I have a 84 F150 300 I-6. The stuff done is a offy dual plane intake, holley 390cfm carb, stock exhaust right now but will have clifford headers and custom exhaust system very soon, and a complete ignition with vacuum advance out of a 81 f150 (i think it's called a duraspark ignition) because with the new carb the stock TFI ignition doesn't work. There is no emissions equipment on it at all and the computer is gone. Now to the point i am getting 6-8 mpg maybe 10 if i ***** it but IMO this is terrible for a 6. Any ideas? Also the exhaust pretty much always smells rich I have not rejetted (which I am thinking will be necessary) and i have the stiffest vacuum secondary spring in.

 

Sounds like your idle screws have backed out. You may also have a linkage issue opening the secondaries before you really want them too.

 

I have my idle mixture adjustment screws as lean as they will go without affecting starting out (manual) abd my idle when choke kicks off is good 800-900 rpm. my secondary linkage is also fine.

 

Then I'd check how the carb is metering the fuel as well as the delivered fuel pressure.

If your pressure is ok then it's got to be your carb. You say it's a new one, how new? It may need an overhaul kit or something as simple as smaller jets.

Hopefully one of carb experts can give you better information.

 

I am for sure going to rejet when once i have time to. I bought it used but it is in very good condition the guy previously had it on a 305 chev but i have no idea what he was trying to do. If smaller jets don't help i will probably try rebuilding. what should the delivered fuel pressure be about? I have a stock fuel pump.

 

bump bump bump

 

Nick, if you have no computer for the TFI ignition then you have no advance. All the carburetor playing in the world will not help. For now try setting your initial advance as high as it will go and still start and run without detonation, you might start around 24 degrees BTDC. Once you get the Duraspark distributor and module you can try around 10 degrees BTDC as a starting point with no vacuum applied. If you can use an adjustable timing light, try to go for 34-36 degrees total centrifugal advance. What you are seeing now is what used to happen with the old Holley "Loadamatic" distributors on the pre-emmision 6cyl. Fords when someone tried a different carburetor setup on them, no power gain and abysimal fuel economy.

 

Its a vacuum advance distriubuter hooked up the vacuum advance port on my holley 390cfm im not quite sure why i would have "no advance".....

 

I have the full duraspark ignition already distributor, coil, and module...

 

I've had to fix two six's with the same problem in the past 4 years. One was a 240 the other a 300. First, don't believe what Clifford tells you. I live some 7 miles from his shop and known his shop from the start.

The carb will never work on the street and deliver anything greater than 9mpg. They just can not be tuned for the street. The first six I had to unscrable was a 300ci in a 65 1/2 ton. My buddy had clifford "design" his engine. Same water heat manifold, same tube headers, cam, carb and retuned dist. It either ran heavy rich or would lean out and stall in the street, at the same time. It started out at 6mpg. That's worse than my Dad's 500ci Chevy 4X4. After wasting enough of my time trying to compensate for the combination I gave my buddy the choice. Don't tell be about it or leave it here and I will fix it, don't ask. He had no choice.

First I sold the carb on ebay to someone else. Next tossed the clifford camshaft, distributor. In went a 254 cam and a napa rebuild 65 dist from a 300. NOW made up an adaptor plate and installed an autolite 2bbl from a 302 smallblock. Wound up using #45 jets and an adjustable fuel pressure regulator set up by the carb, adjusted to 2 pounds, yes 2 pounds. Fuel consumption is related to the fuel pressure. Used the egr plastic carb spacer plate ran that to the rear pcv valve and drilled the oil filler cap, soldered on a stub and ran a fresh air hose to the air clearner dome. Oh yes, an aftermarket FRAM industrial air cleaner element and housing. Did keep the clifford exhaust headers which are a piece of engineering crap. Chronic leaks and vibrations. Had to remove from the truck and hand braze every single flang both inside and outside, every single one. Brazing will flow like water into every crack and leak in a weld and make a curved fillet tht will take away some of the mechanical stress on the joint. That killed one day. Then resurface every flang as they are distorted from the stamp press that punches them out. They still vibrate enough keep loosening the entire exhaust system.

The final part was ripping the gear pod out of the same mustang that had an automatic trans.. Into the truck which was a 4 speed truck. Naturally with the automatic rearend gears it REALLY made the fwy. speed increase but with the modified 6 it pulls just fine.

The results, for this combination and mix of Los Angeles city and fwy driving it gets 18-20 mpg. Even suprised me.

This is the second one I've had to modify.

Zobex

 

Im not running full clifford crap all i have is clifford headers and my dad is the shop manager of one of the biggest machine shops on the west coast so fabbing up the headers to work is no problem for us.

 

I pretty sure I found the problem... I tried rejetting i first tried size 40 jets always ran out of fuel past 2000 rpms. then tried 45's it was better but still had the bog at 2000 rpms. I bought a rebuild kit and rebuilt the primaries and put the size 51's back in and im running a medium vacuum secondary spring it runs good but i still know it's getting bad gas millage. But im pretty sure the problem is not the carb. The problem is I am running size 33 in tires with 3.08 gearing so i am having to lug it and it never is running in the "sweet" spot for the speed i wanna go. Any other ideas?

 

Have you had your speedometer/odometer calibrated for the tall tires or otherwise factored in the difference in calculating your fuel mileage? If not, your fuel mileage may not be as bad as you think. If your odometer is showing you travelled say 90 miles when you have actually travelled 100 miles that would throw your calculation off.

Also, before tinkering too much with the carb you might spend a few $$ on having the exhaust gas analyzed and that may tell you which jets you would best be running.

 

even give 10-20 more miles it's still very bad gas millage for a six. What do you mean by analyze the gas?

 

Exhaust sniffer...like the Emissions Techs use. I'm rebuilding my engine w/mods and in cussin and discussin the carburetor and jet sizes my "advisor" said he would hook it up to his exhaust gas analyzer before I start messing with the jets and go from there.

Offy DP manifold, Holley 450cfm Economaster carb, Comp Cam 260H grind, EFI exhaust manifolds, port match manifolds/head, clean up/polish bowls, Mallory HyFire 6A ignition & MSD 2 coil.