Topic: Honda CB 125 has weird rpm surge

[Shaferw]

Hi guys,
              So I just bought a '74 Honda CB125S. When I bought it everything ran perfect. But now a week into owning the bike I have this problem with the rpms going all over the place while I'm idling and the bike just flat out stalling when I ride for about half a mile. I checked the carb and it looks newly cleaned. The throttle cable has perfect tension. I can't for the life of me figure out what this could be.

I am pretty new to riding, especially with something that's older than me, so I really don't have much of a clue what I'm dealing with.

[RAFster122s]

Check for air leaks around the carb to intake manifold and intake manifold to the head. Carb cleaner will work.  If you have a changing in engine running when you spray one of these areas you have a vacuum leak and your mixture is becoming too lean when it has the unregulated air coming in through those leaks.  Bike might have a bad o-ring (stupid autocorrect had it saying "outing") in the intake manifold to head joint or carb boots may not be tight or may need replacement.

See if this is the cause of your running issues.
David

[DaveBarbier]

Buy a shop manual and go through the 3k mile maintenance check list. I had a ‘74 125 and loved it.

See if applying the choke a little bit helps the situation. If so, you’re looking at a lean condition.

This will be moved into the “other bikes” section soon, so just wanted to get this in before it does. You might want to look around for a dedicated CB125 forum if you don’t get a lot of action in the Other Bikes section.