First guess tight inlet tappet at worse leaking inlet valve. You could also check if the choke has spun in the carb, which is common. Hard to explain on an e mail but if you look down the carb inlets check that the bar going across matches the other carb. It could also be that the mixture screw has broken off at the end blocking the idle tract very common on spanish built webers, once again remove two idle screws one from the offending cylindr and any other to compare, count how may turns it takes to unscrew if you are worried about getting them balanced. The smoke on start up and overrun is oil getting down the valve stems and nothing to worry about. My advice would be take off the head any way it is an easy job always lots of advice on here, lap the valves and put mini oil stem seals on the valves if the oil smoke worries you. I always found that with modern fuels and crossflow heads sealing the valves will always give the engine a useful boost anyway. I suspect from your posting you may be not to confident in doing the work but I promise lots of advice and I am sure you will get help from the local area if you do get in to trouble.
Edited by - Len Unwin on 14 Nov 2006 09:24:51