I've had the stock Cyclone GT RD on my Nishiki, running with a Campy 8sp cassette. I was curious and thought I'd switch out the Cyclone for a shiny and much-touted Superbe Pro RD I had kicking around, but set-up, I couldn't shift the chain onto the 3 lowest cogs.

The Superbe is listed with a chain wrap of 26T, which should be fine with my cassette. Could it be that it just doesn't have the lateral range to get through all 8 cogs? It's not a limit screw issue, just to cross that off the list.

The Superbe Pro should be able to get through 8 cogs. I find it strange that you are having troubles with the last 3 cogs though. What version of the derailleur is it? The friction version (with a matte finish) or the last indexed version (with a shiny finish)? The friction had a 26 teeth max cog size and I think the indexed version had only a 23. If you have the latter, it may be your problem.
The total teeth limit may be also be the trouble too. What kind of chainring setup do you have?
I would try playing around with the B-screw and perhaps experimenting with the axle position in the dropouts.
Good luck!

I used a satin finish (friction, it seems) Superbe Pro derailleur with my otherwise Campy 9-speed setup, and it handled my 12-23 cassette and standard 53/39 double crank with no problems. Is the derailleur or hanger bent at all?

If it's satin finished, as in kind of shiny, it's the last generation, which is Suntour index-compatible. Either way, you state that it handled a 23 teeth cog. This makes sense, as the stated max cog size for that version is 23 teeth. The OP hasn't stated what size cassette he's trying to run, but I assume if it's bigger than a 23 teeth top, that's what the problem is.

That's the first thing I thought of... I can't imagine why it's not working... should be no problem at all.

This may be a dumb suggestion, check to see if anything is stuck in the paralellogram, like a chunk of dirt or a rock. Try to move it by hand too.

Do the shifters run out of range, or is it that the derialleur just won't move any further?

satin finish

the cassette is 25-12

I'm not using matching Superbe shift levers, but they are early Cyclone

I can push the derailleur farther towards the lowest cog than it will go with the shift lever
the shifters, with no cable attached, have a range of more than 180 degrees, but with cable, they stop somewhere around 135 degrees or so.

I'm nearly certain I got this derailleur off a friction-shifting Team Fuji.

If the pivot points need to be cleaned or need oil, the friction could overcome the spring force.

If it is old enough, it could have a weak/worn out spring.

Well dang - indeed - seems like it's got a max cog size of 23. If anyone would care to explain the mechanics of why there would be a max cog size, I'm all ears.

As it is, I guess I'll give up on the Superbe Pro/8sp Campy combo.

Are you saying that it won't move all the way to the inside? or are you saying that the pullys bind on the last cog(s)? I ran into the 2nd problem this past week when I changed to a larger chainring. I didn't take into consideration that I would need a longer chain. Hope this helps.

The maximum cog size is limited by the distance between the upper pivot and the top of the upper pulley.

My guess is that the upper pulley does not have enough clearance to shift onto the larger cogs.If the derailleur has a "B" adjusting screw (the one on the rear of the upper pivot), try screwing that in a few turns. That screw presses against the dropout adapter. Tightening the "B" screw forces the derailleur to rotate backwards which effectively increases the distance between the upper pulley and the pivot.

I bet that derailleur will handle your 25T cog if you're running friction shifters.

Now that it is mentioned, I remember that they had a max cog limit of 23 teeth.

Back when I was young, there was no issue with that! My lowest gear was a 42x21! Which won't work for me anymore....

Hold the phone! I'd been thinking B screw was just another name for either the high or low limit screw. Let me take a gander at what the real B screw has to say about this...

velobase says its listed max cog size was 26. if you still have problems with the 25t after adjusting the b screw, something else is going on ... i had a similar issue a while back and it turned out my derailleur hanger was bent. had it bent back and everything works perfectly now.

I'm going to have the hanger alignment checked, but in the meantime, I switched out the modern 8sp wheelset for a Mavic/Record HF set from '70, and threw a 6sp Winner on it. So I didn't solve the original problem, but I'm going to chalk it up to 'compatibility issue'. I didn't need those 2 extra cogs, anyway!

If you have horizontal dropouts, try moving the wheel further back. You may also be able to get it to work by lengthening or shortening the chain by a link or two.