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2025/03/03 14:46:35

Thornback

I Suddenly Had A Thought . . .

A few days ago, a thought popped into my head. As a teenager I had a Cushman Eagle motor scooter with a 5-HP 4-stroke engine. I used to set the points by rotating the crankshaft around until the cam on the crankshaft opened the points. Then I would set them at .030. So now I'm thinking that if the points opened every revolution of the crankshaft and that fired the sparkplug then the sparkplug would have fired on the compression stroke of the piston and also fired on the exhaust stroke of the piston. IE., 2 revolutions of the crankshaft = 4 strokes, 2 revolutions of the crankshaft = points open twice and spark plug fires each time. I can't get the fact that the sparkplug also fired on the exhaust stroke out of my head. Anyone thinking differently?

3 comments Leave a comment

Conrad Sigona

I'm not familiar with that specific motor, but it's typical that the camshaft turns at 1/2 the rpm of the crankshaft. Are you saying that your motor did not have a camshaft, but opened and closed the points and the valves using cams on a crankshaft?

In any case, it's not uncommon for plugs to fire once per crankshaft revolution. Take, for example, an ignition coil mounted next to the flywheel, where the magnet on the flywheel triggers the spark. The magnet revolves at the same speed as the crankshaft, hence we get one useful spark at the end of the compression stoke and one useless spark at the end of the exhaust stroke.

2025/03/03 21:08:42

SRTsFZ6

"Wasted spark"-

Very common and as noted, ignition is run off the crankshaft.

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2025/03/04 05:59:06

Thornback

Thanks for the two replies. You explained it better than I did. Because I worked on that motor 70 years ago as a teen, I can't remember anything about the camshaft. I just remember removing the flywheel cover which exposed the points and condenser and there was a cam profile on the crankshaft which worked the points. That cam profile could have been manufactured into the crankshaft or could have been a cam shaped ring that slipped over the crankshaft and was 'keyed' to the crankshaft.

2025/03/04 10:39:22

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I Suddenly Had A Thought . . .

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