With the weber 40's i was running the ignition with aldon distributor set at 12 degrees advance @1000rpm.with these bike carbs do i need to alter it.Have noticed a fair bit of petrol vapour blown out of the carbs (without filters) when you blip the throttle.I was thinking this is to do with valve overlap ? anything I should do or not worry

My bike carbs pump out fuel vapor , thet call it stand off, but I have no air filters or airbox.

If you have an airbox or filter it should catch the vapor and let it go back in!

Could be caused by the shortness of the carbs and inlet tract .they tend to be under the ideal length when on a car engine.

Still bloody good though

Thanks for that,I made a four inch long manifold and fitted it to my weber manifold making a total of about nine inches.Read something about inlet manifold length and torque,yet to drive it so that remains unknown.