Hello forum gurus im in desprate need of yalls help i have a 1972 f250 c/s 2wd c6auto 3.73 gears with a mosty stock healthy 390 . Only mods are single chamber muffler edlebrock preformer intake and a quick fuel 750 , my problem is the fuel economy is horendous and i understand its a old truck but i belive 6-8 mpgs is pretty bad. A lot of old timers have told me to go back to a 2 barrel so ive thought about a holly 500cfm 2 barrel but i wanna hear fellow bumpside owners thoughts.

 

That's pretty bad, even for a 4bbl. I'd check into a rebuild of the carb and getting it tuned properly.

 

Welcome to FTE!
Which QF 750 do you have! They are all have their little differences. Switching to a 2 bbl. won't change much, because if your carb is a vacuum secondary you are running on a 375 cfm 2bbl at cruise. I have had QF carbs and out of the box they seem to be calibrated on the rich side, but without an AFR gauge and a good understanding of how they work recalibrating one is a long process.
First thing is to make certian your ignition system is in good condition and the timing is correct. I would recommend setting the initial at around 10 degrees for a start.
Check the float levels if they are higher than recommended it will cause it to run rich. You need to put a vacuum gauge on it and tune the idle to best vacuum. Remember the idle and transition circuits are always putting fuel into the engine. If your carb has 4 corner idle adjustment that idle adjustment can take 30 minutes of fiddling to get right.

 

a. average rpm ,b. does it smell like a gas station when running or more of a...burnt crayon smell and have you checked plugs and wires

 

As a good starting point, that carb is too large for a 390 based off the Carb CFM calculator on Summit's page. As others have said, I would also check the plugs and timing because you are running extremely rich.

 

All new e3 plugs and new wires timing is at 11 initial have put a vacuum gauge on it and she was at 16 pounds at idle as far as exhaust smell walking by the exhaust makes you feel like you smoked a whole pack of cigarettes, I’m just a youngster and was never taught how to tune a carb just trying to do the research and learn , but when I pulled back the fuel it has a awful studder but I haven’t messed with the tuneing a whole ton

 

I think your right even with proper tuneing I believe the carb is oversized for my application

 

Many are switching to the Holly Sniper fuel injection. Carb are getting to be a PITA. Not designed for E in Gasoline for one thing.
You could try a bottle or 2 of Carb out Regain but most likely it has lots of corrosion. Fuel injection is a great option. The sniper kit
and you can include the Sniper ignition stuff also if you're good with some more ideal mods. I wouldn't go with a high lift intake
manifold but one not stock is a great idea with the fuel system change. + the fun of doing the change over is so tempting!

 

I’ve started to look at the efi looks nice and I have a edlebrock proflow so it flows better then the stock iron block for sure

 

I run an Edelbrock 600 on my 460 and it has plenty of fuel to operate, and it doesn't smell like an open gas can. I get 10/11 with a F250 4x4 and 3.54 gears and C6 trans, which is typical for this application. I'd look into doing something different with your carb, personally. That's way too much carb for your application.

 

I just noticed this, if it helps. ..... It's from 2007 so sort of a learning experience moving forward most likely. Fun to know thought.

Ported C7AE-A heads
TRW L2291 pistons, .030" over
Crane 343941 cam (272/284 degree dual pattern, small by some standards, large by others!)
Headers, mufflers
Edelbrock Performer RPM
Among other things.

The system is just about maxed out on this motor, in terms of fuel delivery. I really should have got the 900cfm system with it's bigger injectors. And Holley rates the 700cfm TBI kit up to 440hp, so it makes you wonder just how much my motor is really putting out?

But the options have changed since I did mine! Edelbrock now makes a fuel injection manifold for the FE, as part of their Victor line. This lets you use multi-port fuel injection with out having to modify a manifold. Combine that with one of Holley's universal Commander 950 MPFI systems and you'd be ready to go! Of course, some fuel rail fabrication would be required, and you'd have to figure out what size injectors to run, but neither of those are unsurmountable challenges.

