Itinerant Air-Cooled

2L L-jet: runs fine throughout the range of rpm, but at idle it drops down 700/splutter...rises up to 1000..then down...then up...an even and predictable roller-coaster idle. Acceleration and cruising is fine

???

Classic vacuum leak behavior. Inspect thoroughly all hoses and elbows and boots both surfaces you can see and surfaces hidden from view.
If all checks out, go to injector seals and intake manifold gaskets and seals at the plenum.
Colin

OK Thanks !