CB125S Engine HELP

But definitely try turning off or unplugging headlight first cause thats the easiest.
 
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.
 
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.
 
Dang sorry to hear that didnt work. Let me think on it for a bit.
I am assuming you still got the selenium rectifier?
 
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
 
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.
 
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.
 
Im fresh out of ideas. Seems youve double checked everything.

:mad: frustrating.

Build her back to stock and see if she is still running as good as she originally did.
 
No, they wont run properly without a battery. :eek: Thats the reason they include a battery from the factory.

What type of battery do you have?

Im not familiar w the Radio Shack rectifier. Is is a solid state unit? Regulator/rectifier?
 
Don't remember the part number off hand, but it's a brand new yausa. Same bat it had in it before I took the motor apart and it ran fine. I even unhooked all the lights and ran it with a 12v battery yesterday (google search cb125 12v conversion, totally kosher to do), starts and idles way better, but unfortunately still loses power past half throttle.

Lots of old threads on the net about the radio shack rectifier. Not sure on its specifics but I did it years ago. I think its only a rectifier. Think I am going to toss the motor and stick a 200CC Lifan in the bike so atleast the whole thing isn't a total loss. Total pain in the ass to rip the motor all apart again just to see if it runs correctly stock, got too much other stuff goin on.
 
Can I rule out incorrect setting of the timing chain/sprocket/cam? If the cam was incorrectly set by one tooth on the sprocket/chain would it run so well at idle and up to half throttle?
 
natez said:
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.

At idle, the timing should fire at the F mark. Try to advance the ignition timing until it does. Then the advanced timing should be closer as well. That should improve things.
 
What about air filter? my XL600 stops dead in its tracks at 5000rpm WOT with those popular steelmesh-podfilters.replaced them w longer runners with foamfilter on end-7500rpm clean..

Try removing the airfilter/snorkel.
 
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