ok, im pretty sure its the powervalve i need to change, but im not sure exactly how to choose the correct one.

right now, the motor is heavily OVER cammed. i know this.
i jsut dont have the time to swap cams yet.

carb is a holley 750 VC

i stepped up from my 72 baseline to a 75... it was kinda expected since the larger cam severly dropps the vac signal.

under mild accleration, where im still on the primarys, its smooth, but when cruising, aka maintaining speed, its a lil jerky... its giving me the distinct impression its lean.

my understanding is its the powervalve that richens the mix under part throttle cruise...
but i dont know the proper method for choosing one.

ive read online some about it.. and that i should be able to use a vac gauge to tell me... however, i dont know when i should be noting the vac gauge reading....

cam is huge so vac is always low... matter of fact, until i can get a electric vac pump, im relying on reving and downshifting to get enough vac for the power brakes (not the smartest thing in the world, i know)

when should i look at the gauge/what should i be doing? and how do i use that number to select a power valve?

Don't quote me but i think you take your reading at 55 60 steady mph and half of what the gauge says should be the size of the power valve.

Not sure though.

Well your half Right Bobbyz. You actually take the vaccum reading at idle and then choose a power valve that is half of the reading.

The power valve should not be open when cruising. You should get the cruise mixture right with jets and idle mixture adjustment.

What is this "huge" cam?

Is this an auto or stick?

What is your idle speed and vacuum (both in and out of gear if it's an auto)?

Have you set the secondary idle adjustment such that the primary throttles are near closed at idle, at least closed enough to mostly clear the transition slot?

so in my case, i need a 4.5 powervalve like holley number 125-45

ugh.

i hate this.

theres so much conflicting info.



all i know is, im horribly lean under part throttle cruise.

sorry. i missed your reply when i posted...

cam is solid roller 255/262@.050 just under .600 lift with a 114LSA.

manual trans
"idle" is around 1000. technicly the tach reads 1050, but my stock tach is highly inaccurate
vac is around 9"

the throttle blades are as closed as much as i can. amazingly, it does seem to still be on the idle circuit. i can make it pig rich or lean with the idle mix screws... currently they're 2.5 turns out. on both sides.

i havent even gotten this far yet, but im also lean at WOT, and the secondarys arnt opening all the way.

You need to richen up the jets. Tune for smooth cruise and proper WOT mixture.

Choose the PV to keep it from opening under cruise, as RB said. A 4.5 may work, but if you have off-idle stumble (after you get the jetting correct), go with a higher one.

Why do you say your secondaries aren't opening all the way? How do you know? I assume you meant to say it's a "VS" carb.

yea, i dont know where VC came from, its vac secondaries.

i already richened up the main jets.. they're at 75 now... any richer didnt seem to help.. i could probly put 74s in and it would act the same..

as far as the secondarys opening... well, i suspected it, so i took the aircleaner off (this is before i bolted my hood back on) and drove the car. i could see the linkage.


i'll try jumping the jets up until it solves the lean surge during cruise...
but how do i enrich the back? theres no metering block. no jets. i pulled the fuel bowl, and theres a flat piece of sheetmetal back there.(i didnt take it off, i just put the bowl back on and searched online.. came up with nothing) theres no jets..

You've got a 4160 model, which has a secondary metering plate instead of jets. You can change the plate, but it's easier to convert to 4150 model. This will allow you to change jets just like the primaries, but will also increase the distance between the bowl inlets - so you may need a different fuel line, depending upon what you've got now.

ok, so i suppose this is what i need

are the fuel inlets to my carb regular double flare line? right now i have one that was with the carb when i bought it, but its not flexible, so it wont work when the spacing is wider.

what jets should i stick in the plate to start from?
this should work better.. i can swap jets.. lol.
most of my carb tuning experiance was with motorcycles and other small engines. they only have jets.. (well most of them)

Most people say about 6-8 jet sizes higher than your primaries, I don't think anyone is going to be able to guess exactly what you need,

but then again i'm no expert.

well, i hope thats the right part number.... im going to go order it now at a local store, and they'll have it by the time i get out of work (that way i dont have to drive across town )

im also getting a 5" powervalve and a spring assortment... and i'll go from there.

https://www.thirdgen.org/techbb2/sho...hreadid=244930

https://www.thirdgen.org/techbb2/sho...hreadid=250118

A couple of threads where I've passed along general Holley tuning poop...

Your cam doesn't seem too terribly excessive to me. Get the carb and ignition curves calibrated right, it shoudl be just fine.

i have a crane ignition recurve kit... i just didnt get a chance to put it in last weekend.

thanks for the links. lol, the first one i printed out yesterday..
but to be honest, your method is diffrent then all the others out there... makes sence though.. lean the carb as much as you can, and then use the powervalve for enrichment under power.

