Sometimes I get the older cars mixed up but isnt the 78 450 SEL ( 116 chassis), an Electronic injection system, therefore it does not have an enrichment compensator. The enrichment comp. is only used on CIS cars.
I believe CIS was introduced in 84 on the 201 chassis with a 102 or 103 engine.
Now on the 78, if it is electronic injection, enrichment is done through Injector open duration via the engine control computer.
The first thing I would check is to check fuel pressure at rest and make sure its not leaking ( either an injector or the regulator).
hope this helps!!!