrees usually result in higher RPM’s. Do not confuse this thinking it is what the engine needs. All you will do is lowering the air volume to bring idle down. This will result in very poor power and engine will easily die off when you put a bit of load on it. The lower the engine timing the slower the engine idles and the more idle air it requires. It will also require a slightly richer mixture. Now if you put a little load on the engine you immediately add timing which will make more power due to the rich mixture with more air. Adjusting timing is immediate and there is no mechanical delay. Now when an engine wants to hunt you can effectively control power to it without the idle control valve or throttle movement. Below is some basic ECU settings.

First adjust the timing so that at idle there is 15 degree dynamic timing and no vacuum timing.

Adjust the Idle fuel so that the engine has just enough fuel to idle constant. Do not make it too rich. It should be close to avalanche point where if you go leaner the engine will lose power. The best way to see this point is where the vacuum signal will be at its lowest point for normal engines. Any more or less fuel will results in higher vacuum. Racing cams and matrix tuning will be saver to look at A/F ratio. Idling RPM’s should be on your Target RPM value. In this example 800 RPM. Notice that the green and brown graph below on RPM compensation is at zero compensation. Only the main graph determine idle fuel. Also make sure water and air compensation is zer0.

Now when the engine hunts we will use timing to control it before the idle valve needs to act. First you must understand what happens in hunting. The engine will lose power resulting in an increase in manifold vacuum. The vacuum bar will move to the right and the RPM bar will move to the left. Due to our slower timing we can increase timing to add power which will increase RPM. Adjust the Low rpm timing graph as below.

Lower rpm also brings more air in the manifold which requires a bit more fuel to make the mixture a power mix. Adjust the green and brown graph on the RPM graph at the lower rpm range to add a certain % fuel. Note on the fuel load graph we already should have a slight increase in fuel.

Now the above adjustments will capture falling RPM’s and it may now cause the idle control valve to react as well adding air. This will increase RPM above target. There is an up and down response on the idle valve control settings. As a rule we make up response faster to ensure that the engine does not stall. Then we make down response slower to prevent hunting. Will result in more vacuum which makes the vacuum bar move left and the RPM bar moving right. First we use timing again to make it slower reducing engine power.

Then less air also needs less fuel. This time we adjust the fuel load graph as the engine don’t use this part for normal riding. Note however, that deceleration will cause the vacuum to move in this region. If the mixture is too lean then the engine to being jerking due to too lean mixture.

Automatic Transmissions
For automatic transmissions tuning principles are the same as above. But there are two load steps that differs for neutral and drive. After you do the neutral tuning, then put the engine in drive. The load on the engine from the transmission will cause the vacuum bar to move to the right quite a bit. RPM is controlled and will stay the same. So you need to make sure that the idle fuel will increase for the added load