A few years back Jan Rosenberg created a smoke effect gauge that will emit exhaust smoke when exceeding a selected throttle percent. It was created for FS2002 and works fine in FSX. The installation is pretty easy and it gives you three choices of smoke thickness.
It's called SHOP_FX. Dynamic Jet Smoke Effects Workshop for FS2002. Filename: shop_fx.zip
This can fix the auto-smoke problem with the 727.