94 Merc 25HP timing advance

[AGUS]

I have a 25 HP mercury, 1994. On other motors that I've seen, the timing is advanced, mechanically as the throttle is opened. On this motor the ignition plate is fixed to the block with a link. I'm wondering if and how timing advance is accomplished as the engine speed increases/throttle opened.

The reason for this question is more than just curiosity. I am converting a tiller to remote control and trying to figure out the linkage

Thanks Aaron

 

[AGUS]

Re: 94 Merc 25HP timing advance

I have a 25 HP mercury, 1994. On other motors that I've seen, the timing is advanced, mechanically as the throttle is opened. On this motor the ignition plate is fixed to the block with a link. I'm wondering if and how timing advance is accomplished as the engine speed increases/throttle opened.

The reason for this question is more than just curiosity. I am converting a tiller to remote control and trying to figure out the linkage

Thanks Aaron

 

[Texasmark]

Re: 94 Merc 25HP timing advance

My knowledge of Merc, going back to the '80's as an owner is that they setup the linkage so that at about half throttle or so the carb butterflies are fully opened. The remainder of the rpms come from spark advance from roughly no advance at idle to 20 BTDC or thereabouts at WOT. The spark advance follows the throttle movement all the way.

Look at the butterflies. Advance your throttle. When do they stop moving? If they keep going all the way to WOT then you may not have any. Or you may have centrifugal advance like cars used to have with points and plugs. The weights attached to the distributor would sling out as rpms increased mechanically advancing the timing. You would have to remove the flywheel to see this I would imagine.

I really doubt that you would have either fixed timing around 0 or fixed at 20 BTDC. In either case either the timing would be too fast for idling, or too slow for the torque required to produce the thrust required at WOT.

My 2c,
Mark

 

[AGUS]

Re: 94 Merc 25HP timing advance

It must have the centrifugal advance because there is nothing that moves with the throttle at any time, no linkage connected to throttle. Thanks for the reply.
Aaron

 

[Texasmark]

Re: 94 Merc 25HP timing advance

Well sir, if you feel up to it, pull the flywheel and have a look. I would be interested in your answer.

Mark

 

[AGUS]

Re: 94 Merc 25HP timing advance

Too much work for curiosity sake. We know is has some kind of advance and centrifugal is most probable. I was hoping to get a first hand absolute answer. Thanks Aaron

 

[25xs]

Re: 94 Merc 25HP timing advance

If your trigger advance is "locked" by a short "dog bone" shaped ball & socket link then you have electronic advance built into the switchbox. You can check the idle timing and WOT timing with a standard automotive timing light if you're curious.

Facing the carburetor inlet, the link would be above & to the left of the carburetor. Just behind the manual starter rewind housing mount on the starboard side of the engine. Another way to easily I.D. the system is the switchbox will be a tall rectangle sitting vertical on the port side of the block. The mechanical advance timing ignition system is a smaller, horizontal rectangle and has bullet connector wiring holders built-in on the surface.

That engine can handle ALOT of variation in the timing setting without adverse affects, however setting the base or idle timing can greatly benefit the smoothness at idle speeds.