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Disconnect cable.  Place thumb on lever and lightly push forward against the lever spring and to remove free play twixt actuator and clutch pressure plate.  Loosen locknut.
Adjust pressure plate screw in or out till the sharp end on lever is pointing at the pointer.

​​​​​Tighten adjusting screw locknut.  

Adjust clutch cable free play at the handlebar end adjusters.  Coarse with cable adjuster.  Fine when on the fly with lever adjuster.

Spring #21 pulls the arm back & i guess without cable it goes back until the pushrod aligns nearer the center of the lifting flat "guessing" so in theory the cable connection on the actuator would be a reasonable distance from where it is with cable connected. Not sure if the cable would go out that far but adjusting it on the pressure plate adjuster to take up clearance 'there' i think you'd get very little lift--if it's possible for the cable to reach. Just picturing in my mind how it works. That's why they give the alignment mark so you get maximum lift no doubt.
www.partzilla.com/catalog/yamaha/motorcycle/1978/dt175e/clutch

Seal wouldn't stop that leaking from the screw hole if screw is missing.
Seal is at the top of actuator so oil is in & around the actuator part.
Missing screw probably has a sealing washer of sorts & thickness of washer calculated into the length of the pin piece on the screw so it doesn't bottom out in the actuator groove.
Ok i cheated & had another look & #26 they call a gasket so will be a soft reasonably thick washer.
Have a look under the bench & your missing screw might be there.
If ya keen to go riding-- what i'd do--& i should give bad ideas--is just find a short bolt or screw & cut it even shorter after looking at how many threads are in 'that' hole & make it 'that' length so as not to go in too far & push on the actuator.
Just to block the hole while waiting for a genuine one.
I hope the thread in the hole is ok!!
Sorry--i got a bit carried away there.

I've been lucky not to damage anything by just forcing it at the limit which is probably either pushing the pressure plate into the clutch cover or coil binding the springs but worked for me. I've also turned it out & carefully grabbed it in the vice on the piece that the cable pulls, just grab across the centre & push down while turning to stop it jumping out of the jaws. It'll crack free. Other way I only just thought of is, pull the pushrods out but first measure the rod overhang & place a block of wood in there & tighten cover. Then actuator has nowhere to go moving in so has to crack free. Yep that'd work, forget my other ideas lol.

The clutch actuator adjustment nut and screw are not reverse thread. The nut can become frozen and difficult to remove. It's easily damaged if you put it in a vise. Also the nylon spiral is likely to be cracked if it's original. I use a cordless 1/4" impact driver while securing it in a vise wrapped in a rag. I also have newly manufactured actuators with new old stock screws and nuts.

Hello I have another problem with my bike, now only when it is running oil streams out of a threaded hole right in front of the sprocket. Looking into that hole I can see the clutch actuator rod. Is there supposed to be a bolt in this hole? Like I said it is threaded but the flywheel cover does not have a bolt that goes into it and there was nothing in it when I took the cover off. I was able to slide the clutch rod out but I don't see a seal or any way to replace a seal down the hole. I don't have a service manual but would like to know how to go about replacing the seal.

Welcome from NZ downunder.
Slipping all of a sudden doesn't make sense.
So thinking outside the square, by memory those have the cluch mechanism underneath the motor.
Have you landed on something that might've bent the arm to catch on the case or can the sump guard get bent against it.
Or has someone filled it to the top with oil--when you weren't looking.
Has the chain derailed & bunched up damaging the actuator part of the case.
Has anyone tried adjusting the clutch on the screw that lifts or lowers the arm which increase or decreases your cable freeplay.
If that accentric screw is turned in past its useful lifting or lowering it'll bind by bottoming out in the groove.
Effective movement is all within half a turn each way when it's far enough in to be safe in the groove.
Does the cable move freely & feel like it's free at rest & not holding against the rod.
Sorry lot of questions from me turnng a short answer into an essay lol.
Hope you enjoy your time here.

All,

I know I'll think of things to add until it's sold, so here goes. It's got a Barnett clutch and springs--the mods and Sleeper Pipe ended up producing enough power that it pulled the clutch right out of the bike. Also, the left side case has been modified to use a clutch actuator from an R5/RD-350. Your will NEVER have a clutch pull problem ever again. I'm also running a K & N air cleaner (I know some do not like them), and the oil pump is still installed and working just great.  The bike comes with a brand new battery with the electrolyte, AND if desired, a capacitor to run it without the battery at all. Look at it, you won't be sorry!  Spares include a spare rear cushion drive hub and a complete foot peg bar.  It also has allen case bolts everywhere.  Oh, the handlebars are CR-low bars as well--I liked them a lot, and it has both a working stock compression release and a secondary release in the spare plug hole.

Hammer

Without looking, but been there many times--The DT1 ball is a big 5/16 one & only goes in from the clutch side ahead of the mushroom. Hope i'm not treading on toes.
Just to add, there is one 1/4 ball in the actuator where the cable connects but that is sort of retained by three pop markls or something to stop it dropping out.
photos.app.goo.gl/Hi7dpHCFi6vy24ZA7
Mark can you load the pic for me please if pos. Cheers.
[img]photos.app.goo.gl/Hi7dpHCFi6vy24ZA7[/img]