Hello!

Got a funny issue with my 84 duke 4sp. Start up is fine, drops to correct idle. Get it moving in gear and it's fine upto ~30mph, then the idle kicks up to 1500-2000 and refuses to come back down until the car comes to a complete stop. Speedometer works 100% fine.
Disconnecting the VSS or the panel connector to the speedometer board makes the idle act naturally, but then I got no speedometer!
Thought it was the board itself so I ordered a new used board to compare from TFS and plugged that in, with minor success (worked for half of one drive).
All the connections are clean, and connectors fully plugged in, my only thought from here is that it could be the ECM (someone did replace the original with a 86 3 speed ECM at some point during its life).

Please let me know whatever thoughts you might have!

quote Originally posted by Csell2: Hello! Got a funny issue with my 84 duke 4sp. Start up is fine, drops to correct idle. Get it moving in gear and it's fine upto ~30mph, then the idle kicks up to 1500-2000 and refuses to come back down until the car comes to a complete stop. Speedometer works 100% fine. Disconnecting the VSS or the panel connector to the speedometer board makes the idle act naturally, but then I got no speedometer! Thought it was the board itself so I ordered a new used board to compare from TFS and plugged that in, with minor success (worked for half of one drive). All the connections are clean, and connectors fully plugged in, my only thought from here is that it could be the ECM (someone did replace the original with a 86 3 speed ECM at some point during its life). Please let me know whatever thoughts you might have!

sounds like you figured it out already, the automatic ecm tried to raise idle in what it thinks is drive to keep the transmission crawling

The rolling idle is programmed to be higher than the stationary idle.

The ECM doesn't know if your clutch is engaged or disengaged. So as a precaution the ECM keeps the idle speed higher so there is less chance of an engine stall between shifts. Most people don't even notice it. But yes, you will notice the ECM drops the idle speed to the warm idle of 1000 rpm at something like 5 mph.