what size main jets came stock on a 1975 ford f 150 390 6.4 the carb is a motorcraft 2150 and can you tell me the best ones to use for good gas mileage and how do i tell the size of a main jet

 

I'm sure there is a chart somewhere that tells what jet is suppose to be in there. Not sure myself.

The jet will have a number stamped on it. This does not always denote the hole size, but just the "part number" it is.

You can lean it out a bit, but you need to watch you don't put too small a jet and start burning pistons/valves from being too lean.

I've found the best way to get mileage from an FE....keep your foot out of it and stay at the speed limits. With 3.25 gears I can run 65/67 mph at around 3000 rpm. Much more and it really sucks the fuel. It starts working too hard to keep that mass moving at a high rate of speed.

 

for what it's worth...
my '69's stock 2150 had #54 jets in it.
I recently went to a Holley 2bbl and have ended up with #78 jets in it to achieve 13.5:1 a/f ratio. I may end up with #80's in it before the end of summer... we'll see how detonation goes.
I do not know how the motorcraft jets compare in size to the Holley but have read here they do differ.
I'm with Freightrain, leaning it out may not be in the best intrest of your motors longevity(sp?). Best of luck.

Out of intrest, what is your mileage now?
I was getting 10.5mpg with the stock carb, but in a heavier truck.

 

i am getting about 5 miles a gallon

 

oH LORDY!!!!

Man, you need to look into that. My 352 gets 10 pretty regularly.

 

Ford use to color-code most of the parts in there Factory Holley carbs. As to what jets came in it from factory, would greatly depend on the location/dealership is was being sent to....example. They wouldn't put #78's in it if it was going to be sold in Denver.


I love my holley carbs, but if there is one thing I've learned over the years, it's that tuning them is often done by trial and error. I live at about 4900 feet, and I'm running 68's for my primary, and since I'm running a 4160 body, I'm using a plate for the secondaries (134-21, equals 75 jet size I think) and it's running great. Most importainly start out with new spark plugs, drive it around for a while then pull the plugs, note the color of the porcelin. if it's bright white, your lean, if it's black your rich, light brown/tan, your set.

Best rule of thumb, is increase/decrease 2 jet sizes per 2000 fee of elevation. I've also found changing 2 jet sizes at a time will show more of a change anyway. There are usually only 4 to 6% difference between one size and another.

You should have a number stamped on the carb, or a tag somewhere. If you get that I might be able to cross it in some of my books.

Hope this helps.