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Author Topic: CB125S Engine HELP  (Read 1202 times)

Offline trek97

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Re: CB125S Engine HELP
« Reply #20 on: Apr 02, 2017, 19:25:53 »
But definitely try turning off or unplugging headlight first cause thats the easiest.

Offline natez

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Re: CB125S Engine HELP
« Reply #21 on: Apr 02, 2017, 19:25:56 »
Yea I did a compression test the other day. Rock solid 155psi.

Charging system issue is def a possibility, and thats where things start to get confusing. This bike has a funky 6 volt electrical system. Won't run properly without a battery, burns out highlight bulbs if you do try to run it without a battery. These engines always run alittle weird with a bad battery but mine is brand new, and I have checked it and such. I am about to go try to run it without the head light plugged in but not sure its going to make a difference. It is the only light I have on the bike except on very small LED brake light. If the charging system is shot, do I replace the stator or somthing? When I took the engine apart I tested the stator as per the maint. manual and it checked out fine.

Offline trek97

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Re: CB125S Engine HELP
« Reply #22 on: Apr 02, 2017, 19:27:59 »
I gotta go fetch a pizza...try the headlight and report back.

Offline natez

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Re: CB125S Engine HELP
« Reply #23 on: Apr 02, 2017, 19:43:14 »
Tried it. No change.

We can rule out the timing being off when I set up the cam sprocket right? Would it idle and run from idle to just under half throttle so well if it was off? I have also tried different points plate positions to change the timing either way (now back to stock position) with no change (only thing that would change is it would be very hard to get started in one direction). So it's either got to be the advancing unit not advancing the timing properly (inspected the unit, cleaned, lubed etc) or its got to be some sort of coil/power issue. Wires on the coil can't be reversed, I never disconnected the coil when I did the engine, stayed on the frame, only disconnected the spark plug wire.

Offline trek97

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Re: CB125S Engine HELP
« Reply #24 on: Apr 02, 2017, 21:19:01 »
Dang sorry to hear that didnt work.  Let me think on it for a bit.
I am assuming you still got the selenium rectifier?
« Last Edit: Apr 02, 2017, 21:21:06 by trek97 »

Offline natez

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Re: CB125S Engine HELP
« Reply #25 on: Apr 03, 2017, 20:50:23 »
I converted it to the radioshack one a while ago....

Offline natez

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Re: CB125S Engine HELP
« Reply #26 on: Apr 03, 2017, 20:57:32 »
If the timing (talking about the timing chain/can sprocket, not the points plate) was off one tooth in either direction, then the engine would run like crap at all throttle settigs righr? I was to rule that and/or anything else that I may need to disasemble the engine for.... like i said it starts and has great power up to half throttle position

Offline teazer

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Re: CB125S Engine HELP
« Reply #27 on: Apr 03, 2017, 23:29:19 »
Could be any number of things but I'd start with taking off the carb and cleaning every orifice again.  Try it without a main jet and see if the stumble is the same or if anything changes.  Have you tried putting on the choke when it's running to see if that made any difference.

I wasn't joking about trying it with no main jet.  At half revs the plug should not be hot enough to burn clean, so the color suggests that it's way lean.  It could be a bent float and partially blocked fuel supply.

I didn't read what you you found with a timing light.  You may need to get a spare alternator cover to drill a hole in to see where the spark is timed without spraying oil everywhere. On Hondas with wet alternator it is a good idea to drill a 1" hole in a spare side cover  to reduce oil spray. And then use an automotive timing light to strobe set the timing.  AT tickover the static pointed should point close to the F mark or T mark and by about 3500 revs it should be between teh two fiull advanced marks.

Offline natez

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Re: CB125S Engine HELP
« Reply #28 on: Apr 06, 2017, 17:26:45 »
Right but can I rule out incorrect setting of the timing chain/sprocket/cam?


Offline natez

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Re: CB125S Engine HELP
« Reply #29 on: Apr 06, 2017, 18:12:49 »
Just ran it with the timing light. The static pointer is dead on the T mark at idle. When you rev it up timing advances to the left hand of the two timing marks. It strobe doesn't miss or anything when you start to rev through the bogs so I guess we can rule out an ignition issue.... That damn carb is clean as a whistle, float is set right. I haven't tried it with no main jet yet, but when you move up in jets from 100 (100 is what another person with the same motor and carb went with after he did the same cylinder conversion) to 125, the miss/bog remains the same however it begins to smoke a bit more. Pull the plug and its darker/sootier.