Stumped

[eil]

Before you start on the engine take the compressor inlet filter right off, just in case you are creating a vacuum, then open the little unloading valve on the side of the Gennie valve it will look like this

 

[hillman]

I used to have a petrol compressor when i was rally servicing. As the previous post, there should be an unloader valve which needs to be opened before starting, otherwise you're trying to compress the air as you're starting it.
I suppose it's like starting your car when it's in gear, the starter motor would be trying to move the car forwards as well as start the engine.

 

[gordon stephenson]

Could it be something stupid, like if it's a points motor, the load of the belt on the motor pulley is taking up any slack in the crankcase bearing and altering the points gap etc?