On my 71 F100, 351W, AOD, 3.7 gears, 600 Holley, 235/75/15" tires. Etc. I get about 15 mpg around town driving normal. When i get on the interstate and run average 65-75 mph I get roughly 13.5-14 mpg. I would have thought I'd get better on the interstate. A buddy has the same truck with a 390 with the 3 sp. overdrive and he gets about 18 mpg on the interstate. At 70 mph I'm turning about 2k.
Could it be that I'm lugging my engine too much? Any thoughts or suggestions on this fellows. Not really complaining, it's just my next goal on my truck to increase the fuel mieage while maintaining the drivability.
Clint

 

Thats weird, could there be slipping of some kind in the tranny? I am clueless so don't think I am making a diagnosis just throwing out ideas. Could the carb be off just enough that your provoking it to drop a little more fuel than needed when your running 2000 rpm steadily? I am interested in the fix to this one. Can't say I have heard of better mph in the city than interstate. Sometimes fuel economy can be from poor igniton timing, make ya wanna grab the 1/2" wrench and start messing with the timing? haha.

Side note if I were getting 14-15mpg with a carbed 351w engine I would care less how or where it got the best. Then again I am a little like yourself, if things aren't just so then I want to make it right. later

 

Is holley vacumn or mech secondary? Secondary,s open a little at cruise when they dont need to be? Maybe try a plug instead of running a power valve. You could also try smaller jets. Just some ideas.

 

Circledirt
That is what I was attempting to say but well I have trouble with the wording sometimes. Oh well, the secondaries are either but being he has a street motor I would say his are vacuum. Later

 

yea, they are vacuum sec. on a Holley 600. Was running 67 jets in the front but I replaced them today with the 70's again. Sparkknock clears up with 70's
Clint

 

Have you tried running a vacuum gauge to determine the throttle opening while on the road?

 

Hooked it up last night. Going to run it a while and see what it looks like.
Clint

 

71swissaqua
I thought I had my motor pegged as far as timing as I mentioned a while back. The other day I was driving up a hill and heard one little ping. I am sure my face turned red, you know the feeling of working on something having it run good then the problem reoccurs and you think to yourself sob when is it gonna end. haha. I went one stage rich on my metering rods and it solved the problem and that bad boy runs stronger than ever, I kinda figured the sudden temperature drop outside had a little something to do with it and kind of expected it so it wasn't too bad. Let us know what you come up with with the vacuum gauge. Later

 

JWT starts talking about metering rods, and I was just about to ask what size power valve he was running. Talk about getting derailed.

Anyway, that 600 isn't a double-pumper is it? Those are great carbs for track performance, but they are thirsty little monsters. The power circuit in the DP's is richer for greater power, unfortunately it consumes a great deal more fuel. I think you should try a smaller power valve, because at 2K, you may be dropping enough vacuum to actuate the power circuit, which is where all your fuel is going. If you have a 12-13, go for a 9 or 10. You'll have to get on the throttle a bit more, but you won't be power-cruising on the freeway. I'd also step up your tire pressure to 35 psi all around, I've found that to be worth a 3-5% increase. Yeah, they wear quicker, but a 3-5% fuel economy increase will easily offset the 10% tread life difference in driving value.

You're a brave man for setting up an AOD to a Holley carburetor. I was reading into the swap, and after finding out more about the electronic OD's, I'm waiting to find a 4R70W instead. 4R70W's use a TPS instead of a TV, which I'd feel a whole lot better about fabricating. How does it shift, and did you keep the lockup feature?

 

You might try a stiffer secondary spring in the vacuum pod. 2K RPM is pretty close to the stock Holley opening point (where it STARTS to open).

