Fuel stains running down intake tube... WTH

[Rick of Austin]

It surprised me to see blue avgas stains running down the intake tube as if the fuel was coming from the intake pipe to cylinder head gasket. This is a cylinder with a primer line to it.

Is there a common reason and solution for this? It would seem that with the intake always being under a vacuum fuel couldnt get out.

Thanks in advance guys
Rick

 

[zav6a]

Stains

When the intake valve closes there is a short period of positive pressure due to ram effect.

 

[Ron Lee]

I had the same thing on a rebuilt engine. Put in a new gasket and properly torqued the bolts. Problem gone.