what is the function of the spark advance lever

astampfel

Petty Officer 2nd Class
Joined
Aug 2, 2010
Messages
100
In my Johnson manual, 1990 90 Hp, on page 1-36, there is a section called, "Cam Follower Pickup Timing". In that procedure, it tells me to move the spark advance lever until the cam follower is aligned with the center of the mark on the throttle cam. I have done that, but I don't see how the 'spark advance lever' has anything to do with the spark advance when it is controlling the throttle cam and is not even hooked up to the timer base! The only lever that is hooked to the timer base is the 'spark advance rod'. So, is the purpose of the 'spark advance lever' to get the embossed mark aligned to the center of the roller? Next in that procedure, it states for me to adjust the wheel on the 'spark advance lever' to mess with the timing. Won't this just open or shut my carb while I'm at idle? Shouldn't my carb be closed at idle? Why do they want me to mess with my carb position at idle? If I move the thumbscrew on the 'spark advance lever' then it moves the embossed mark away from being in the center of the roller, which is not where the roller should be at idle.
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Re: what is the function of the spark advance lever

I don't know what is causing the confusion, but at slow idle the carburetors are completely closed and the rpm is controlled by spark advance/retard. As you advance the throttle control, first the spark advances a bit, then the cam contacts the roller and the carbs slowly open as the spark continues to advance. Once the spark reaches full advance, it stops moving and continued control movement only opens the carburetors to wide open.

All this should be obvious if you slowly operate it by hand with the control cable disconnected. By "operate it by hand" I mean by hand at where the cable was disconnected. What you just wrote doesn't make sense. Go back and take a second look.
 

mrcj001

Petty Officer 1st Class
Joined
Jul 25, 2008
Messages
313
Re: what is the function of the spark advance lever

When you give the engine throttle, the spark advance rod moves before the cam follower starts to open up the carburators throttle valves. If it dont /or isnt, the engine will fall flat on its face. The carbs idle with the throttle valves closed. Engine idle is adjusted by spark advance.

Its best not to mess with the spark advance adjustment under the flywheel unless you have serviced the stator or other components.

The engine syncro can and is a bit confusing.
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Re: what is the function of the spark advance lever

Thinking about it, the spark advance lever and cam rod/ lever are interconnected by a coiled spring inside where they pivot around the mounting bolt. If the timer base isn't moving freely, the spring isn't strong enough to make it move. Perhaps that is what is going on here??? Maybe the timer base is sticky.
 
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