beetle intake manifold

I have a 1970 VW beetle with a stock 1600cc engine. Since it has begun to get cold outside I ab having a problem that seems to recur every winter. When it is cold outside the car will start fine , but once it has been running for 10 or so minutes it starts going dead, it will keep running as long as my foot on the gas, but the second I let off it goes dead. I have noticed when I examine the engine I notice that the intake manifold has iced over as if the fuel going through the manifold is acting like refrigerant. Any ideas? Thanks

Your heat risers are clogged or restricted with carbon. The only cure for this is to remove the manifold and ream out the risers. An old piece of clutch cable on a variable speed drill works good for this.

Soaking in a good carbon solvent (from any auto parts store) before reaming will help, and will also remove any stubborn carbon deposits after reaming.

Or get a new Intake manifold.

Your heat risers are clogged or restricted with carbon. The only cure for this is to remove the manifold and ream out the risers. An old piece of clutch cable on a variable speed drill works good for this.

Soaking in a good carbon solvent (from any auto parts store) before reaming will help, and will also remove any stubborn carbon deposits after reaming.

Or get a new Intake manifold.
Thank you soooo much for the info.