Also make sure that the vac can is functional and adding an appropriate amount of timing, like 8-12 degrees depending on what the motor wants. On my engine, it's worth about 5-6 mpg, so worth getting right.

Before that, get all the usual tune-up stuff done as outined earlier in this thread. Ie plugs, wires, carb cleaned thouroghly and checked for leaks, timing set correctly, etc etc. A good tuneup will do wonders for power, mpg, and heat. See where that gets you before buying new carbs and dizzys.

 

Fast- awesome. That is quite helpful. I can definitely relay a lot of this information to someone who can work on it, i'm sure with all the expertise you have all shared that this will be able to be working well soon! Hopefully within a few weeks i'll be able to give an update

 

Alright guys who haven't abandoned me at this point. I took it to Tony G in orange park, florida and had him tune it- the statistics said my fuel and oxygen mixture are correct and optimized for fuel efficiency, and i'm getting about 190 HP from the engine, about what he'd expect from a 5.0 crate engine. So....gas mileage is still an issue. Something else which might be affecting it is the Idle. It Idle's a little high in park, a touch under 1000 RPM's. Not ridiculously high, but im thinking 650-700 is probably closer to where it should be. Forgetting that though, putting it into drive and just letting it idle, the thing slowly picks up speed on flat ground literally shifting itself into second gear eventually and continually cruises without touching the accelerator at about 15 MPH and around 1250-1500 RPM's. As I said before, I don't know a lot about cars, but it seems like something is wrong here, something which COULD be a factor in this gas mileage nightmare saga.....thoughts?

 

Yes the idle is too high; should be 750ish out of gear, 600idh in gear. Very simple easy fix: one tweeeek of one little screw. Now, not to be mean or anything, but if that intimidates you to the point you "take it" somewhere for it, then you don't have any business driving a 30-odd-yr-old car with substantial induction system modifications in the OLD direction. (that carb is a 50 yr old design) All the posting on the Internet that you and all of us can do, won't overcome that degree of lack of preparation.

So now that the timing curve and fuel curves have been optimized, what is the actual gas mileage?

I'd say if it's any better than 15, you've got about all that particular carb is going to give.

 

So you took it some where and paid money for someone to "tune" it and they didn't even set the idle? That is one of the first things that should have been done.
If I remember correctly, the correct setting is 650 rpm with the transmission in drive.
It can be adjusted with a screwdriver by backing out the idle stop screw.
Easier to do with 2 people, 1 person in the car to hold their foot on the brake and watch the tach, the other person working the screwdriver under the hood.

 

This is not a worry, it's a relief. This is what a nice tight stock torque converter is supposed to do when idling the engine so fast. Most crapanese junk won't do it, but they don't have even half the displacement, either.

 

Well sofa- I took it to the best tune shop in Jacksonville (there aren't that many) and he checked it, did a few pulls on it, and the test showed it was already tuned properly, so all i paid for was the diagnostic, $60- he said it should have pretty decent gas mileage as it is, and sent me on my way. So he didn't tune it, he said it only had room to maybe run it 2% more lean which wouldn't really help anything. So no change equals continued shitty gas mileage.

I've got it running at about 650 rpm while in park and in drive after checking it just now...although occasionally after giving it gas, the idle doesn't return back under 1000 rpm. I went under the hood and revved it there, and then it fixed itself and went back where I had it. around 600-650.

Quite honestly, I'm thinking next year of just finding an old stock motor and putting it back in and getting rid of this aftermarket crap as a swap (i don't mean its crap, i mean since i don't know what the ****er did to the old engine, the headers, etc..... me trying to get the gas mileage even over 10 is more trouble than its worth.) I don't need the power, i don't want the power. I have a mustang for that.

 

Thanks Josh- it was pretty easy. I had to deactivate the high idle screw first, then adjusting the main idle screw showed me pretty quick results, it only took a few minutes to get it to idle lower.

 

Glad to know Atilla. I've never seen any car do this, japanese, european, or american of any age, so it was a curious. I'm guessing now that the idle speed has been fixed, that won't happen anymore. I can't drive it anywhere to do any REAL checks cause I have the whole interior ripped out, finishing up the last seat re-upholster now, and plan on getting the new carpeting in within the next week or two.

 

really all he did was throw it on the dyno, and did three pulls

 

I don't believe your tuning guy knows jack about tuning, but consider these facts: First, headers don't hurt MPG. Second, the Mustang has a few fatal flaws that can't be fixed: It has more of the engine weight forward of the front axle centerline. It's narrower, it has worse aero and a far worse rear suspension, it can't take an S60 nor anything comparable, it can't fit 285s on 10.5s front, it can't fit 315s on 11s rear, and it costs far more to put a 1200-horse twin-T76 4.8L in it. Now tell us which car is better suited to performance?

 

Atilla- I live in Jacksonville, FL. If you can point me to someone better i'll gladly go. I lived on forums for months to find one guy that was highly recommended who had a dyno....and I took it there. Arguing with me about the car with better performance upside when I couldn't care less about HP for either car is pretty pointless. My mustang is fast enough that I don't want or need anything faster, and that was my point.