The holley site says 2" mercury less than idle. That being said, the conversion from inches of water column to inches of mercury is what? I have 14" WC at idle in gear. There is a 6.5" in my carb right now. Do I need to go lower? Basically, I have a "dead zone" between 700RPM (my idle speed in gear) and 1000RPM. Engine responce above 1000RPM is very crisp and strong, but off idle to 1000 seems to be not as sharp. I was told going to a different PV could help this.

1 atmosphere = approx 14.7 psi = approx 32' of water = approx 30" of Hg

You don't have 14" WC of vacuum idling in gear, I hope. Engine vacuum is usually measured in mercury. If you did have 14" WC Of vacuum, that would be just over 1" of mercury.

If you have 14" idling in gear, you need to raise your power valve number. That number is the vacuum at which the power valves closes. In other words, you're sitting there idling, the PV is closed; you give it a little gas, the vacuum drops a little, say to 10"; the PV stays closed; the engine wants more fuel but doesn't get it; it stumbles. You have to REALLY open the throttle to get the vacuum low enough to open the PV without the flat spot.

Try a 9.5. You should be able to lean the jets. If it helps, try a 10.5, and lean the jets out another notch or 2.

The way to calibrate a Holley main system for the street is to drive down the road at a fairly high speed, say 50-60 mph, in high gear; and see how the engine runs. Ignore how it accelerates for the purposes of this step. If it sems to run OK, lean the jets 3 sizes. See if it still cruises OK. If so, drop them 3 sizes more. Eventually you'll get to a point where the car surges driving down the road, which is where it feels like it's stumbling or stuttering or runs unevenly in little bursts, especially as it starts to go up an incline. When you reach that point, go back up 2 sizes, and see how it does. If it runs smooth in high gear without surging while climbing the largest hill you customarily drive up, it's good; if not, go back up 2 more sizes. (1 jet size is almost imperceptible; 2 sizes is a fine step; 3 sizes is an average step; 4 sizes is a coarse step; 5 sizes is huge.) Once you get the jet size right, adjust the PV to the lowest number that dosen't give you a sag or a flat spot when you open the throttles slightly during the same test. Start with the value calculated by the 2" less than idling in gear method, it usually gets you close. You'll probably need to re-adjust the idle speed and mixture after doing this. Adjust idle mixture for highest idle vacuum. You may have to drop it one or 2 numbers from there, if the cruise vacuum in high gear ishigher than idle vacuum; definitely run a vacuum hose inside the car so you can watch the vacuum as you drive. That will tell you alot about what the car (gears, converter, weight, driver habits, etc. etc. etc.) makes the engine do.

You'll usually find that you need a MUCH higher PV number than Holley typically calls for in a "performance" carb. Holley expects every car to be set up with far too much cam, and therefore to have way too low idle vacuum, and calibrates their carbs accordingly. If you've got 14" of vacuum idling in gear, then you've got very little more than a stock cam, and their calibration is WAY off.

Incidentally, I've seen people pick up as much as 8-10 mpg when I've re-calibrated their carb like that, compared to the out-of-the-box setup they put in some of them; along with better driveability, longer plug life, less fuel in their oil, and better emissions. The car will even sound happier in the exhaust; it will sound crisp and responsive, especially near and just off idle, instead of that usual Holley loaded-up kind of sound.

Wow, lots of good info.

I've got a 670 Street Avenger.

65/68 jetting. I have no idea what that means. Is that high for a 300-350HP application?

I'm definately going to get a jet kit and rework the carb, I'm getting like 10MPG....so it sounds like this should help me out.

You need to read the Holley information more carefully. What you quoted was for a race/competition engine with a long duration high overlap cam.

For you, they say:

"A stock engine, or one that is only mildly built for street use, will have high manifold vacuum at idle speeds. To determine the correct power valve the vehicle should be driven at various steady speeds and vacuum readings taken. The power valve selected should have an opening point about 2" Hg below the lowest steady speed engine vacuum observed. "

http://www.holley.com/HiOctn/TechSer...o/TI-222A.html

Well driving around my vacuum normally hung anywhere from 14"-17". 17" being 60 MPH cruise on the highway, with fluctuations from 14"-15" in city speeds. So I am looking around 10"-12" power valve instead of the 6.5" currently...which should hopefully sharpen up my engine response? Why on earth holley puts such a low PV in a stock carb.

Will the PV change help fuel milage?

