Re: 1974 Evinrude 9.9 won’t idle at low RPMs, need help

[optsyeagle]

No, not the point gap. The points are under the flywheel,

That metal bracket (throttle cam) is how you sync the carb. It will push on that roller as you turn the twist grip of the tiller arm. The first thing is to see if the indented arrow on that cam lines up in the center of the roller JUST as the cam starts to push on the roller (throttle butterfly starts to open). This is what they call the carburetor synchronization and it sync’s the carb to the spark timing of the motor.

In the quote you linked, I was referring to the fact that when the motor is running properly, as you reduce the throttle to idle the motor’s RPMs down, there will be a point where the throttle cam stops pushing on the roller. When that happens the carb’s throttle butterfly will be completely closed. As you reduce the throttle some more, the only thing that changes, to allow the RPMs to decrease anymore, since the gas from the carb is now constant, is the spark timing. Just so you know, on these motors the spark timing is changed as the plate under the flywheel is rotated. They call it the timing plate. As you throttle up, the spark timing is advanced a little ahead of top dead center to deal with the faster RPMs and as you throttle down, the spark timing it is retarded back again. This is because a plugs spark is not exactly instantaneous and therefore precisely WHEN it sparks, needs to be adjusted for the different RPMs of the motor to optimize its performance.

That change of spark timing is also going on at the very low RPMs required for idle. If the timing is also changed due to improper point gaps (another spark timing mechanism) then it can cause problems when trying to idle the motor.