Hanging idle, but with a twist. M750

[TheInsider]

 My '99 m750 has a strange hanging idle issue I'm having trouble pinning down. You will see after I list the facts..

1. The idle ONLY hangs when revving the engine to 5000 rpm or above. You can literally ride all day around town without encountering the hang if revs are kept below ~5k. The engine runs perfectly otherwise. No fouling or backfiring or lean pops when engine braking, even while idle hanging, and no loss of power.
2. There are no leaks between the engine side or the carbs to the intake on the heads. This has been checked and rechecked and rechecked yet again using a variety of methods. (for sake of arguement lets say 100% no external intake leaks)
3. Idle is set to 1300rpm. It hangs at 2000rpm. If not revved to 5k or over it instantly goes back down to normal idle. The hang occurs for a few seconds before returning to normal idle. I first noticed this after coming off interstates onto the off ramps and it hanging as i pulled up to the stoplight.
4. Carbs are jetted and have new needles and emulsion tubes. Carbs are synched nicely.
5. 17k miles. Replaced vacuum fuel valve with a manual, no leaks here. Checked and rechecked, ad nauseum

Any help would be appreciated guys!

[Howie]

Lower the RPM to 1200.

[TheInsider]

Lower the RPM to 1200.

Interesting, may I ask for some elaboration? Possibly a timing advance issue?

[Speeddog]

Yes, it's the timing advance, at ~1500rpm it starts advancing quite rapidly.

[brad black]

for a start, make sure the return cable is backed all the way off, just in case.