most likely the RH crank seal (clutch side)

Can you smell petrol in the gearbox oil ? unscrew the filler and have a sniff

is the dipstick level dropping?

is the smoke foul smelling?

Dip stick level is dropping. I set the oil level with the stick sitting in the hole but not screwed in, is that correct or should it be screwed in? I'm just wondering if the smoke could be from overfilling? Will check out the gearbox and smoke smells in the next day or two.

Its an easy fix you need to take off clutch the basket & primary gear, pull out old seal , and push in the new,,

Any special tools required for that job and can that be done with box/motor still in the bike? I have a repair manual for the 74-76 DT175, would the procedure be the same for my 96 model?

you need to put the bike into 5th gear when undoing and re-torqueing the primary pinion nut, Get someone to sit on the bike with the brakes on. I find that a small self tapper screw into the face of the seal gives something to pull on.

***Watch out for the small plate washer on the tacho drive it often falls off when the clutch case is taken off***

Paul " show him your tool , . aye the clutch basket holder,[ home made]

Here's a photo;

thanks

Found a parts list and owners manual online for the 1996 DT175H that shows it having a vacumn type fuel tap, so looks like the pipe that is blanked off should go to the fuel tap. Have you got a photo of the fuel tap? What position do you run the tap on as with the pipe disconnected it should only work in the "PRI" position. As Cynic has said looks like the 2T pipe is in the backgound.

Tony