280z slowly goes back to idle while driving

[Mattbarnes]

Hello, I have a 76 280z and I noticed when I drive it, it very slowly declines in rpm's back down to where it should be around 900-1000. It will rev around 3000 and ever so slowly go back down. I think it may be something vacuum related but Id like to hear anyone's advice. Not sure if this will make a difference but upgrades are 6:1 performance cams, cold air intake, 2.5" exhaust and 5 speed swap from a zx. Thanks for your time!

[PapaSmurf2k3]

Yeah sounds like either a vacuum leak, or sticky throttle.

[evildky]

I assume you mean that the idle sticks at high rmp then slowly returns to normal. This could be a linkage issue, a dirty sticky TB or a dirty sticky IACV. Could also be a bad TB, If memory serves the 76 had a rather complicated TB with a decel valve built onto the bottom of the tb, might also just be dirty and sticky. I'd start by cleaning and lubing the throttle linkage. Then clean the TB IACV and decel valve with carb and choke cleaner, make sure everything is clean and free to move.

[Mattbarnes]

Thanks for your suggestions they are appreciated!