77 Inconsistent, Fuel or Spark or?

[redbike]

The first time this happened it felt like I was out of gas. I had enough gas, I changed the fuel filter at the end of the day. The next day it seemed fine at the start, later in the day the issue came back.
1. The issue is inconsistent, sometime it runs as it should.
2. The majority of the time the issue is when I am between 2800 and 3300 RPM.
3. It can run and start fine, then in the middle of a ride the issue can show up.
Anyone else with similar? The concern is with inconsistencies is how to find it (when its not there) and do I start with the carbs, fuel pump etc? Or, the ignition system?
The bike does have an aftermarket electronic ignition.
Thanks in advance!

[Fred Camper]

Seems like ignition to me. I had a aftermarket (Dina) ignition in my bike that would drop two cylinders once in a while. Carbs generally do not do that. Electronic ignitions do get hot that close to the engine so I would start by replacing it as you really cannot diagnose it when it is behaving well. On a car I once put an led on the component and grounded the other leg and routed the LED to the dash, and sure enough when I had the issue the LED would go out. As Gord says below, a gas cap that does not vent behaves the same way. If you pop the gas cap when it happens next and it solves it, a new cap is your solution.

[Rat]

Check gas cap venting ?

Gord(been there)Jones

[redbike]

I'll try the gas cap first. Can't get much easier then that!
Thanks for the suggestions. I'm hoping to not have the change the ignition, the inconsistency makes me believe it is a spark issue.

[CYBORG]

Had a similar problem. Turned out to be the fuel filter particularly blocked

[gltriker]

The first time this happened it felt like I was out of gas.
1. The issue is inconsistent,
My concern is
The bike does have an aftermarket electronic ignition.

Seems like ignition to me. I had an aftermarket (Dyna) ignition in my bike that would drop two cylinders once in a while.

I'm hoping to not have to change the ignition, the inconsistency makes me believe it is a spark issue.

20220317_191536_1647613874595_resized.jpg

When the engine is running, correctly, individually probe at both test points.

**note: low watts = an old school incandescent small flashlight bulb

When both test points present a steadily flashing light in synchronization with their individual coil's spark plugs, its good.
If the running engine suddenly/seemingly loses spark, probe both test points, again.
Find one NOT flashing... that is the typical example of a mysterious Dyna S' ground path switching Module failure.
Its 2 individual Modules each performs an "Electronic" simulation of a contact breaker points function.
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Remedy? a dyna s clone is available on ebay. (search for DS1-3) less than $45
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20221025_170030 (1) - primary ignition wire circuits test points with low watts 12vdc voltage probe light.png

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AN EXAMPLE OF HOW THE TESTER FLASHES AS THE CRANKSHAFT IS SPINNING
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20221025_152106_1_1.mp4

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last frame of the video

Screenshot 2025-06-16 020237.png