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Author Topic: 72 CB100 engine issues?  (Read 193 times)

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Offline JWorsech

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72 CB100 engine issues?
« on: May 07, 2025, 05:11:39 AM »
I picked up a running CB100 a few years ago, It needed a little work so i put new points, ignition coil, cleaned the carb. I set the timing and got it running again for a few days, albiet not running entirely smoothly. Then it just stopped running all together. No amount of starting fluid was firing it up, not even a pop from the engine. A friend of mine helped me go over everything and could not figure out why it wouldn't run. So we did a compression test on the engine and found that it was only getting to 120PSI after a dozen and a half kicks. I saw in the manual it should be at 170PSI. So any help in figure out what to do next to get this little bike up and running would be very welcome.
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Offline MauiK3

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Re: 72 CB100 engine issues?
« Reply #1 on: May 07, 2025, 07:52:18 AM »
Sounds as if you have an ignition issue, it should run at 120 psi, which will likely improve if it is run some more.
Check all the wiring and see if you have spark with the plug removed.
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Offline HondaMan

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Re: 72 CB100 engine issues?
« Reply #2 on: May 07, 2025, 11:02:40 AM »
By chance, did you also replace the condensor with the points? If so, re-install the old condensor. They don't ever wear out from use, and most new ones are coming from China and are JUNK right from their packages.
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Offline PeWe

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Re: 72 CB100 engine issues?
« Reply #3 on: May 07, 2025, 11:51:41 AM »
Compression test with wide open throttle?
If not, lower value.
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Online WhyNot2

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Re: 72 CB100 engine issues?
« Reply #4 on: May 07, 2025, 07:56:13 PM »
Need to have a fully charged battery as well.
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Offline Ozzybud

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Re: 72 CB100 engine issues?
« Reply #5 on: May 07, 2025, 09:11:37 PM »
Are you getting a spark?  Check for a fouled plug. Easy stuff first.
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Offline JWorsech

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Re: 72 CB100 engine issues?
« Reply #6 on: May 08, 2025, 07:03:13 AM »
Quote from: MauiK3 on May 07, 2025, 07:52:18 AM
Sounds as if you have an ignition issue, it should run at 120 psi, which will likely improve if it is run some more.
Check all the wiring and see if you have spark with the plug removed.
Wiring all seems good, and it does get a spark
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Offline JWorsech

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Re: 72 CB100 engine issues?
« Reply #7 on: May 08, 2025, 07:04:42 AM »
Quote from: HondaMan on May 07, 2025, 11:02:40 AM
By chance, did you also replace the condensor with the points? If so, re-install the old condensor. They don't ever wear out from use, and most new ones are coming from China and are JUNK right from their packages.
I did not replace the condensor, just the points, spark plug, and ignition coil as far as electrical goes.
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Offline JWorsech

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Re: 72 CB100 engine issues?
« Reply #8 on: May 08, 2025, 07:05:11 AM »
Quote from: WhyNot2 on May 07, 2025, 07:56:13 PM
Need to have a fully charged battery as well.
Got a new fully charged battery on it as well.
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Offline low-side

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Re: 72 CB100 engine issues?
« Reply #9 on: May 08, 2025, 03:42:42 PM »
If I recall correctly, those can jump several teeth on the cam sprocket when the cam chain is loose.  Might be something to check into if fuel and ignition are good.
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Offline carnivorous chicken

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Re: 72 CB100 engine issues?
« Reply #10 on: May 08, 2025, 07:24:11 PM »
Spark first, that's easy. Blocked jet too. It'll run on 120 and might go up some.
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