I know that gas mileage is touchy sudject around here, I know its a truck and its not a sports cars but I was just wondering my truck can get better than 8mpg. This is my situation, ive got a '70 F250, 390 with a 600 holly 4bbl, C6 trans, 2in dua tail pipes, and 3.53 rear end. I did have a 4.10 and was bearly making 6mpg, when i made the swap it raised up to around 8. I am thinking that it mite be the carborator, when the engine was rebuilt 2 years ago the guy that rebult it said that one side jet adjustment woudn't tune up. any help would nice.

 

A hotter coil pack will improve gas mileage.

Convert ot duraspark II electronic ignition system.

Sounds like the carburator is malfunction, either R&R carburator or replace the carburator.

Keep the truck running at 55 mph or lower, this will improve the MPG's.

 

I'd give that carb a proper rebuild and add electronic ignition. I'm a fan of Ford's Duraspark system. It was used on FE's in '75 and '76.

 

I forgot mention that I have all ready put a pertronics ignitor II and there Flame thrower II on it works. no gain in mpg's. I rebuilt the carb once already but it still has the same problem. I think am just going to replace it with a 570 cfm holly street avenger. Would I gain anything by using better spark plugs like the bosch platnum ++4 or something like that?

 

I would start with carb work first. It's possible better plugs may help, but Autolites have never let me down.

 

How does it run? Ever do a compression check?

 

it runs good ecept at idle. sometimes it idles too fast, sometimes it dies on me, but most of the time it just idles rough, not like its missing, its just rough. the compession should be good the engine was rebuilt 2 years ago and ive only put like 4000 miles on it maybe more. do you think that the 570cfm holly street avenger would be a good replacement for the old carb? and with the hotter coil would you ajust the spark plug gap bigger or leave it be?

 

Running that ignition, I would go with about a .045 gap on Autolite 45 plugs. If the truck has a detonation issue, try a 44 or 46 (I don't remember which one is colder). I would avoid the platinum plugs, their self-cleaning temperature is higher than what it would see in that engine, so they will foul out and cause problems.

 

thanks, ill stay away from those bosch plugs. i did gap the autolite 45's that are in there now to about .045 or .044, some where in there. no problems with it so far. what about the street avenger carb thou?

 

Loose the holly. I have a stock 67 F250 Cruise-O-matic with a 4:10 and I get 11 mpg, city, highway, summer, winter... I know the C6 is not an efficient transmission, but it sounds like the Holly is your weakest MPG link.

 

so when i replace the carb should i put a carb spacer in to? if so what kind? and what configuration?

 

Take it to a reputable repair shop and have then use a gas analyizer to check how rich the exhaust is. I would diagnose the problem beore spending money on a bunch of parts. Maybe all you need is to rejet your existing carb and that's a lot cheeper than a new one.

I have a 390 with an S intake and a Motorcraft 4300 4-V off a 1974 460, C6 and 4.10 gears. I have only put 6-700 miles on the truck and the first trip was towing a 19' travle trailer with about 1200 pounds in the bed. We traveled 300 miles including a canyon with 5-6% grades and on the flats we ran between 65-70 mph and got a little over 7 mpg. Have no idea what it gets around town.

 

Check your compression and test with a vacuum gage and see what it says:

How to Read a Vacuum Gage http://www.users.bigpond.com/ergoff/vac1.htm

Just because the engine was recently rebuilt does not mean it does not have internal problems. It could also have been rebuilt with a large cam that will totally destroy your gas mileage.

 

i just so happen to have a vaccum gauge in the truck it reads at about 18 when at idle. i will call the guy that rebuilt the engine to see what kind of cam he put in. i posted the thread about the redish black goop, it seems to be coming from the carb somewhere. i have already replaced the carb, the old carb as i have found out from my grandfather is about 15 years old and has been rebuilt several times always having problems with it, thats the main reason why i replaced it. i love he new carb i put on so far, i haven't checked to see how it has effected my mpg's yet.

 

Try adjusting the fuel mixture screws your self first, if it don't stay right, then you can pay someone to do it.
You will be suprised how easy it is. Start with the engine off. Turn both air fuel mixture screws in (clockwise rotation) and do not tighten the screws, just to were you barely feel the screws bottom out (overtighten these screws can ruin them). Then back out each screw 1 1/2 turn (counter clockwise rotation). Start the engine. Adjust your engine idle to about 600 rpm's. Then at 1/4 turn intervals set to bothe air screws in a counter clockwise rotation. Listen to the engine till the engine idles smoothly and does not idle roughly. Might need to keep setting the carb to 600 rpm's. If you notice the engine smooth out and then begins to idle rought as you adjust the air mixture screws, you have set the air screws to far out. When the engine idle will smooth out, then Gas and Rev the engine and it should not have a noticable dead spot on the carb when the air mixture is set properly.

Having good electrical contact for the charging system does affect how well your engine will run and can save you on fuel costs.
Keep all battery cables clean at battery terminal.
Check and clean and sand the ground cable where it make contact at the engine block.
Clean and sand all metal contacts at starter relay (seloniod).
Clean and sand all metal contacts at voltage regulator including ground wire.
Add ground wire from engine block to frame rail.