Dynamo problems

I have a problem Doc, well thats another story...But here is the real problem///I was asked to test a 6v dynamo [gen] to the US. It would not produce a charge when driven on the bike, Did the pol sparky bit ,trying to excite the field...but nothing. wired the D and F together and earthed a bulb...Sod all,
So i removed the dynamo and drive it on the bench with a drill...and yes it was going in the correct direction...Presto! Voltage soared and lit up a 12 volt bulb...great its working!
So put it back on the bike with the D and F still connected, started the bike....Nothing. Nix ,Zilch!! ERM........took all the tests...Nothing,
So back off the bike...drill turned...Presto nealy burnt my finger holding the bulb!
Back on the bike..reving at 4000 Nothing, and again off the bike and no problem ...so whats going on?
I have a idea that the start up low voltage is being earthed though the field coil, to a degree it will not "kick start" the process. Doe'snt happen on the bench..only when earthed to the bike.......what do you old timers think...is it possible.

i am going to test the field holm resittance on friday..but before then is my thinking sound , or just clutching at straws?

One of the old codgers over on the Real Classic message board will probably know: http://www.realclassic.co.uk/messages.html

Remember to double check the rotation on the bike, it has to go through several direction changes from the crank through the cogs...

There may be something that gets grounded when it is mounted on the bike (armature :?: :?: )

Jean