Panhead ignition timing question - Harley Davidson Forums: Harley Davidson Motorcycle Forum
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post #1 of 7 (permalink) Old 08-31-2019, 03:38 PM Thread Starter
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Panhead ignition timing question

With a single points setup, do both spark plugs fire at the same time at the same camshaft position (when points open)? If so, wouldn't that result in the rear cylinder firing during the intake stroke? For example, if the front cylinder fires at 20* BTDC (compression stroke), the rear cylinder would fire at 25* AFTER TDC (intake stroke), due to the 45* difference between cylinders.

I don't even own one of these, but the curiosity is killing me! And my internet research skills have failed me.

Thanks in advance for any answers,
Al
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post #2 of 7 (permalink) Old 08-31-2019, 04:40 PM
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The breaker points are operated by a cam with a narrow and a wide lobe. The narrow lobe times the front cylinder , the wide lobe times the rear cylinder. A single ignition coil fires both spark plugs at the same time, one will spark in the exhaust stroke , while the other fires the combustible gases in the other cylinder to produce the power stroke. Timing is advanced or retarded by manual rotation of the circuit breaker base in relation to cam.
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post #3 of 7 (permalink) Old 08-31-2019, 06:03 PM
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Quote:
Originally Posted by al68 View Post
For example, if the front cylinder fires at 20* BTDC (compression stroke), the rear cylinder would fire at 25* AFTER TDC (intake stroke), due to the 45* difference between cylinders.
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post #4 of 7 (permalink) Old 09-01-2019, 06:51 AM
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Both fire at same time, one on compression the other is on the exhaust stroke.
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post #5 of 7 (permalink) Old 09-01-2019, 02:03 PM Thread Starter
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Quote:
Originally Posted by bobf View Post
Both fire at same time, one on compression the other is on the exhaust stroke.
Except that the rear cylinder will already be on the intake stroke when the front cylinder fires, since they are 45* apart. Hence my question.
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post #6 of 7 (permalink) Old 09-01-2019, 02:27 PM Thread Starter
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Originally Posted by Kainam View Post
Thank you, I read the article. It does say that when the front cylinder fires the rear cylinder is on the intake stroke, as I said above, but the article then says that the intake valve is not yet open on the rear cylinder. This makes no sense given the valve timing. The rear cylinder will be 45* after TDC (intake stroke) when the front cylinder is at TDC (end of compression stroke). Even at 30* advance, the rear will be 15* after TDC on the intake stroke, closer to 40* after TDC at idle. The intake valve will have been open for ~15-40* depending on timing advance.

I must be missing something, that's why I asked the question. Unless there just isn't enough fuel/air drawn into the cylinder yet at 15-40* ATDC to hurt anything.

Last edited by al68; 09-01-2019 at 05:43 PM.
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post #7 of 7 (permalink) Old 09-09-2019, 06:12 AM
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Same game in a "nose cone" points-triggered system.
Waisted spark is of no harm.

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