 

Ran it with the vacuum gauge today and might have found the problem As stated, I thinks it's my powervalve. I'm running the 6.5 and while cruising the interstate pulling a long hill the vacuum was dropping real close to 6 and considering the abuse this gauge has had in it's life there's no telling how accurite it is. Guess I'll pick up a 4.5 and see how it does.
Get about 17 at idle in gear but it looks like I might be in the powervalve too much just running down the road. thanks for the help fellows as several were right on the mark.
I just love tuning on this thing. I might have to try me one of the performer 600's one day and see how it does. Just don't know anything about them. i can rejet and change the powervalve in about 5 minutes on the Holley now. lol

 

I tell you one thing the edelbrock is a very user friendly piece. With your experience especially with the holley you will more likely than not find it to be a breeze. Hope you found your problem. you can go 4% rich or lean with the edelbrock with no more effort than loosening two screws on top of the carb you remove nothing but the air filter slide the little cover over pull out a metering rod then slide it out put whichever you want in then put it back in and tighten two screws easy huh. Should you need to change a jet you take out like 8-10 screws can't remember exactly but around that many then you remove a little tricky I will call it a cotter pin then raise the top of carb with fuel still in it unscrew two jets then reinstall appropriate jets then reverse the previous order. Still not too bad. nice part is the carb is still on the truck and you don't have gas everywhere. What do you think about that?

 

Jw, you seem to know a lot about tuning edelbrocks. I've got a couple of questions. My 79 F100 (see gallery) with 351w (5000m on rebuild), C6, 3.70:1 gears( soon to be 3.50:1 trac-loc) gets about 8mpg city and 10 hwy. I was wondering what changes I could make to the carb to improve this. It has a 625cfm carter afb (looks just like an edelbrock). I also wanted to know if an edelbrock tuning kit would work on this carb or where you can get the carter kit as I am having no luck finding one. Thanks for any info.

 

have you tried summit? The kit should work as the performer and as you mentioned the afb are very similar if not identical, my performer 600 has carter stamped on it. This is all dependant on your having a close to stock cam heads etc. Either way your best bet would be to make sure your timing is between 8-10 and your vacuum advance is close. Then trim your idle mixture screws until you see your rpm gauge start to drop slightly or fluctuate, it should drop a little first. Turn one side until your rpm drops just a little like 50 rpm then bring your rpm back up to what ever you keep it at 650 rpm is recommended then do the other one the same way. If your carb is stock (otherwise untouched internally) then see if this makes a difference in mpg. Should you find your mpg is still not improving then you might opt for the metering rod and jets kit as that is about all you can do short of metering rods and jet changes. You will need the book (owners manual) as it directs you to certain reference numbers which provide a recommended jet and metering rod. Then it is just a matter of going lean until you feel flat spots surges pinging lack of power and any other out of the ordinary type problem with the motor. With a poor tune and heavy foot on a stock 351 w/t a 3 speed and 3.55 gears I was getting 11mpg and that was with me tinkering around the entire time short trips and hard not crazy accelerations so I am anxious myself to see what improvement I got from starting over and tuning it the right way as the cold starts and power are great. here is the number for edelbrock if you wanna call them for more info as far as an owners manual for reference numbers and interchangeability 1-800-416-8628 I am with you they are probably the same carb different provider. Should you call them let us know what kinda differences if any their are between the two carbs. Later

 

Summit does not sell the carter kit, only edelbrock and there are different ones for each model. I found a carter kit on ebay so I will know in a few days if that works out. I just want to get a little better milage driving back and forth from college. If it doesn't get any better I may be temted to start driving old "white lighting", a 1983 f150 I6 with a homemade flatbed and 200,000 hard miles. Runs good but increasing rust, a ratty interior, and a leaky rear main compensates for the 5mpg I'd be gaining. It has developed a whistling exhaust leak that sort of sounds like a turbo, kinda cool but it still doesn't go any faster. So I'll keep driving my 79, besides I need to have somthing to compete with the guys around here whose dad gave them a new Lexus or BMW for for graduation. The way I see it any dope can make payments but it takes talent and hard work to restore a vehical and its nice to not have to look at 10 cars identical to yours while in the parking lot.