I bought my Holley 750 double pumper from a guy a few miles away. He bought it brand new and never finished the engine it was to go on. He got low on money and sold the engine, but negotiated a lower price and kept the carb. I came along and simply got the carb for a great price.

The 750 has the 4 corner idle with a proform main body and is without a choke. I took the carb apart to make sure there were no shavings or debris in right after i had got it. I also took note what parts were in the carb but i don't have the paper with me right now. I know it has a 6.5 power valve. I believe the primaries were 72 or 73 and the secondaries are 83? I'll make another post with exactly what I have after i get a jet kit and start swapping them.

Im new to carbs and I want to get more mpg's from the double pumper. Ill get a wideband O2 sensor to tune with.

The main question i have is this: Ive heard that you can't just lean the carb out, you must do some mods to the main body and the metering blocks for adjustable air bleeds. Im not completely sure on what to do so thats why Im posting this.

Could someone please go step by step or post a link to a guide on how to do the mods to carb so i can get better fuel economy?

 

MPG and a double pumper are kinda mutually exclusive.

 

your in for some work a better choice for MPG's would be a vacuum secondary as a starting platform. SA Designs has a book out just for the holley (Super Tuning and Modifying Holley's) its a great read to help understand that particular carb. I recomend getting the book before you start otherwise you could be chasing your tail on the jet kit.(you can get the book at Hasting's or Barnes and Noble)

 

If I sell my current carb, I can get a pretty decent bit for it. If i get another carb, I think im getting a quadrajet.. Do you know where I can find one in great condition?

 

http://www.jetchip.com/jet_carburetors.asp

 

Sorry, guys, but those statements are absolute nonsense.

Holleys tend to be calibrated "fat" for general performance usage. Mostly to keep ill-informed users from hurting their engines. Getting better mileage is a matter of tailoring the tune for it. Being a double pumper doesn't make it inherently "bad" for economy.

Start with the basics - ignition. If you don't have a vacuum/mechanical distributor, you'll never get decent mileage. You need vacuum advance, and you need it hooked up. Then, you have to have the timing properly set up. Until you do these things, nothing you do to the carb is going to produce satisfactory results.

The next steps are getting the main jetting proper. A wideband O2 sensor will help immensely with that, but they aren't cheap.

After that is power valve selection. You don't want it opening under cruise, but you want it opening up soon enough to avoid off-idle dead spots.

Idle adjustment is also critical. The reason for that is your idle circuit is supplying a good bit of the mixture under cruise conditions. About all you can do with a "standard" double pumper is adjust the throttle blades properly (see the Holley tuning sticky) and set the idle mixture screws from there. 4 corner idle is a plus here (all Holley 4 bbls have idle circuits on all corners, but many only have adjustable mixture screws on the primaries).

To get much farther than that requires fussing with things such as idle and high speed air bleeds, idle feed restrictors, and power valve restrictors. All of these are "set" with the typical universal Holley, although the idle and PV restrictors, as well as emulsion bleeds, are replaceable with billet metering blocks.

You can get your Holley to be much more efficient with these basic adjustments for less money (including the wideband O2) than a quality q-jet will cost you. But, to really get down to the gnat's hairs will start to cost you, regardless of which route you go.

Have you considered EFI?

 

http://www.nationalcarburetors.com/

http://www.bob2000.com/carb.htm

http://www.viragotech.com/fixit/Fuel...encyPower.html

 

Sorry, guys, but those statements are absolute nonsense.
UM even Holley recomends the vac secondaries for fuel efficency.
now you got me confused

 

Ignorant statements like Duckmans are why I don't come around here much anymore. I have to put up with it too much at work.

It doesn't make any difference In MPG whether you have a Vac Sec carb or a DP. If you are concerned about MPG then you are definitely not gonna be into the secondaries at all, so its a moot point.

Listen to 57s advice he is 100% correct.

By the way I get 24 mpg hwy and about 19 around town with my 383, 850dp, properly setup ignition system, a t-5 and 3:23 gears. How? I keep my damn foot out of it. I can drive like a maniac and get less than 10mpg, but why would I.

Seth

 

Ignorant statements like Duckmans are why I don't come around here much anymore.

I'm not Ignorant, stupid or any other thought that comes to your mind, I have dealt directly with Holley and they have recomended thier vac secondaries for MPG's over thier DP's. Please keep your hater comments to yourself as they are not helping the O/P in any way.

 

You ARE coming across as ignorant by telling the OP that he should switch to a VS (carb for a tow truck) to get better MPG. That is "complete nonsense" because there is not one difference in the primary side of a DP and the Primary side of a Vac sec carb if they are tuned the same, and like I said if you are concerned about MPG then you are staying out of the secondaries so they are out of the equation.

Holley recommends a whole carb to you because they are trying to make a bigger sale than selling you a handfull of jets, and probably figure that you don't know the first thing about tuning a carb so you should just swap it.

OP - here is a decent article on what you're trying to accomplish.
http://www.chevyhiperformance.com/te...age/index.html

 

(carb for a tow truck)

OK what post did you get this information from? cause its not in any of the above except yours.
yet I will grant you the good information link.

 

All I am saying is that a VS carb is more suitable for a application like a truck not a camaro. Since the op has a 383 then I would assume he is after a more performance oriented carb, so since the primaries are what we are trying to stay in when we are after best MPG and the primary sides of the 2 carbs are the same then the obvious choice for performance would be the DP. Do you understand a little better now?