Yeah; as I posted in one of those threads, Holley "performance" carbs are built assuming that they are too big for the motor (threfore requiring richer jetting), the motor has too much cam (therefore requiring richer jetting), it's in front of a stock converter (therefore requiring a low-number power valve, and therefore requiring richer jetting), and that people will complain about cold-weather start-up and driveability problems (therefore requiring ..... you guessed it ..... richer jetting). So when you buy one, you automatically get jets that are too rich, a power valve with too low a closing point, a fat idle circuit, etc. etc. That creates the typical gallons/mile fuel consumption, blows out black smoke at idle, tip-in sag, have to stomp it to get power at which point it seems to suddenly "turn on", fouls plugs every 2 weeks, etc. etc. etc. In a word, all the familiar out-of-the-box Holley characteristics.

If you just forget everything you ever heard in the McDonalds parking lot about Holley carbs, and go learn how one actually works, maybe read Alex Walordy's book on Holleys, it all becomes crystal clear. Problem is, too few people have done any of that, and so we hear the same Friday night McDonalds parking lot monkey-spank over and over and over, and nobody ever learns to properly set up a Holley for the street.

at the mickyDs here, noone knows what a holley looks like.
but they do have some kickass wings. (not chicken)

Doesn't some division of Holley make coffee-can fart pipes? They might all be thinking that a Holley is a horsepower tripling exhaust tip, to go with their look-like-*** wings.

This seems to fit in here... I was asked for a rebuild kit for a carb today for "... an aftermarket carb on my '79 Camaro..."

I asked if it was a Holley, and the guy says "Who's she? Should I talk to her about that?"

Ugh.....

At checker last week it took the guy behind the counter 3 tries to get a me a 4bbl square bore carburetor gasket.

... a great example of why parts stores like the one I work at have such a reputation of incompetence

2 weeks ago.

i call advance auto because i KNOW they stock holley parts and they're on my way home from work.

i ask for a holley trick kit.. i even give the part number.

i get a, "we dont carry it"

i say (nicely using diffrent words) bull****. look on the shelf behind you.

they say its not there. they dont carry it.

i ask to talk to the manager. he gets on the phone, and i give him the part number. he also says they dont have it.

i nicely remind him of the huge banner behind him saying how they carry the entire holley line.

he repeats he doesnt have it. and tells me "oh, we only carry most..." or some BS line like that.

disgusted, i call another place, drive out of my way and pick it up there.


fast forward to last thursday. im in there picking up somthing small and i look behind them.
sure enough
on the shelf
is not one
not two
but FIVE boxes. all say holley trick kit on them. big ****ing letters.
3 of the 5 are the one i want. including part number

but i goto the PC and it doesnt come up... ahhhh, thats how come you couldnt find it.
because you carry the ENTIRE HOLLEY FU*KING LINE AND YOU KNOW IT! BUT YOU'RE TOO DAMN LAZY OR STUPID TO JUST LOOK SOMEPLACE BESIDEDS THE COMPUTER..



bastads.

i worked for one summer at advance auto parts (same one im bitching about above)


i fondly recall one day where i had a line of about 7 customers, all of them regulars. when the other two parts people called next, none of them moved...

when the manager asked them why they wernt helping anyone, they said because the customers were waiting on me.

he turns around and asks me why im holding up customers...

and one of the guys in the line just grabs the managers attention and says somthing to the effect of, "because hes gives us the right parts", and the 7 other people in line agreed.


made me feel good...
still didnt get a raise.

There was an old workplace "cartoon" with Charlie Brown that said, "Doing a good job around here is like wetting your dark-colored pants - it gives you a warm feeling, but nobody notices."

How's the carb doing?

i have a wierd surge under light cruise.

i have yet to figure it out.
its still there when i take the vac advance off.
its gone under anything other then a very light cruise. if i go up a bridge (no hills around here) or acclerate or decel, it goes away.

its worse when i cruise at about 35.

still donno what it is. im basicly ignoring it right now.

Main jets too lean. It's going away when the PV kicks in.

yea.

i know it seems that obvious... just rejet it a lil richer right?

i tried that.

it needed a 80somthing to stop it. and was pig rich everywhere else.

somthings dirty or wrong.... i donno.

i tried and tried to find a vac leak.. i even pulled the intake, and reinstalled it.
behaved exactly the same.

im pretty confident its somthing in the carb.

Might be time to mess with the air bleeds.

(Also about the point where I'm starting to get over my head...)

Personally, I've always jetted mine where it runs best at the track. On the street/freeway, it doesn't behave quite as well. I was having to get into the PV in order to stop the surging, but the plugs would be rich on the street and okay on the track. It was then I was reminded by a TGO member that the Proform main body doesn't provide vacuum to the timed port on the metering block. It's much better now that I'm using manifold vacuum, and I actually richened the primary jets and leaned the secondaries to maintain the balance at the track. And, the plugs stay cleaner on the street. Still has a slight surge, though. Since I'm more concerned about strip performance, I haven't played with it beyond that (have other automotive repairs in the family job jar needing attention, too).

honestly, i dont know what the air bleeds are. where they are, how they work, or what they do.

ive gotten curious before and searched online, but it seems to be one of thoes things that people want to sell you a service or book to do... i havent found any real info on it.

if nothing else, its keeping me very motivated to get the EFI on. heh.

