Just had a call from PJME informing me that the cylinder is ready to be shipped. Great timing since I am on holiday next week.

I suppose I had better strip and rebuild the oil pump with the seal kit I bought, remove the stator, replace the std generator side crank seal with the IT175 one I bought (thanks for the tip guys) and break out my dial gauge to reset the timing.

I have just been giving the offending bike a sour stare as I walk on by for the last month.
Guess it is time to roll my sleeves up and get stuck in.

This bike is well and truly cursed. I think someone was killed on it and that is why it was parked up as a more or less brand new bike with just light accident damage to the left side for over 40 years....
The ghost rider is not pleased that the family finally disposed of it and is leading me a merry dance....

Rebuilt the oil pump this morning, refitted it. Got the plunger to the bottom of its stroke so I could bleed the oil lines right up to the carb end, filled the oil tank with just enough two stroke to bleed the system and observe.

Instead of oil dripping out of the injector line end, it was dripping under the bike. What the frikkin hell.  I have not disturbed any oil lines other than inside the pump hosing.  No need to remove the clutch cover, the pump driveshaft oil seal was replaced at engine rebuild.
Oil tank outlet spigot has decided to crack by itself while parked and a full 3 months after being parked up. I may have knocked it when removing the exhaust, but the exhaust was removed extremely carefully so as not to scratch any paint. No sign of leaking after being left undisturbed for weeks after.

The oil pump leakage problem was found to be a tiny speck of dirt in the outlet check valve. I guess when the weather got hot, the oil thinned and the very slow drip turned into a much faster one.  Valve seat thoroughly cleaned, new gaskets, oil seals and new ball and spring fitted.

But I wish I never set eyes on the damned bike. Never known anything like that bikes absolute refusal to be completed.

Now I have to either find another tank or figure out how to repair the spigot. I am thinking of drilling it out larger and reinforcing it internally with brass tubing epoxied into place.

I have been a career technician for 40 years, over 12 of those as a senior motorcycle technician.  You would think this is the first motor vehicle I ever tried to repair.  
Ye Gods.
My patience is well and truly tried.  Anyone got a box of matches?

If the tank is plastic, make a new threaded outlet spigot that can be fitted using a nut, and sealed with orings. If its metal, try silver soldering. The chances of getting a new one are very low.

I have drilled the spigot to 4mm, ordered some 4mm thin wall brass tube and some 2 part polypropylene glue to hold it in place and glue / seal the crack.

Original ID at 3mm restored.

If it's a plastic tank, you will be very lucky if glue results in a permanent repair. The problem with that, is that if it fails while you are riding, the motor won't be getting any oil, and if you are unlucky could be very badly damaged.

A threaded fitting would be much safer, and I would guess it would be pretty easy to find something ready-made?

The reason why the spigot broke is that HDP tends to get brittle over the years, and as the broken part may well have been like glass, will have snapped or cracked off very easily.

All monoshock DT175's had plastic tanks... is yours on the right or left side of the bike? (Yamaha made a change in the US in 1980, not sure if/when UK bikes got the new air filter and oil tank arrangement.)

It is cracked and dripping, it is not broken off.

I am using loctite 3 glue specifically for gluing high density polyprop and all other plastics to adhere and seal the crack.
As said, the tube, suitably glued into place within the spigot adds reinforcement, provides a leak proof path past the crack and maintains the oil flow. The original spigot maintains the outer diameter only.

Left side Mark.  Close by the silencer which is why I am wondering if I inadvertantly tugged on an oil pipe when pulling the exhaust.

Its the one with the wing nut at the front and hinge at the rear.

That's the "early" style. I'm not sure I have any of those lying around. They do just starting leaking sometimes. Not sure why. I have a later style that started seeping from a hairline crack after a few months in storage. Hadn't been touched. So don't blame yourself! I will keep an eye out for the early type tank and if I have one you can have it for the price of shipping if your repair doesn't work...

(I think your repair will work great!)

Old plastic tanks sometimes leak, as they go brittle as they get older. Spigots tend to crack or break off completely.

Damaged spigots are easily repaired, with threaded fittings, as long as it's possible to get the fitting through from the inside, so the nut can be screwed on.