My setup is in the sig. I questioned Holley about why my gas mileage is so bad. I'm getting about 8.5-9 MPG on a tank of gas without flooring it at all. I'm pulling around 8-9" of vacuum in drive. I have 72 and 80 jets in there. My initial timing is 11 with total at 39 coming in at 2500 RPM.

They said:
"Hello, if the plugs are tan to brown in color and you have adjusted the idle mixture screws to peak vacuum and the power valve is matched properly to the engine vacuum I dont think your going to do much better than 8.5-9 MPG. iF Your vacuum at idle in gear is 8-9" install a 4.5 power valve. You can also jet the primary down until the plugs are light brown or tan for some better milage. Your timing sounds fine."

Now I'm no expert but aren't you supposed to have a power valve 1-2" below idle vacuum in drive? Also, 8.5-9 MPG just seems insane for my setup. I should be getting more, shouldn't I? ANY help would be greatly appreciated. Thanks in advance.

 

Your PV sounds correct for your app as per Holley & Demon.They say on a healthy street motor to install one that is around 2" below hot idle vaccum in gear (auto) or nuetral(stick).I hot idle around 11" so I run a 9.5".If the plugs are coloring fine with no surge (lean) or clouds (rich) I would have to say your jetting has to be damn close to what you need it at.With my Demon 750dp I get about the same mileage with the same heads and cam..rear gears..but I run stick.You are gonna be hard pressed to squeeze any more mileage out of that combo.It is not INSANE for that combo..unless someone has done better.It has taken me two years to get my mileage where it is thru many many hours tuning the carb and timing.

 

I'm no expert but aren't you supposed to have a power valve 1-2" below idle vacuum in drive? Also, 8.5-9 MPG just seems insane for my setup. I should be getting more, shouldn't I? ANY help would be greatly appreciated. Thanks in advance.

No, Holley is correct. You want the PV to operate at the lowest vaccum point possible for good economy, so If all you driving is done at 8inv and up you should select a PV that is at least 3 below that or the lowest point vac when you accelerate. Holley is probably figuring a vac drop from idle in drive to initial acceleration, so Idle 8/9in press gas and vac drop to 6.5 or 7 in of vacuum so 4.5 would be an alright choice might be a bit high but.......... Id personally try advancing the timing up just a tad if the MCS are set correctly and bring to vacuum to at least 10in in drive.

 

I never had much luck getting mileage from my double pumper. Its just not the nature of the beast. I jetted the primaries down so low it almost became a plug cleaner!

 

Gee. Those old 800 CFM Rochesters are beginning to look pretty good right about now, eh? Pull one off an old 455 factory setup and you should have an 800 unit.

 

I agree!!! What he said

 

Yeah yeah, I know I shouldn't be bitchin' about my mileage running a Holley, but who can blame me? In the words of my friend (in response to my mileage complaints) who is a hardcore drag racer....."welcome to the world of race cars."

 

I'm no carb expert, but I've never seen anybody brag about their great gas milage with their double pumper. A more traditional holley carb will provide equal performance in *most* cases, and can also provide better milage.

 

My suggestion is get a 3310 750 vacuum secondary for your setup. I used one of these with really good mileage on a 350. I have since gone to a 750 double pumper. This double pumper is killing my mileage but I like the performance advantage over the 3310. Since I don't drive it everyday I'm not too worried about it. If it were my everyday car I'd jerk the DP off in a hurry and put on the VS 3310. I don't know what your 1/4 mile times are but a ran a 12.36 with a 1.64 60' with my 3310. The 60' shows that there is no hesitation at all. This is just my experience.

 

Well, I'm going to rebuild my double pumper and tune it for best track MPH. After that, maybe I'll try swapping over a vacuum secondary Holley. Do you really think that will make a big difference in gas mileage? Also, are the double pumper and vacuum secondary jets interchangeable? Thanks.

 

I have a 81Z with no overdrive(4-speed) and a 205/216@.050 cam and 3.42s.....with my 650 Double Pumper I would get 11 miles to the gallon, switched to the 3310 750 Vaccum Secondary and I get 13-14 mpg...car is much more responsive when acc. from cruising but I prolly am losing something in the 1/4 mile..I'm gonna go to the track and test em back to back to find out.

Another thing I am thinking about doing is using the Double Pumper except with the Vaccum Secondary main metering block on it...the Idle Feed Restrictions are noticablly smaller in the VS carb cuase the DP is used for race like engines with big loopy cams that need a lot of fuel at idle and my engine was running way too rich at idle no matter how I adjusted the idle mixture screws...then I'm gonna get a tight spring and put it on the secondarys to make sure I am not opening them up just cruising around...trying to get the best of both worlds

 

Damn, and I was complaining about only getting 14 - 15mpg in the city with my new motor. I used to get 19 - 20 with the stock L98 350 (after the headers and six-speed swap). You carb guys have it rough!

 

Well I just talked to Air Flow Research and Demon about my problems. AFR said to go with a Speed Demon 650 with mechanical secondaries. Demon said to go with either the 650 Mighty Demon with mechanical secondaries or the 650 Speed Demon with mechanical secondaries. The difference between those two is that the Mighty Demon has no provisions for a choke. I only drive my car during spring and summer up here in northern IL, but I still don't see how my car could survive without a choke. Would going to one of these Demons help my mileage at all? Thanks again.

 

You have a few things working against ya.
large(er) 195cc heads that have low volosity at part throttle cruise rpm. {Relative to stock heads (165cc)}

A air gap manifold that has a non heated plenum.
a manifold with a heated plenum would vapourize the fuel at part throttle much better and allow leaner jetting.

A double pumper which has a "rich" idle circuit designed for
the typical race car/ motor they get bolted to.
You could lean out the idle circuit ( idle feed restriction/ air bleed
once you heated the manifold. A hot water bladder stuffed under the air gap would help and you could shut it off with a vacuum valve for racing. Hooking up a PCV system will lean the mixture a little too.
A camshaft that has too much duration and overlap to be efficient at cruise rpm. A milder cam would create more cylinder pressure at part throttle-cruise rpm allowing a lighter foot on the gas.
A cam with about half the duration or 215@.050 in 220ex would really help and not cost you much peak HP.

All these things are calibrated more for horsepower than mileage.
You make no mention of vacuum advance. Very important.
Should add between 10 to 20deg of additional PORTED VACUUM advance at high vacuum cruise.

I would first get the vacuum advance working. Then try leaning down the carbs primaries from 72 to 66 -68 jets.
then I would try leaning down the idle circuit with a thin piece(s) of wire stuffed in the idle feed restriction hole ( metering block)
A Air/fuel ratio guage will help you there. You won't get it to idle at 14.7:1 but you will get it leaner and get the cruise leaner.
You want full throttle to remain the same. You can correct this later by enlarging the powervalve channel restriction after you find out what smaller primary jet it likes.
Once you get it leaned down for proper efficient cruiseing
the mileage will be much better. It will want a functional choke
for cold starting with this leaner jetting. Useing the choke will help warmup mileage.
When trying different leaner jetting make sure the motor is FULLY WARMED UP. ( it should stumble when cold/cool (with out the choke)

The carb is not to blame. It is doing the job it was calibrated to do. (work well on a race motor out of the box.)

 

what they suggested should help mpg but like everyone has been saying you might lose a little time in the 1/4 mile. you should probably go with the choke but its up to you.