TOPIC:

I have a 1978 Z650 B2 in the U.K. but it is a us import, it is almost impossible to start the engine unless I spray easy start into the air filter housing, then it starts quite easily and runs perfectly, ticking over around 1000rpm, I have checked the timing with a strobe and it is fine, the carburettors have been vapour cleaned and re built with new jets and gaskets, once it is running everything is fine, any advice very gratefully received.
thanks 

Poorly functioning choke system - check all the plungers ar fully lifting when on full choke. The lifter arms arm fairly weak and bend over time. (Assuming vm carbs) 
Renew the o rings on the plunger brass bolt mountings. 
Low compression - 650's should be 160- 180 psi when warm. 
Check voltage at the coils. Should be be full 12+ battery volts. 
All the above will have negative effects on cold starting. 

Pleas update us. We need correct information to help you. 

May 2022 you post pictures of sooty plugs and claim it’s brand new Boyer bransden micro digital system. 

Then on October 2022 you post sooty plugs again but claim it all standard ignition. 

 

Did you put clip in the center grove? Did you do the coil mod. These all will relate to your continuing situation. 

The choke pickup passage in the float bowls may be blocked.  Or, the passage in the carb between the brass choke pickup tube, and the choke chamber at the plunger.



 

Nessism  gives a very important  info about the cold start procedre (richer air/fuel ratio).
These special points of cleaning the carb enricher circuit is often missing during a "carb rebuild or cleaning".

Thanks 

I couldn’t get it to run with the electronic ignition system so I put it back to standard with points and mechanical advance retard unit, the plugs are still sooty 

Thank you, back in October it was recommended that you clean and service the choke circuit, and check, adjust if necessary the vale lash. Was this ever completed. Again these will cause issues your experience. This forum is fush with very experienced people, they can help diagnose and fix just about any condition, if you choose not to follow the recommendations, your 100% guaranteed to have ongoing issues as your experiencing.


EDIT; Vapor blasting the carbs, if done correctly only changes the visual appearance on the outside of the carb bodies. It’s not a process to deep clean them. A soak in carb dip, then spraying out each and every passage way as suggested is the only way to ensure you have clean correct functioning carburetors. Did you note the Jets sizes? Are they Mikuni brand? Mikuni logo is a small square in the corner of a larger square. Previous owner may have swapped in aftermarket Jets and this also will contribute to tuning issues.

I had a similar problem when trying to start my CB750 that I purchased not running. I had to put gas directly into the carbs to get it to start, I changed pilot jets from 35 to 40, problem solved.