Throttle cam and cam follower

[eli_lilly]

For my 1985 Johnson 90, the manual indicates that the throttle cam mark should be in the middle of the cam follower (roller) when the "first embossed mark on the cam contacts the cam follower". What has never been clear to me is whether or not the cam and the follower are supposed to touch when the engine is at idle. What got me thinking about this again was a post here from last week where someone said that the timing should advance before the butterflies open. The way I have my sync set, that wouldn't happen, as the cam is against the follower, at the mark, at idle. Any increase in throttle moves the plates immediately. My question is whether or not the cam and follower should be touching at idle.

Thank you,
-E

 

[ob]

Re: Throttle cam and cam follower

When the controls are in the neutral position the cam follower (roller) should be on center with the scribe line of the throttle cam and "just touching" As in you should be able to easily roll it with a light finger touch.This insures that the throttle plates are not being held cracked open when in the neutral position.Subsequently ,when the throttle is advanced to full throttle position ,the carb flaps should be fully horizontally open but not past full horizontal ,otherwise they begine to overshoot to a closing position.Otherwise insure that the timer plate advances smoothly throughout its travel until it reaches its stop screw.It will attain this full advanced position well before the throttle plates are fully open.

 

[F_R]

Re: Throttle cam and cam follower

Eli, perhaps this will explain:
1. At idle speed, the throttle plates must be completely closed, and rpm controlled by spark advance only. So, no the roller is not supposed to be in contact with the cam at slow idle.

2. As you slowly advance the control to speed up, the cam contacts the roller when aligned with the scribe mark.

3. Now, at this point the spark advance and throttle plates move in unison at a pre-determined rate.

4. Once the spark advance gets to full advance, against the rubber bumper, it cannot move any further. And the carb plates are partially open.

5. Any more movement of the control beyond this point opens the carb plates without any more increase in spark advance--till the plates are fully open. That's as fast as she goes.

6. The problem with the plates opening too soon can be simulated by advancing the control while at the same time obstructing the movement of the vertical arm that moves the timer base with your finger. If you do that, you will see the plates being cammed open, but the spark advance is being held back by your finger. You are duplicating a condition where the spark advance is being held back by stickiness in the mounting of the timer.

When that happens, the motor is getting too much air for the degrees of spark advance and it will fall on it's face, maybe even quit running.

 

[eli_lilly]

Re: Throttle cam and cam follower

Eli, perhaps this will explain:
1. At idle speed, the throttle plates must be completely closed, and rpm controlled by spark advance only. So, no the roller is not supposed to be in contact with the cam at slow idle.

2. As you slowly advance the control to speed up, the cam contacts the roller when aligned with the scribe mark.

Thanks F_R. Now I am wondering at what point during throttle advance the roller would contact the cam? How much throttle movement is supposed to be present before they would touch? Based on OB's suggestion, I am guessing nearly immediate, that they would be a hair apart at idle?

-E

 

[F_R]

Re: Throttle cam and cam follower

As you retard the throttle (slow down) the cam leaves the roller at the scribe mark. From there, any more reduction in RPM is achieved by further retarding of the spark. So, every motor is slightly different in how far the spark is retarded to acheive proper idle RPM. But however far it gets retarded is however far it takes the other way to get back to the scribe mark.

 

[eli_lilly]

Re: Throttle cam and cam follower

As you retard the throttle (slow down) the cam leaves the roller at the scribe mark. From there, any more reduction in RPM is achieved by further retarding of the spark. So, every motor is slightly different in how far the spark is retarded to acheive proper idle RPM. But however far it gets retarded is however far it takes the other way to get back to the scribe mark.

I get it now, thank you!

-E