Also, I was looking in summit and the jet kit starts at 64 and goes up. My carb has 65 primary jets and 68 secondary jets. So how much leaner can I go on primaries to get better gas milage? It seems like I'm already at the bottom. The secondarys don't really open under normal driving....I've got 3.23 gears. so even doing 70MPH on the highway I'm not taching high enough for the secondaries to open.

I'm getting around 10-11MPG right now in the city...not sure how much more I can expect to get out of the engine...but I was hoping to sqeeze like 13-14MPG out of it. For *** sake, my bosses 440 big block gets 11MPG and it doesn't even have OD.

Ignore the numbers on the jets, as far as trying to predict what you need. Don't worry about how "low" it "seems" they are. Just give the engine what it wants.

If the "kit" doesn't include smaller jets, then you'll have to buy pairs of them individually. Keep in mind, the "kit" is chosen tp include the parts that would be needed by people who are putting too large of a carb on too small of an engine, that being the way to get the most power; it's not necessarily chosen to fine-tune driveability. The "too much carb" scenario will require larger jets than what comes in a carb, usually.

The reason you have to jet it leaner is because you have a carb that's too small for the engine; therby producing a larger-than-expected "signal" at the venturi. In other words, your 305-sized carb thinks that all that air flowing through it, means that your foot is on the floor; when in reality it's just because you have a larger engine than the carb was designed for. One of the most amusing myths in all of carb tuning is that if you take a given carb and put it on a smaller motor, you arbitrarily lean the jets, and if you put it on a larger motor, you richen them That's backwards.

I told you why Holley does what they do. You now understand what the problem is, and what the cure is.

I can't predict what your gas mileage will be. I can however predict that if you calibrate the carb properly to match what the engine wants, that it will improve.

The PV change, by otself, will not help fuel mileage; in fact, it would hurt it if anything, by causing power enrichment to occur more of the time. Where you will pick up mileage, is in not having jets that are too large, to cover up a PV opening point that's too far removed from cruise and idle vaccum.

Go try it. We can talk about it all day, but talk won't help your car.

Ahhh, I got ya. The idea of going 670 instead of 750 was I would get better responce down low. But the result is the stronger signal is pulling more fuel....sounds logical to me. So there are smaller jets than 64 I can go and buy? Any suggestion to what I should start with? Drop from 65 on primaries to 62 and work from there? Hopefully holley sells a kit of smaller jet sizes.

I don't know if they sell a kit like that; but they sell jets well down into the 50s. The spreadbore carbs like the 6210 end up with sizes like that sometimes.

If you have 65s now, get yourself some 59s and some 62s; see where that takes you; then buy more if you find that you need to go farther, or others in between, or whatever.

You'll probably find that something like a 59 or 60 jet and a 10.5 PV will be about right. I seem to recall ending up with a combo about like that more than once before.

This has been a very informative thread for me.

I didn't start it, but found it with a search.

Thanx to all for the good info.

Got my carb running good yesterday. It was running real rich. Backed the idle screws down to 3/4 turn out, and installed a 8.5" powervalve (I need to get a 9.5 or 10 but 8.5 is all I could find without ordering one). I also had my floats set wrong, I had them at the middle of the sight plug, and set them so that I can barely see the fuel at the bottom of the plugs. Now my vac has jumped from 9-10" to a good solid 12.

opps double post

Hey guys I have a holley 700cfm double pumper. How many jets does this carb have? Is it 2 or 4? What size are the jets that come in this carb?

Look on your carb at the circled place and get that number, the "list"" number; then go here http://www.holley.com/HiOctn/ProdLine/CarbList.pdf and look it up.

holy crap! this thread just saved me a TON of money. i still know jack squat about holley's, but it makes me a little more confident in tuning mine! 4150 600 vs on a 305. i need to get a vaccum gauge so i can start to really dial this thing in. so far i am getting roughly 20-22 mpg on the highway. i would like to get the best driveability possible.

Hey RB83L69,

I looked the number up on my carb and it was 4778-8 or so it says but when i look at that graph it doesnt show one with an 8 but it does have one with a 3. Could it just be a misprint or do i have a freak carb. Gosh I hope not lol.

But anyways here are some specs on my carb.

Primary jet=122-69
Sec. Jet=122-78
Primary powervalve=125-65
primary discharge nozzle=.028

Do i follow the same steps in jetting my carb down or do I go differently? My primary jet is 69 what size jet should I go down too? my secondary is a 78 also what size jet should I start with?