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Got a old 4 stroke motor from the early 80s i beleive, slide valve carb, has hp marked on the head and carb, has fc 25 on the case, any info???

Hirtenberger HP VT 25 Car. VT stands for Vier Takt, or Four Stroke.

I can't tell from the pics, is that a rotary valve? I always thought the rotary valve was a great idea as the parasitic drag would be far less. Wonder why it never caught on

Yes, these are rotary valve engines.
I can't answer your question, maybe more difficult to seal properly?

The rotary valves are used on airplane motors all the time, but I think the slide valves are used mainly in car applications as it gives you the most mounting options. The rotaries need the throttle linkage in line with the motor (behind or ahead) or a bellcrank to change the throw direction 90deg. Having the slide simplifies linkage attachment.
Cool old 4 stroke.

Not the carb, the valvetrain

gomachv wrote:Not the carb, the valvetrain

That's what I get for skimming when I'm supposed to be working.