If this is a new installation then I suggest you have a look at my Tech web page for guidelines for initial setup and starting:
Weber Carbs Technical
Have you checked that fuel delivery pressure is 3 to 3 1/2 psi? Have you checked float levels, these are best set on a running engine but until then check them with engine off. If fuel pressure is too high you may be getting a way too rich idle mixture and killing engine that way. The CIS fuel pump delivery pressure is way too high for carbs, PMO sells a regulator that will solve this issue. It could be that initial starting is with less than fully filled float bowls and second starting attempt the fuel pressure is beginning to flood the bowls and enrich the mixture to a point that it won't burn.
You will need to get a float level gauge and an air flow meter (I like the STE the best of what I have tried) for your future tuning efforts.
Again, the air correction jets and the fuel valves for the accelerator squirters may be swapped, if you rebuilt the previously running carbs then it could happen, one type is a check valve and the other is a metered jet.