Stay away from the Holley Pro-Jection injection systems. They are non-adjustable speed density systems. You have ***** you can turn. This just doesn't work good. Equally, stay away from anything that says it doesn't require a laptop to tune. 90% chance it's a non-adjustable speed-density system. The Commander 950 is a speed-density system, but it is fully adjustable. Trust me when I say fully adjustable!

Here's what I'd get, if I had to do mine over again:
http://www.holley.com/534-183.asp <-- See the instructions for what I mean by "fully adjustable!"
http://www.edelbrock.com/automotive_...d_bb_vic.shtml <-- #29365

WAY BACK THEN PROGRAMABLE WAS IN THE PICTURE.

 

This is what a stock 390 would have come from the factory with. 390-2V had a 351-cfm carb, 390-4V had a 446 cfm Ford (Autolite) carb, 390 GT, 4V had a 600 cfm Holley carb. That is from my 1970 Ford muscle parts catalog. But Ford recommends using their aluminum manifold with a 735 cfm Holley (C9AZ-9510-N) carb. But with all that work and expense you only gain 17 HP and that is Fords words.
Running a carb that is bigger than the engine needs isn't going to make one run rich. It is the tune of the carb that determines the mixture. It is very common for a carb that is sized too large to actually run on the lean side above 3000 rpm because of insufficient airflow over the boosters.
I run a 780 Holley on one of my 351 Clevelands and after a proper tuning it works just fine. But to get it right it required changing both primary and secondary main jets, the power valve and the power valve restrictor channels and the idle air bleeds and the pump squirter. That required taking that carb apart about 6 times.
Also, if you have an aftermarket cam with a lot of overlap you will likely never get it to idle clean mainly because of lower vacuum at idle and reversion.

 

16 inches of merc is a little low? should be like 19 but it doesnt super matter unless that needle isn't dead steady, and she's lean at idle so get some non flammable brake clean and shoot around the intake to check for any rouge leaks, especially around the base of the carb. pull a plug and post a pic so I can see how it's burning and I would like a bit more information on what it's driving behavior is

 

ok I’ll post a plug picture later, but as far as driving behavior it’s pretty good at higher rpm but it’s the take off or starting that makes it run rough or almost stall out

 

I have been TRYING to restore my truck , for 7 years or more . during this time I have bought countless FORD manuals , Hot Rod manuals , most of my television/youtube watching is Automotive related , Years on this Forum
EFI is overrated , especially for the price . Amazing professional marketing campaigns via TV shows & youtube influences strengthen this !!!
We have trucks , not 8 second dragsters . a clean , properly tuned , sized carb along with the WHOLE ignition system operating well do great , PROPERLY being the KEY to it all
As a previously poster said , check vacuum , another said do the calculator to get right size carb , there are many things that must all work TOGETHER , fuel SYSTEM
I highly suggest you start from the beginning , because I believe 16 lbs of vacuum is low , I think it should b around 20 or 21 at least .
New spark plug wires , or check wire resistance , new stock gapped , PROJECTED nose plugs , check for vacuum leaks , accelerator pump lever , strong battery , tight connections , carb linkage opening & closing COMPLETELY .
fuel filter , tune distributor , ETC...ETC...ETC...
I have seen several $30,000 or more engines never getting COMPLETELY TUNED CORRECTLY because it was so complicated , actually it blew up at least once so that was MORE money .
Any way , later Then you can determine if u need a hotter or colder plug , & more simple teaks if needed , return fuel system , A/F tuning..............power valve change ...etc...
later-gator...........

P.S. Yes EFI has good traits , but also has more things to go bad , plastic computers , plastic tuning remote , . Now the computer is interrogated into the carb witch I almost know heat/vibration will cause this to fail in a year or two.............or 3.........
I recently saw a N/A engine in 32 ford run over 150mph - 8sec. in the 1/4 mile............(non-EFI)