omc 4hp timing marks

[frankr]

quote Wedgie:

This 1974 motor runs pretty good now, but I would like for it to idle a bit lower.

I set the points using a timing fixture and a test lamp. I set the points to break in the middle of the two lines on the mag plate base. When it is running, the strobe indicates it is breaking on the left side line. Being that the points are used, using the timing fixture, should I set the points to break on the right side line ?

Thanks Wedgie, that is something I’ve wondered about but never took the time to investigate. It takes time for things to happen, once the points break. I think that is why you see the spark occurring just a hair later than what you set it for. I’ve wondered just how much time, and you have answered the question.

As far as what it all means in the actual world, you set the points to break at the proper time in relation to the magnets passing by the coil, and to get them 180 degrees apart. The purpose is NOT to time it before or after TDC. That is controlled by the throttle rotating the armature plate. And at idle speed, the armature plate position is what controls the RPM solely by the amount of spark advance/retard. Again, if they are 180 degrees apart, and sync’d with the magnets & coil so you have best possible spark, that is about all the points setting is going to do about the quality of idle. Any effect from advance/retard of spark in relation to TDC caused by points setting is a drop in the bucket compared to moving the armature plate.

BTW, the strobe timing light is indicating the time the spark occurs in relation to the flywheel and armature plate relationship. It is NOT telling you when the spark is occurring in relation to TDC. That would require a timing mark on the block to indicate TDC. And there is none.

[fleetwin]

OK, now I am getting confused. The flywheel has only one mark, the mag plate has two. Wedgie: go ahead and set the points to start opening at the right hand (in your picture) mag plate mark.

[crosbyman]

seems all complicated.

I just set my engines at 0.20 on the high point of the the cam .

My 5.5, 7.5, 4hp all run fine and great for trolling

not scientific one bit but it works for me.

Joining AOMCI has priviledges 🙂

[Mumbles]

The timing fixture and the flywheel keyways both have a miniscule amount of slop in them otherwise they wouldn’t fit over the key. Turning the motor over by the fixture takes the slack out of the keyway fit and true timing could be slightly retarded compared to the indicated timing on the marks. Allowable factory tolerance in the machining of the flywheel could also affect this. For everyday use, if the flywheel mark lines up between the two marks on the plate when checked with a timing light, the motor should run fine. Finer tuning of the mag will place the marks where you want.

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