Motorcycle Repair: cb250k4 running badly, point gap, air leaks

Question
Dear Bill
I recently bought a 1973 honda cb250k3/4 which is running badly
I have replaced the jets in the carbs the diaphragms look good.and they are both very clean
the bike will start easily but tickover is not constant .sometimes it will die sometimes it will race away to about 4000 rpm.when trying to ride it,it will not pull at all and dies. I have noticed the left carb is flooding and leaking through the float bowl joint.also when making adjustments to the air/mixture screws on the right carb this has no effect at all to the running but doing the same on the left carb alters things significantly ??

Any advise gratefully appreciated
kind regards

stuart.

Answer
Stuart, Start from the beginning.....  Adjust the valves, check compression to see if you have something worthy of spending your time and money on.

If you have about 160 psi in each side, then proceed to remove the spark advancer, hidden behind the points plate and inspect/lube the points cam and advancer shaft. The spark advancer needs to be able to move forward when the weights swing out and then return smartly at idle. The point gap should be about .014"-.016" with the contact faces nice and clean/shiny. Then, set each point set so that they just OPEN at the F and LF marks respectively. Watch the points, while the engine is running for signs of arcing. Heavy arcing is a sign of a bad condenser, loose coil wiring connections at the condenser or a poorly grounded condenser to the chassis.

Uncontrollable fast idles often come from sticking spark advancers, but once the timing is set and the advancer is working properly, you have to start looking at air leaks at the manifolds next. Other causes are the throttle cable hanging up under the fuel tank or the cable strands are fraying inside the housing.

Fix the problem with the floats next! Either the float has pin holes and is sinking down so it won't close the float valve or the float level is not set correctly or the float valve is leaking due to wear. The float level setting is 21mm. The float tang should just be pushing the float valve closed, but not compressing the spring-loaded tip of the needle.

The carburetors should be cleaned completely. There are emulsion tubes up above the primary main jets which need to be gently removed and cleaned thoroughly. All jets with cross-drilled holes must be clean and clear.

Finally, make sure that you have a fully charged battery in the bike and the charging system is functioning normally.

Don't ride it until you fix it, otherwise you are exposing yourself to any number of potential dangers due to stalling or catching fire!

Bill Silver