1987 Volkswagen Van Hard to cold start

I have 87 Westfalia. For over months, I have cold start issue. We are in California, so temp is never too low. But even now after I park the van for a day then to restart it, I need to put more gas to get higher rpm and stay there for 30 seconds or more.
I have used a meter to check the battery voltage before start and a Pep Boys store checked it too, it is normal, and the engine normal idle speed is about 900.
When engine is warm, there isn't any problem to restart it.