Topic: 1975 CB125s

[chriscraft42]

Looking for suggestions about an issue I am having with my CB125s. I recently replaced the carburetor and all my problems seemed to go away. The bike is idling at 1200 rpms and running decently if I am riding for short runs in the neighborhood. If I take a longer run going over 40 mph the bike starts to backfire and eventually stalls. I have tried to adjust the throttle screw and air screw. ANY SUGGESTIONS!    Thanks...

[kirkn]

Intake valve clearance too tight?

When the motor heats up, the clearance closes up and intake valve leaks?  Just guessing.

Otherwise, temperature-related problems can be condensor or ignition coil. 

Hmmm.....

[SteveD CB500F]

Fuel starvation?

You wouldn't notice until the engine starts to demand a higher rate fuel flow from the carb.

When it stops, undo the float bowl drain and see if there's any fuel in there. If not, check the fuel pipe for restriction. If you use a fuel filter, clean it...

[mattcb350f]

I agree with Steve, however, the location of the backfire is important.

If it's backfiring through the carb (sounds like a poping sneeze...sort of), it's fuel starvation. This is caused by lack of fuel entering the carb/engine. Check the fuel line routing, filter, and gas cap vent for blockage....and also check that there is enough fuel in the tank 

If it's backfiring out the exhaust than it's over fueling, or flooding out. This is caused by too much fuel entering the engine....usually an internal carb problem.

 Matt.

[fuzzybutt]

make sure the fuel tank vent in the cap is open too.