Hi gang, happy to report that I've had the leaking head gasket replaced on the GB250 Clubman, and the bike has just passed its annual Ministry of Transport safety test with flying colours. It still has a slight carburation problem, (I think). It starts ok, hot or cold, and when driving goes very well, but, after slowing on a closed throttle it is almost impossible to carefully open the throttle again without a vicious snatch, which is not what you want as you exit a corner! Have never had a vehicle with CV carb before, so not quite sure where to look. Any help would be greatly appreciated. Thanks, Mike.
That problem usually indicates an incorrect mixture screw setting. It's either too lean or too rich on the pilot circuit. Check how many turns out the screw is set (2 as an example) and try half a turn in or out from where it is currently set. Good pilot mixture screw setting is in the range of 2-3 turns out from lightly seated. If it needs more than that, it has a blocked pilot jet, or the pilot jet is too small. If it needs less than 2-3 turns to run right, pilot jet is too large. You should also carefully check that the pilot jet is of the correct type, as there are bleed and non-bleed Mikuni pilot jets. The bleed types can also have different numbers of holes in them, there are multiple kinds again.
Or ..... Check for something obvious like the throttle spindle or throttle cable binding. I have seen the throttle butterfly stick in the carb throat before. Check that your idle speed adjustment is set correctly. Make sure there are no air leaks (that could be being compensated for with idle mixture screw adjustments and the throttle butterfly being close too hard.
