Problems with Jet engine smoke

I am running windows 7X64 Ultimate and FSX-Steam.  Flights with the JF DC-8 and  a freeware B-52 both have Jet engine smoke in their respective Aircraft config. files. However when advancing the throttles on either Aircraft the requisite smoke appears as programmed, but

within a minute or less the smoke stops.  If one you hold the "i" key down the smoke appears, but when you release the key it stops.

Now all Aircraft in my hanger (turboprops or jets,) are doing the same thing.

anyone have any ideas regarding this??

Thanks for considering this problem

Bests; Zepnick