91 300E Poor Mileage

I have a 91 300E which I recently had the heads rebuilt on due to oil consumption. Prior to the work my mileage was 20 - 22. After it has dropped to 15-17. Sent it back to the shop and they adjusted the lamda (sp) control to increase the vacum. My economy gage now runs in the middle even at idle.

Any thoughts would be appreciated.

Clark