Help needed 1978 xs650 se

[ROYSTON]

First time out after rebuild.
Starts up & runs fine, carbs have been rebuilt & synced, fuel levels set.
Bike has no power & dies as if starving of fuel.
This bike has been converted to electric ignition (don't know if that is a factor)
Any ideas ?
Thanks.

 

[ROYSTON]

Just dismantled the gas cap & I think it may be the problem.