Weird Idle/Bouncing Tach

[Follybob]

OK, I have a 79 150 Johnson. It has been running well after a recent (6 trips ago) carb rebuild. I went out today and noticed the tach oscillating back and forth on the way out.

For example if it was running at 4000 it would bounce quickly from 3500 to 4500. When I got to the honey hole, and for the rest of the day, and was idling around I noticed the idle was not as smooth as before. It wasn't rough but was 'hunting' so to speak. I don't think it was missing or skipping. The rest of the RPM range also seemed affected as well but it is harder to observe at speed. I replaced both power packs with CDI units last year.

Top speed is unaffected. No other symptoms but the unsteady idle and tach. Has anyone seen this before? The motor was running awesome before this.

Rob

 

[emdsapmgr]

Re: Weird Idle/Bouncing Tach

The tach receives it's signal from the charging rectifier. When the tach starts to become erratic, it is likely the rectifier going bad (not charging.) If you don't replace the rectifier, the battery will eventually go dead. This tach issue is usually a separate issue from the way the engine runs.

 

[Follybob]

Re: Weird Idle/Bouncing Tach

OK. Are there any specific tests I should perform? Could a failing stator cause this?

Rob

 

[emdsapmgr]

Re: Weird Idle/Bouncing Tach

Yes. The factory manual will give you specific electrical tests for both the stator and the charging rectifier.