omc 4hp timing marks

[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 ?

[chris-p]

Were points new?

[fleetwin]

Well, having the points start to open at the second mark means the point timing is at the retarded end of the spectrum. I would try to set the mark more towards the middle between the two marks. New points are usually set to the first mark (right mark in your picture) to account for initial rubbing block wear.
All that being said, I just wouldn’t expect a 4hp to idle as nicely as the older 3hp models, but resetting the points might be worth a try, too bad you have to remove the flywheel again to do it.

[legendre]

Any motor with used points, that you can reasonably expect to remain relatively stable, should be set for the "correct" timing. As fleettwin mentions, the other mark is there to account for initial break-in of a new set, which prevents the engine from running at a retarded angle once the new points settle.

Static timing with a continuity light / buzz-box is really just an initial ballpark, enough to get you up & running close to the mark. Final timing should always be set dynamically, using a strobe, if at all possible.

[wedgie]

quote Chris_P:

Were points new?

No, I just polished up an old set. I think I have a new set here somewhere. Maybe I’ll put them in just for an experiment.

quote fleetwin:

Well, having the points start to open at the second mark means the point timing is at the retarded end of the spectrum. I would try to set the mark more towards the middle between the two marks. New points are usually set to the first mark (right mark in your picture) to account for initial rubbing block wear.
All that being said, I just wouldn’t expect a 4hp to idle as nicely as the older 3hp models, but resetting the points might be worth a try, too bad you have to remove the flywheel again to do it.

Using a timing fixture I did set the points to open in between the two marks, but when running, the timing light indicates it is firing on the leftmost mark. That leads me to believe I should set the points to start opening at the right mark, or do I have it backwards Don?I might be "at the retarded end of the spectrum myself" 😆

I’m just playing around for practice with the timing fixture and to see if it really does make a difference on idle.
I don’t mind taking off the flywheel again. Might be a different story if it was a big twin that needs 105 foot pounds torque.

[legendre]

With the engine rotating in the normal running direction, the first flywheel mark to cross the datum is the most advanced. Any additional marks are progressively less advanced (or more retarded – whichever way you prefer to view it).

[wedgie]

quote legendre:

With the engine rotating in the normal running direction, the first flywheel mark to cross the datum is the most advanced. Any additional marks are progressively less advanced (or more retarded – whichever way you prefer to view it).

Thanks Legendary . That makes it easier to understand 😉

[legendre]

quote Wedgie:

quote legendre:

With the engine rotating in the normal running direction, the first flywheel mark to cross the datum is the most advanced. Any additional marks are progressively less advanced (or more retarded – whichever way you prefer to view it).

Thanks Legendary . That makes it easier to understand 😉

Not "legendary", but legendre.. it’s a surname. I’m neither a legend in my own mind, nor in my own time.. 😉

And I’m glad it is helpful, but I should have qualified it.. a more accurate statement is "With the engine rotating in the normal running direction, the first flywheel mark to cross the datum, after the reference cylinder has traversed BDC, is the most advanced". In other words, the first mark to pass the datum as the piston is heading back up to TDC.

Any gearhead should have understood, but it could be plenty confusing to the newbie.

[wedgie]

So just for fun I set the points to start opening on the left mark. With the timing light, the strobe flashes when the flywheel is past the left mark. It does Idle slower now.
I will re set it now on the right mark and see if it opens in the middle.

I assume the timing light should be set at 0 degrees ?

[legendre]

quote Wedgie:

I will re set it now on the right mark and see if it opens in the middle. (…) I assume the timing light should be set at 0 degrees ?

Not sure what you mean by "set at 0 degrees". The light flashes when the plug fires.. and only +if+ the plug fires.

(Some folks don’t know this, but the inductive timing light is a great diagnostic tool. Even if you’re not checking timing, it will tell you if the plug +actually+ sparks when under compression.. A coil impulse alone isn’t enough, the plug has to actually fire across the gap for the light to flash. Just because a plug will fire in free-air, doesn’t mean it’s actually firing under load. It’s great for t-shooting ignition issues, and has served me very well over the years.)

ETA: Some setups (like automotive) allow you to adjust timing on the fly, but most of the time, when you’re dealing with small engines or motorbikes, you have to T&E it. Nudge it one way, and re-check.. repeat until the timing is as close as you can get it before you totally run out of patience.. 😉

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