1973 Honda ST90 - doesn't seem to shift at higher RPM

I've only ridden one of these things for perhaps 10 minutes total, so bear with me if I ask some dumb questions.

Got one of these 3 speed 'semi-automatic' ST90's the other day. 3 down, neutral up.

It doesn't seem to want to shift gears unless the RPM's are down a bit. I'm cruising along, let off the throttle and try to shift and nothing seems to happen unless the engine winds down a bit more.

Wondering if this is typical, or if there are adjustments to be made somewhere. I don't know enough about the way the transmission works in this bike to understand how mechanically it would not try to shift the cogs over simply because the RPM's are a little high.

Also, when shifting into first from neutral, clearly I'm able to hold the bike in place pretty easily either with brake or simply with my feet firmly planted. In this situation, is the clutch just slipping, or is there something else happening? We had a little Vino 50cc scooter for a while which was engineered to do the same, but it was a centrifugal clutch from what I gather and perhaps a little different.

Just looking to learn a bit about how these transmissions work before I start adjusting things. Would like to understand better what to expect when riding it.
The first thing I would do is change the oil , the engine and transmission share a common sump

Then after another test I would adjust the clutch , it is done with a flat screw driver and a wrench

Looses the jamb nut then turn the srew until you feel resistance, back it of a quarter turn and tighten the lock nut

There are some good you tube vids on this
Oh, I'm comfortable with adjusting the clutch, and absolutely will go through the gamut of tune-up / basic maintenance including the timing, valve lash, cam chain tension, clutch adjustment etc....

I was mostly curious about anything specific to the semi-automatic transmission. Near as I can tell it's as simple as the gear shifter actuating the clutch when you're changing gears - which is why I don't understand the relationship between RPM and the mechanism.

But I'll change the oil, adjust the clutch and see what's what.
I've only ridden one of these things for perhaps 10 minutes total...
It all sounds normal to me.

I have a couple of Honda C70s with the same setup as your bike. They are super-low mileage bikes and in great condition. I ride motorcycles all the time, but had not ridden one w/an auto clutch for many years before I got these. At first I thought something was wrong because the shifting is really notchy feeling They shift much more smoothly if you back off the throttle and really let the engine slow down between shifts. After awhile, I became accustomed to it. This kind of setup just doesn't like to be revved hard going up through the gears. They are much happier if you short-shift and get into top gear as soon as you can. They also don't downshift too smoothly. I usually just roll up to stop signs in 3rd gear and shift it back to first once I've stopped.

The transmission is not semi-automatic. It's a normal constant-mesh transmission with a centrifugal clutch instead of one you release manually with a lever, like bigger motorcycles have. At low RPMs a spring holds the driving and driven elements of the clutch apart. Spin it faster and the centrifugal force overcomes the springs and the clutch engages. You should be able to sit still with it running and in gear.

I'd be surprised if it need adjustment. Changing the oil is a good thing. You shouldn't use regular motor oil in motorcycles because it has additives to make it more slippery, which can be bad for the clutch. Motorcycle-specific oils don't have as much of that stuff. Bike oils are also formulated to handle the sheer forces between the transmission gear teeth.
Thanks for the clutch explanation. I think there's also a clutch mechanism triggered when you shift, but presumably that's to separate the plates a bit when the gears are changed under load.

Good to know it can sit still in any gear - wasn't sure how that worked.

I've got nothing but motorcycle specific oils in my shop. I've got a bunch of older bikes, most with wet clutches.

1966 Honda C95
1971 Motobecane moped
1972 Yamaha XS650
1973 Honda ST90
1975 BMW R75/6
1978 Yamaha SR500
2011 Voodoo Vintage custom rigid w/ 1978 Yamaha XS650 power
2012 Suzuki TU250X
I would definitely start with an oil change and a clutch adjustment - it can't hurt. I have two ST90 bikes and two CT90 bikes with the same clutch style as yours.

After that, I would look to see if the shift drum stopper or star plate are loose under the main drive gear. You'll need to remove the riders right side engine case and it takes a JIS #3 screwdriver to remove those screws. They are not a Phillips head. Then you will need a JIS #1 screwdriver to remove the countersunk screws in the outer clutch cover. Then you will need a clutch nut socket to remove the clutch after tapping down the keeper tabs on the nut.

Use a snap ring remover on the main drive gear and remove the gear. Finally, you can get to the star plate and the shift drum stopper on the end of the shift drum. The roller should follow the edge of the star plate. Use some red Loctite on the star plate screw after making sure it is indexed to the pins properly.

Hopefully, it's just an oil change and clutch adjustment that you need because it's a major chore getting to the shift drum. But at least the cases don't need to be split.

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Most of the "clutch problems" on my bikes have been slipping due to worn out discs , but are cheap and easy to replace
I usually order mine from DrAtv , had good luck even with the no name brand
Much thanks for the tips and step-by-step instructions. Going to spend a bit of time this weekend fiddling with it to see what's what. Like I say, it runs and drives fine, and for all I know is exhibiting perfectly normal behavior.
Did a little adjusting here and there and it's working a little more smoothly than before. I'll probably never get used to having to let the engine wind down to shift - keep trying at the top of the RPM range to shift into a higher gear and it doesn't do anything (the mechanics of which still elude me).

At some point I might grab a 140cc Lifan crate motor and drop it in. 4 speed manual transmission and an extra 50cc should make it fun! I've already got the engine mount adapter kit on order from TBolt and can get the engine locally for $400, which doesn't seem too bad especially if you had a non-runner and were facing a rebuild. I think I'd buy a crate motor before bothering to rebuild, but I'm not a purist at all.
Did a lifan 125 swap in an sl70 frame with my bud Ron , it turned it into a very streetable little bike
We did it at first with the 6 volt to 12 adapter from Dr atv , but have since gone full 12volt