I am in the process of rebuilding my 1990 DT125R which I have owned for around 2 years now. The head gasket went and I decided to completely strip the bike down and rebuild from the bottom up.
The previous owner had decided to remove the oil pump and I had been running on premix. However, I know wish to refit the oil pump. I have purchased a second hand pump, oil tank and sender.
I took the pump apart to have a look inside and subsequently forgot which way a couple of the washers inside went!
I wonder if anyone has a technical drawing or previous experience of taking one apart as I want to make sure I have put back together correctly.
When I went to Yamaha and asked them to rebuild my oil pump, they refused saying that none of the parts were serviceable items. Abd that the pump must be replaced all together.
I don't know how true that is, simply going by what they said.
In any case, is the engine modified or is it standard? If the barrel has been ported then you will need to run premix, if it is standard then you will be alright.
In a similar position myself, here's a exploded view of the internal components:
I had to stretch out the plunger spring as it lost some of its original force and wasn't returning down.
Replace the internal seals also. They are cheap from simplybearings.co.uk, just standard shaft lip seals, sizes are 4x9x3 and 14x25x5
p.s. always take photos as you disassemble stuff, for this exact reason!