Probably time for new clutch plates. Mine had the original friction plates, and was slipping very badly after I got her up and running, and with only some 350 miles on it. If you have oil in the case, they're oiled up.

I guess I'm as Newbee as it gets! MarkT is on line..... knolageable!! I'''''ve got a quick question......
1971 RT1B 360 reconstructing....shop manuals in hand are a combination of MX DT and Hanes .... for various models....... Prime(bleed) oil pump wont work.... the worm gear shaft is fixed with the crank gears.....???? no oil(in crank) and in neutral ???? whassup??

Ok. Just friction plates? Sources? Thanks for the help!

These are what I got:

Xttps://www.ebay.com/itm/184969870432 (change the X to and H)

However, they do slip a bit when the engine is cold. Once it's warm, they're fine. Hopefully others will weigh in, but you might want to buy OEM friction plates. Also, the springs may be under spec, and mine were. In the manual it shows how long they should measure, and when to replace them. My neighbor, who works on hot rods and old bikes, also suggested roughing up the steel plates a bit with sandpaper (this was after I already installed mine).

First thing might be to check your pushrod adjustment, if you haven't. It's in the manual. Maybe it's not fully releasing?

Firstly be sure its got clearance on the actuator adjuster on the left & also a less than perfect cable can give a false impression or even hold a light preload on the actuator & it doesn't take much to give grief, Even to the point of [if zero clearance] & the helical [whatever they are-angled teeth]; The angled clutch gear tries to drag it all sideways. And if any slight side movement of bearing in the case then the clutch frees a whisker & slips under power. I doubt your plates are toast & i'm going to the extremes with my comments but have had that happen.
But wait there's more lol--on my 175 [AT1--175 top end] it slipped like crazy under power. [clutch never been off since new 1970] but recently [vid up here somewhere of mine] i recently beat the s--t [snot not s--t] out of it around a track & it damn well came right unbelievably. Might come back to bite me but is ok at the mo. So check its not very overfull of oil then go ride it again. If you need to go 'in' then leave out the rubber rings under the plates & de-glaze the fibers & scratch the steels.
Edit--Also actuator arm needs a good spring to keep it right back with clearance & doesn't take much of a cruddy cable to overcome the spring.

Just my ideas--but--
Chipcat #12, i'm sure Mark will be along. But just my two cents worth. We might need a bit more info. There's a small screw on the side to free flow bleed. When you're happy with 'that' & it'll never look perfect but so long as plenty & constant slow flow. Then pull the pump cable out of the adjuster so pump is on full delivery. Then with line union removed at the cylinder just revolve the plastic wheel which should only go one direction. Oil should pulse [not much oil] out at the union. Hold the union vertical & watch it give little pulses straight up corresponding to every time the pump plate compresses. With cable back in & on minimum delivery the pulse will be barely visible--at finger speed. Safe to start the motor on premix & play with pump if you want. Just put a 6mm screw in the cylinder feed hole to stop air causing a high idle.
I think Welcome from NZ are in order. Enjoy the forum. Be a good idea for you to start a separate post for your problem. Unless i'm confused--am old & confused lol. Cheers.

I didn’t read in your posts if you changed the transmission oil, start with the basics first, if you did change the oil which I would hope you did after as you say the bike sat for 45+ years, did you put in the correct type of oil designed for “wet clutch” use? Starting with the basics first is easier than jumping to changing clutch plates.

A little bit of an update on the CT 3: I found a replace left switch on eBay (pricey), got new clutch plates from Yamaha dealer, and had them install. Tech said clutch plates were backward, causing them to slip. I guess someone had bee in there before. While bike was in shop, I had a little hospital stay and recovering. A friend brought it home. He rode it around without the seat (it’s still in upholstery shop). Maybe I will be able to ride by the time the seat is ready. I am also restoring an 83 PW50, that I bought new and has been in my barn unused for 25 or so years. I am studying the forum for all kinds of useful info. I think I have the enduro bug

Many thanks to all you and your knowledge.

Not sure what plates in backwards would look like but hey if its fixed then that's the important part.

 
couple of pics of it so far