I’ve got a leak from my petcock and removed it for inspection. The o ring looks serviceable so I’m thinking it must be the screws. I don’t understand how they could seal with them being inside the wet fuel area and nothing to seal them?
 

Are the sealing washers for the screws still intact?

Hhhmmm, there is only metal washers on the screws. 

They should be a fibre type material - see link below. I would imagine that they are still available from Yamaha.

WASHER, PLATE(3MB) for RT2 1972 USA - order at CMSNL

Washers are the same as even a modern YZ85 and Yamaha does have them still.

I've also used the K&L petcock washers that have a rubber seal bonded to it on one side.  They stopped a leak the new Yamaha ones wouldn't.  (I might be wrong, but I thought the original Yamaha petcock washers when these bikes were new had a rubber seal?)

Link to K&L petcock washers on ebay

Just install the correct Yamaha sealing washers and i also use a small amount of permatex no-2 non hardening form-a-gasket sealant on the threads to help seal the screws. let it dry good. Never had one leak. 

Thanks gentlemen! Just wanted some input, didn’t want to be doing this twice.