[Healeys] Overdrive and RPM

[john spaur]

I searched the archives and could not find any posts to help me.

Driving in gear at 3100 RPM with the overdrive engaged, the tach will 
drop about 1000 RPM when I take my foot off the accelerator. When I 
put my foot back on the accelerator it is the RPM comes slowly up 3100.

When I do the same thing with the OD disengaged, the tack will show 
very little decrease in RPM's. When I press on the accelerator the 
RPM's come back to 3100 very quickly.

What might be causing the RPM drop with the OD engaged?

John Spaur
'62 BT7