MPG and Exhaust Manifold Gasket

[Soonersteve]

I could use some advice from the knowledge-base here. I have a '75 F100 with a 360 in it. I have been averaging about 13 miles per gallon for the better part of a year now and finally got around to fixing some exhaust leaks. I picked up some Fel-Pro gaskets and replaced them in your standard fashion (remove exhaust manifold bolts, removed old gasket, placed new ones in, replaced the bolts). Since I did this, I noticed a drop of MPG down to approximately 10 mpg (from 13 = 23% reduction) and a smell of unburned fuel. Is it possible to "run rich" after replacement of these gaskets?
Any advice would be helpful.

[idaho_cowboy]

Interesting question, and observation.

But I'd guess you're on track.. Could be rich now.

What's your timing set to? (total, and initial, if you know..)

- Joseph

[tnlprt]

As dumb as it sounds the exhaust is no longer as free flowing as before

Goes to show what happens when you reduce back pressure in the exhaust system Then place a cork in it

[Soonersteve]

Boy, long time on this thread, but thought I would close it out with an update:
-- The entire exhaust system was loose, so I got some longer grade 8 bolts with lock washers for the manifold bolts (went from 1' to 1.25'). That fixed the manifold leaks. Installed new rings and tightened everything down tight. Put in a flowmaster super 44.
-- Adjusted the timing (it was way off) back to 8 degrees BTC.
-- Adjusted idle and carb.

Long story short, I am about 12 MPG on the highway. Guess that is pretty much par for the FE course!

[MadJoe]

What was the timing set to when it was "way off"? If 10-12° BTC, it was probably done intentionally. That seems to be the sweet spot for these 360's in terms of drivability.

-Joe