I need some ideas. I have owned my mustang for 5 years now and it has usually gotten 12-18 mpg depending on how I drive it, no big deal I expect such numbers. The problem is that ever since I did a few minor mods it has dropped to 8mpg which seems a little excessive to me.

It has a 302 with E7 heads, a mild cam with .469 lift and .266 (I think) duration, duel exhaust, four barrel intake and carb. Now the changes I made were: roller rocker arms with a 1.7 lift changing my applied lift to somewhere around .498, and I went from an Edelbrock 600cfm carb to a Holley 600cfm carb. Changed because Edelbrock was causing problems with a vacuum leak at the throttle shafts that new bushings didn't fix, and when I went to holly I changed from mechanical secondaries to vacuum. I wouldn't expect the lifter ratio would cause that big of a difference in gas consumption and I figured with a carb that didn't have a huge vacuum leak would help efficiency.

I have not done much of any trouble shooting aside from pulling a spark plug which showed to be a little darker than the desired light tan. I have herd that holley's tend to be touchy when it comes to fuel pressure but have not checked the pressure my pump produces. My understanding is that I need 6psi for a holly correct? I have not tweaked the holly other than setting the idle air mixture and the choke, but didn't think it would be that bad out of the factory. Other than psi, what should I be looking at to make sure I am not wasting gas? If I am simply using that much gas then so be it, but I don't want to waste any.

Thanks, Brian.

 

That's a pretty stout cam, but you should be getting a LOT better than that. I'm guessing a combination of carb tuning and distributor curve tuning will get you to the 12-14 range, better in the highway. What size is the carb?

 

I'm pushing 16-18mpg on .561 int and .563 exh with the 1.7 rockers...everything roller. 1970 351w heads with 1.90 intake valves, RPM Airgap intake and 650 Holley double pumper with mechanical secondaries and manual choke. I'm with 2+2GT, 8mpg is a little low...on top of carb and distributor tuning also look at the engagement speed of the secondaries. If they're kicking in too soon then it'll really kill your gas mileage.

 

What's it running for a temp? This can make a huge difference. I cut my mileage in half by choosing a lower temp thermostate and promptly switched back.

 

Its been running with a normal 180 thermostat since before I noticed the bad mileage. Though I have replaced the rad in the last 6 months because of a progressively large leak that was developing in the old one for the last 4 years, so it is possible that it runs cooler now that have a rad that seals.

If I remember correctly I set my base timing to 12ish and the mechanical curve is a straight curve to max advance at about 2500 rpm (I don't remember what the max was). The current settings I have on the dizy have allotted for the best performance I have been able to obtain from it but maybe I will reset the ignition back to stock settings and try that.

When I got the carb there was a note attached to the carb where Holley insisted that their "factory tune" would be adequate for most street applications and would not need adjusted. With that I decided to give the factory tune a chance, and would like to make sure everything else is in order before I start messing with it.

I have not dealt with vacuum secondaries before this, so how will I determine if they are engaging too quickly or working properly?

 

What Tstat did you switch to that cut milage in half? Which tstat do you run now?

-Gun

 

What temp were you running and what temp are you running now?

 

Well thats funny that we asked the same question.

 

i bet you he had 185 and changed to 160. My gas mileage went down with that as well although I never realised this was the problem. I had heat issues and as part of the troubleshooting I went for a 160 or 165. never changed back, and I should.
I believe it is part of the problems I had with lean issues. I have to run richer to compensate, when I have A/F at 14 it stumbles just off idle. cured with higher float levels and richer, but that can't be the way to go

Anyway if the plugs are black my first guess is that engine might run rich.
you should check on two things
a) overall and idle mixture. you see that on the metal ring at the very top of the thread. this is an exact copy of what your combustion chamber looks like. black deposit there-> rich
b) on a WOT run. jst after WOT run switch off engine remove spark plug and check at the very inside. the bottom of the ceramic (you need a light to shine down there). There should be the WOT fuel ring, that should be 3/4 of the way around.

check for obvious things as well:
fuel pressure. yes, 6psi is bang on for a holley
fuel levels. on straight level check on the fuel bowls. they usually have a screw or sightglass. if screw remove screw and adjust so that it just starts spilling out of the bowl when running in idle. if not spilling then so that if you give the car a very slight push from the side that fuel dribbles out. be careful on the rear fuel level. most carbs don't use fuel from rear bowl in idle. so if you go up with the level you will see the effect immediately. if you go down with the level then you need to get rid of the excessive fuel in there first. so if it's a double pumper slowly squirt some fuel from the secondary after adjustment. only then the level will go down when you adjusted downward. same applies for the front, but there you can just give it a bit of throttle.

Kalli

 

Ford engine designers specified a 190°F thermostat for best engine performance and economy.