I personally wouldn't mess with jetting to start out, although I know people who swear by it.

Take the carb off the car, turn it over and look at the primary throttle blades. Be sure the little transition slot is not too far uncovered.

While the carb is still off the car, hold the throttle plates all the way open (like your flooring it) and make sure your accelerator pump linkage has a gap of no less than .015.

Put the carb back on the car and then turn the mixture screws in until they lightly seat, then back them off 1.5 turns.

Start the car, check float levels to be sure they are proper, warm it up completely, affix an accurate vacuum gauge to a manifold vacuum port then adjust idle to the proper speed. Then adjust mixture screws for the highest vacuum reading possible. You may have to mess with both the idle speed and mixture screws to accomplish this.

Now take the vacuum reading at idle in gear and at cruising speed while driving. Note the lowest reading you get (do not count acceleration readings!). The power valve you should start with would be 2 numbers down.

with the exception of the accel pump linkage check, thats exactly what i did.
when i first drove it, it was so lean under light cruise that i couldnt drive it down the street and back. it was bucking like it was in the rodeo. lol

I'm kind of late to the party...but you identified the problem in your first post. That cam is simply too big for street use, even on a 400 ci motor (Quote: Cam is solid roller 255/262@.050 just under .600 lift with a 114LSA. ). You know this of course.


What is happening is that you are not getting enough " Booster " signal on the Primary side. Booster signal is the most critical element of any Carburator ( Check out articles by Doug Roe and David Vizard ). Anything that can be done to increase Booster signal will increase drivabilty.

Holley's for the most part are what we call two circuit carbs...exception being Dominator series and some HP models, which are three circuit carbs.

3 circuit carbs have a separate idle circuit, a separate " Low" speed circuit that incorporates the Transition circuit and a separate " Low" speed circuit, and finally the third circuit, the main circuit. (We are talking Primary side circuitry). A 2 circuit carb combines the idle and Transition circuits together as one circuit and then you have the main circuit. It is purely a cost saving measure. A 3 circuit carb will deliver a much more accurate fuel curve, particularily on the low and mid-range.

When you put in too large of a carb or too large of a cam...you kill the Booster signal. Because of the low Booster signal caused by the larger cam, the mains are not coming in fast enough. What can be done to correct this?

Change the cam...obvious answer...so lets go to the less obvious.

1: Change Primary Boosters to Annular boosters. This is perhaps the best mod that you can do to ANY Holley carb.

Holley uses three main types of Boosters. In order of booster performance they are:

(A) Straight leg...Cheapest to produce...so guess how they perform? Yep...they suck. Least efficient design.

( B) Dog Leg . Used on higher cost Holley models, such as the HP series. More efficient than Dog Leg. Pulls a higher Booster siganl at same airflow speed.

Interestingly the early 3310-1 series of 750VS carbs ( actually rated at 780CFM because of less restrictive booster design ) cam with "Dog Leg" boosters. All other series of 3310-X came with straight leg boosters.

(C) Annular boosters....now you're getting into the good stuff. Annular boosters provide a much, much stronger signal than any other type of booster. Atomization is also far superior. They cost more to produce so you only see them on " Top of the Line " models.

I would recommend sending your carb to an " Experienced" carb shop and getting Annular Boosters installed . You will not believe the difference in driveablity and throttle response. As an added bonus, fuel milage can be increased as Accelerator Pump shot can be reduced ( Holley's use a big pump shot to cover the " Hole " in the fuel curve left by the absence of the third circuit ) and main jetting can usually be reduced two to three sizes because of the superior atomization of the Annular boosters. The " Carb Shop " has what they call " Super Boosters "....these are superior to Holley's Annular design. They would be my #1 choice. " TMP Carbs" can also install Annular Boosters. They can also convert your carb to use " Weber Power Plates....Full 3 circuit design using all Weber Components. This is what I have. Check out this article: Engine Masters


2:Increase Booster signal by other means. Several ways to do this. Some easy...some hard.

What type of manifold are you running? Dual plane manifold will give a stronger signal than a Single plane.....but you may not want to do this if you plan on running at the track.If you are running and open spacer under the carb...switch to a four hole spacer. This will increase Booster signal slightly.

3:Remove choke assembly and install a K&N stub stack...this will increase Booster signal slightly and also increase CFM capacity of carb by about 20 to 25CFM.

4: Increase airflow through Primary venturi. Thinning throttle shafts and installing low profile " Button Head " screws help a little. Can gain 10 to 15 CFM and icrease Booster signal slightly.

Really, with that cam, you are going to have a SOB time getting it to run decently on the street....But Annular boosters , and a K&N Stub Stack should help considerably. Good mod's even if you do install a smaller cam in the future. Good Luck.