Suction at tail pipe

[BigRatFink]

My 78 300D won't start. Everyone and their cousin has looked at it, but no one seems to know what the problem is.
During one of the tests, my neighbor suggested putting my hand over the exhaust pipe while he cranked the engine.
I felt suction, alternately with exhaust.
Anyone have an idea as to why that would be? Timing belt issue?
The engine won't even fire with WD-40.
Any suggestions will be greatly appreciated.