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Author Topic:   Two-Stroke Ignition Systems
duece posted 06-12-2007 10:06 PM ET (US)   Profile for duece   Send Email to duece  
wecome all i joined this site because i saw some great ignition info for mercs i just had a meltdown on my 70 hp three cyl the stator tested bad as well as one channel on the switchbox i replaced everthing with a red stator kit which includes a stator rectifier and a voltage regulator and a new switch box the only thing original is the trigger timerbase but it tests out good now i have good spark on all cyl but the motor runs like crap has anyone done this job ?does it throw the timing off?what does the trigger fire from the stator or the flywheel
jimh posted 06-13-2007 08:20 AM ET (US)     Profile for jimh  Send Email to jimh     
The usual arrangement on an outboard motor for developing the trigger signal for the spark ignition is to use a set of coils (called a timer base or trigger coils) which are arranged to be excited by a set of magnets in the flywheel. The coil assembly is usually made to rotate with respect to the crankshaft over a small arc, usually about 20- to 25-degrees. The position of the coil assembly is mechanically linked to the engine throttle by a series of cams and levers in such as way so that as the position of the throttle is advance, the position of the timer base rotates so as to advance the timing of the spark ignition.

The electrical signals developed by the timer base coils from their being excited by the flywheel magnets are routed to a electrical amplifier (called a power pack or a switch box), where they are boosted in level and applied to the primary windings of the spark coil.

If you have replaced any of these components it is a good idea to check the engine spark timing. The usual procedure for checking the spark timing is to:

--verify the position of the number one cylinder at dead top center using a piston stop gauge;

--verify the alignment of the timing mark with the engraved timing numbers on the flywheel;

--connect a timing light to the number one cylinder;

--operate the engine and observe the spark timing;

--adjust as necessary to conform with the specifications for the engine.

The factory service manual for your engine should contain a detailed procedure for adjustment of the spark timing.

In many classic two-stroke motors the spark timing is intentionally retarded quite severely in order to permit the engine to idle down to a slow speed. As the throttle is advanced, the actual carburetor throttle linkage may not even move during the initial portion of the throttle advance, and only the engine timing is affected. As the engine timing is advanced the engine speed will increase. At some point, typically around 1,500-RPM, the linkage will be adjusted so that further increase in the throttle will produce an opening of the carburetor throttle plates in synchronism with increasing spark timing advance. This will continue until the ignition spark timing advance reaches a limit, typically around 15-degrees of spark advance, and then further movement of the throttle only produces more carburetor throttle opening.

Verify that you have the throttle linkage properly adjusted. Consult the service manual of your engine for details.

jandrewg posted 06-13-2007 12:35 PM ET (US)     Profile for jandrewg  Send Email to jandrewg     
I just changed the stator on my 1995 Mercury 125hp and used the red stator conversion kit. No, the stator doesn’t have provision for adjusting the timing; only the trigger will affect that. However, be sure that you positioned the stator correctly when you changed it. Actually, it’s pretty straightforward as there are arrows taped to the top of it, showing how it should be oriented. The correct arrow for your application will want to face aft.

The trigger has the role of supplying voltage for ignition, but the stator plays a role here too, as my old stator failed on the low rpm winding side (the blue and blue w/white stripe wires) and this meant that I didn’t have spark to start the motor.

Jim is right, by all means get a manual. I did, and it is tremendously helpful. The manual for your three cylinder is the same as the one for my inline four. The correct OHM values are given for the electrical components, as are step-by-step procedures for diagnosing issues such as yours.

Remember, it’s not black magic, just a mechanical problem. Given the right information to find it, you will.

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