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Discussion Starter · #1 ·
Hi guys,

I'm having problems with my CB125 K5 as its not running smoothly at all.
Little bit of backstory: bought the bike in a horrible mechanical state. Got the head rebored to.75 oversize, new pistons, piston rings etc. Valve clearance adjusted, timing is correct. Rebuild the carbs using one of those Keyster refurb kits. I cleaned the idle passageways with tiny wire brushes and carb cleaner. But the bike just isn't running right. I checked the throttle valves already, they're inserted the correct way. It runs in idle without choke bit the revs are just all over the place, I don't touch the bike at al and it jumps between 1000 and 3000 rpm. It revs up and down randomly. It's so weird. When I apply throttle, the right exhaust puffs out white smoke with a little bang, it's exhaust pipe is also clearly hotter than the left, maybe something that's stuck inside the pipe itself?
After 5 minutes of letting it run that way the plugs are really soothy and dry. Pitch black.

Any thoughts of getting it to run nicely?trying to get it to run by the beginning of summer.
 

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Sensei
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27,173 Posts
Hi guys,

I'm having problems with my CB125 K5 as its not running smoothly at all.
Little bit of backstory: bought the bike in a horrible mechanical state. Got the head rebored to.75 oversize, new pistons, piston rings etc. Valve clearance adjusted, timing is correct. Rebuild the carbs using one of those Keyster refurb kits. I cleaned the idle passageways with tiny wire brushes and carb cleaner. ( doubt you had brushes small enough to do this, and some "rebuild" kits contain the wrong or "junk" jets.... Clean and reuse the original brass...) But the bike just isn't running right. I checked the throttle valves already, they're inserted the correct way. It runs in idle without choke bit the revs are just all over the place, (air cleaner clean?...Float height correctly set? shut-off needle and seat working correctly?) I don't touch the bike at al and it jumps between 1000 and 3000 rpm. It revs up and down randomly. It's so weird. When I apply throttle, the right exhaust puffs out white smoke with a little bang, it's exhaust pipe is also clearly hotter than the left, maybe something that's stuck inside the pipe itself? (more likely excess fuel burning inside the pipe, or mistimed plug firing)
After 5 minutes of letting it run that way the plugs are really soothy and dry. Pitch black. (again, air cleaner?....Fuel height in bowl correct?... Main needle clip in correct notch?... Mixture screw set within specs?)

Any thoughts of getting it to run nicely?trying to get it to run by the beginning of summer.
See suggestions in blue above......
 

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Discussion Starter · #3 · (Edited)
Thanks for the feedback!

Do you suggest getting the carb bodies ultrasonically cleaned? I'll look for the original jets.

I "cleaned" it with compressed air but a conversion to a foam element is on the list.

The timing was just set correctly by a retired Honda mechanic whom I trust.
I synchronised the valves as best as I could using the top cable screws. The valves cut off nicely and rise evenly. Cable's not stuck.

I haven't checked the float height! Thanks, I missed that! Needle clip is on the correct notch and mixture screws were screwed 1 and 1/2 turns out.

This is so valuable to me, thank you so much!
 

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Sensei is right.

Rich mixture You will get due to fuel overflow (float height, needle notch positiion) or restricted airflow. Usually old air filter will result in this...
 

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Sensei
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Best explained by a phone call.....Too much involved to type at the moment..... 540-525-5199
 
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