eating gas 1990 gmc sierra 350

I replace my intake manifold gasket.After replaceing the gakent I noticed that my gas mileage decrease hugely. What should i check or what is my problem??

Is the check engine light on? Did you disconnect the EST bypass when you set the timing? Timing set to 0 when EST bypass connected? All vacuum hoses reconnected?

The 02 ground is grounded at the intake manifold, did you reconnect all of the grounds?

//2000CAYukon