I finally been able to get out and work on my 78 f100 and have got alot done as far as it not running so great. I had problems with it starting acting like it was out of time and getting 10mpg. I have replaced all the vaccine lines, tees, and caps. I have replaced the fuel filter and a couple pieces of rubber fuel line. The carb has been rebuilt within about a year. New air filter, and I just installed a DUI hei distributor. I have Taylor wires that are probably 6 months old and the plugs look perfect. No catalytic converter. The only problem I am having is that my gas mileage is still horrible!! I am averaging 10-12mpg. I am running stock tires(235/75r15) and the stock 2.72 gears and the truck feels likes it is running great. I used to get about 17mpg what could I be overlooking?? Thanks!!!

 

Late timing would do it. Also vacuum advance inop or, a vacuum leak? Attach a vacuum gauge to a source of manifold vacuum and note the vacuum indication at 500-600 RPM. Should be 18-20 at sea level with a healthy engine and engine timing where it needs to be.

 

Gas mileage is a hobby of mine. See if this thread will give any ideas?


https://www.ford-trucks.com/forums/1...-9l-300-a.html