 

Come on guys.. lets all get along here!

I realized that the proform main body does have the replaceable air bleeds from the factory. I had to take the car a few hundred miles yesterday. I tuned the idle screws the best I could with a vacuum guage until I had it pretty decent. I screwed it back out a tiny bit so it wouldnt run too lean after I test drove it. I had a problem where it just didnt want to stay idling until the temp was all the way to opearing temp. The little extra cured that.

I finally gave up on the advance canister and did something on my own. I took an eraser and stuck in between the rod and counted how much vacuum the total was giving me. Im now getting 12 vacuum advance and 38 degrees mechanical so I have 50 part cruise. The car drives fine except for bucking forward but its always done this since the manual swap. Ive noticed that the springs that are the carb gives me about 23 inital and about 38 total?? Ill check again tomorrow. The engine isnt hard to turn over now, but when i do really get on the gas, the car chugs alone. It doesent want to pull like it used too. I guess I need more tuning.

 

Come on guys.. lets all get along here!

yeah your right and I appologize for having a heated disscussion with Seth on your thread, were all just one big happy family. Now if I read your last post correctly, then you backed your timing down to 38* total on the mechanical side and have about 12* worth of vacuum, giving a grand total of 50* during cruise. yet now your running a bit sluggish. what are your cam specs that give the effect of needing more timing? I know your trying to get more mpg's but it sounds like you have one heck of a 383. what mpg's are you wanting to reach? and do you still wish to maintain max power? Both should be atainable just need to tune all aspects,the wideband will help a lot.

 

Here are some timing notes I keep handy that have served me well, long as I member to subtract the initial.


"Most V8's run best at WOT, with a total timing of 30-36*, all in at 2500 RPM, or less. The easiest way to determine your total advance is to use a dialback timing light. You simply connect the light, plug your vacuum advance, and have a second person slowly rev the engine. With the dial back feature, you adjust the light to keep the timing mark in sight as it rises. When the timing mark stops moving, you hold the RPM's steady, adjust the dial until the balancer mark lines up with the 0 on the timing tab, and read your total advance off the dial.

To do this with a conventional timing light, you need to make a 30* mark on your balancer. With a 6 3/4" in diameter balancer. Circumference (360*) of a circle is pi(3.14) X diameter. 6.75 X 3.14 = 21.195"/12 = 1.76" (30*). Looking at the engine from the front, measure exactly 1 3/4" clockwise around the balancer, and make a second mark. This is your 30* mark. Connect up your timing light, and watch your 30* mark as you increase the RPM's. At some point, your 30* mark will stop rising, and move no higher. This is the RPM, where all of your mechanical advance is in.

[SUBTRAC INITIAL OF 10-12*]

At this same RPM, with the distributor loose, adjust it so that your 30* mark lines up with the 0 on the timing tab. You now have 30* of total timing. Line it up with the 2, 32* total, ect.

Keep in mind that a stock distributor usually has stiff springs in it, that don't allow full advance in until 4000 RPM or more. For best performance, you want your advance in at 2500 RPM, or before. The easiest way to do this is to purchase a Crane adjustable vacuum advance kit. It comes with 3 sets of springs to allow your advance in as early as 1600 RPM, or as late as 3200 RPM, or anything in between. For points distributors (Jegs part # 270-99601-1,) (GM HEI, 270-99600-1). What I did was purchase the kit, and install the lightest springs(2 yellow). I used these springs to adjust my total timing, that way, I didn't have to rev the motor very high to see my total. Afterward, I installed the springs that brought my advance in at 2200 (2 silver)

One important note is to make sure the timing is returning to the initial setting, when the engine is idling. So when setting your timing, pay attention to when the advance starts, as well as when it is fully in. Having the distributor in the advance curve, at idle speeds, can cause excessive rpm drop with an automatic trans, with some camshaft/converter combinations. Generally, the more agressive a camshaft you use, the more important this will be. Advance curves should generally start at around 1000- 1200 rpm, when your idle speed is around 800 in Park."

 

good info from GUMBY, would like to see mechanical all in at 2800-3000 and total of 34- 38* without vacuum.
a spring kit from crane costs about 15 bucks, an adjustable vacuum advance can maybe 20. best 35 0r so bucks you will ever spend if you want a really good running engine!
try one blue and one silver spring to get mechanical advance all in at 2800-3000 rpm
to get your mechanical advance set; first plug the vacuum advance, set mechanical advance to 34* and make a full throttle run in high gear. if you hear detonation, back off 2 degrees, if not, advance 2 degrees. keep advancing until you hear the "rattle" then back off 2 degrees. re-attach vacuum advance line
for vacuum advance remember that the adjustment is backwards, ie; screwing in the allen key decreases total advance. find a steep hill and approach it in high gear at low rpm. if you hear the rattle turn the adjustment in 2 turns at a time until it goes away then 1 additional turn. if you don't hear it, turn it out 2 turns at a time until you do, then in one turn.
the hotter the day the more detonation prone your engine will be, so a really hot day is best for an all year setup. also keep in mind that adjustments to your idle mixture and jetting will affect your timing settings.
i'm sure 57 kid will chime in if i screwed any of this up. he is a very knowledgable and helpful poster on this site.
these recommendations will put you on the ragged edge of detonation and should be approached with caution!

 

A side not that just hit me, if you have a MSD box, they need the regular old cheap timing light to read right from the multi spark of the box.