I want to ask everyone's opinion about how to properly size a carburator.
I also would like to know what are the effects of having one too large (CFM wise), for example if an engine is deemed that a 600 cfm is what is required what would happen if a 750 cfm is installed.
This being said, if it won't hurt anything being a little over sized can the 750 be jetted to properly feed the correct amount of fuel?

Street driven, not race.

Thanks,

iirc,,, holley sizes/calibrates a carb at 1.5 inch vacuum at max rpm. most say that max rpm power comes in at arround .5 if vacuum or a slightly less.

as you make the carb bigger than it needs to be,, at low engine speeds,, the vacuum signal/air speed through the venturi's gets weaker,, till the rpms come up,, in where it won't want to draw fuel as easily/early in the rpm band. richer jetting will be needed to get the carb to start flowing fuel through the jets/pump shot. big carbs on a small engine need to be enrichened in the main jet and pump shot,, contrary to what the mind would think. although the idle circuit can be trimmed down with the idle mixture screws,, the transition slot/circuit may be slightly rich with a bigger carb and the lower vacuum at part throttle opening bringing in the power valve earlier from lower part throttle vacuum with the bigger carb.

i've used the holley box math for years. cubic inch x max rpm / 3456 x v/e = cfm required. some will say that it's very conservative. if v/e is correct,, like it's a number aqquired from a dyno,, you can get much closer to the "right" carb size with less guess work. knowing true v/e is the key if you're not wanting to try a mess of carbs to find out,, and tune each to the engine. for a street engine,, the holley box math gives a good carb signal for a heavier car,, and that will increase milage. hope it helps. andy

If using entirely on the street, a carb that is sized to the small side of requirements will give you better throttle response, economy, and better overall drivability. Holley has a requirement calculator and Wallace is always one of my go to's when calculating automotive stuff.

http://holley.com/TechService/
http://wallaceracing.com/Calculators.htm

kik,, picture a sand blaster gun/nozzle. it has a air entry point in the gun that releases the sand into a chamber where it has a step up in size,, which lowers the pressure inside the gun,, in that chamber. now change the sand to fuel. now,, start with 100psi of air pressure and pull the trigger. LOTS of sand/fuel flow out the nozzle.

ok,, now reduce the line pressure to 30psi. not as much sand/fuel. it's the actual air speed past the inner gun chamber/venturi that draws the sand/fuel through the orrifice/sand hose. a too big of a carb has less vacuum depression/air speed past/through the booster venturi,, and therefore draws less sand/fuel,, and needs more engine rpm to pull the same amount of sand/fuel. just an analogy. maybe puts things in perspective a bit. hope it helps. andy

the thing with a carb is that the outside air pressure is pretty much stable,, to an extent. barometric pressure that is. so you can size a carb with a vacuum guage at max rpm to show 1.5 down to .5 of vacuum for performance. up to 3 inches for a roundy round car that likes alot of midrange torque to get off the corners. that would be a restriction on a drag car though,, and really limit top end power.

iirc,,, holley sizes/calibrates a carb at 1.5 inch vacuum at max rpm. most say that max rpm power comes in at arround .5 if vacuum or a slightly less.

as you make the carb bigger than it needs to be,, at low engine speeds,, the vacuum signal/air speed through the venturi's gets weaker,, till the rpms come up,, in where it won't want to draw fuel as easily/early in the rpm band. richer jetting will be needed to get the carb to start flowing fuel through the jets/pump shot. big carbs on a small engine need to be enrichened in the main jet and pump shot,, contrary to what the mind would think. although the idle circuit can be trimmed down with the idle mixture screws,, the transition slot/circuit may be slightly rich with a bigger carb and the lower vacuum at part throttle opening bringing in the power valve earlier from lower part throttle vacuum with the bigger carb.

i've used the holley box math for years. cubic inch x max rpm / 3456 x v/e = cfm required. some will say that it's very conservative. if v/e is correct,, like it's a number aqquired from a dyno,, you can get much closer to the "right" carb size with less guess work. knowing true v/e is the key if you're not wanting to try a mess of carbs to find out,, and tune each to the engine. for a street engine,, the holley box math gives a good carb signal for a heavier car,, and that will increase milage. hope it helps. andy


Wow Rat! Learned more in one post on carbs than I've learned in 40+ years!

Thanks!

Now I've really got some questions...
1. V/E-- I know what it is but would you consider a new engine at 100% or is there any engine that is at 100%?

kik,, i can give a couple of (uneducated) guesses. most engines seem to make about 1 hp/per cu. in. at arround 5500-6000 rpms with arround 9.5-10.0 compression and cam,, all setup for that range,, with stock heads that have a slight bit of work to them. with decent flowing exaust,, etc..... so i'll say that's about the 100% v/e mark for a guess. knocking on that door anyway. so,, if you cam/compression it less,, or run lower quality heads,, the v/e number goes down.

a better head design that makes the power per cu in at a lower rpm would have a better than 100% v/e when comparing to the example above,, at a lower rpm. if you build the engine and components matched to turn higher rpms,, the v/e can increase with shock wave tuning,, to the point where the head,, or something else becomes the choke. the faster moving air speeds through the engine has the ability to create more inertial ram effects/carb signal as you turn it faster.

seems that it's much harder to create that same situation at lower engine speeds with smaller ports/intakes/headers,, etx....,, to gain over 100% v/e,, therefore sizing the carb slightly smaller can compensate for a street engine/heavy car/low numerical gearing/low stall. better signal at the carb.

in my old 70 chevy truck,, the last engine i sold from it,, i ran the 70 LT-1 piston with 487x heads,, pocket ported with stock valves. heads cut .040 with a 230@.050 cam. had 1 1/2" headers with a 2 1/2" exaust. i used a wiand action plus intake with a 500 carter competition series 4bbl carb,, with 2800 converter and 3.73 gears and 295/50/15 tires. the setup started choking off at 5800,, but was a torque monster through the torque peak and above. the carb signal was great. the other trucks arround here used bigger carbs,, headers,, intakes. they never caught me. when they went to aftermarket heads,, they came by me,, but not by much. i've never liked the bigger is better syndrome. i think the good velocity of the smaller carb/intake/headers mixed the fuel better throughout the power band on the street,, with the stock runner heads. i don't think the v/e was over 100% though. just guessing at best. but the guy i sold it to is now using a offy port-o-sonic single plane intake with 1 5/8 headers and a 750d/p carb,, and can't get it to run like my setup did. feels mushy throughout the power band in comparison. also he's got 3.42 gears,,,same stall range. don't know if that answers your question,, but hope it helps. andy

Rat, I've got a 406 with Dart heads and dished KB hyper pistons, 5.7 rod and everything else pretty close to stock. I do have headers and good dual exhaust.

Trying to get a tune up on it with a Holley 750 double pumper with mechanical secondary and manual choke.

I've got a few known issues (incorrect power valve and maybe jets too small) after reading all of this and I'll correct them and then re-group...

I'll be bringing this back to life after I correct them.

Thank you very much for taking the time!

your welcome kik. sorry it was such a mouthfull !!!! maybe have a read at what i posted here about initial setup on an unknown setup,, or starting from scratch. might be of some more help.
http://www.nastyz28.com/forum/showthread.php?t=70315&highlight=butterfly

Wow...People can never say too much when they know what their talking about! right click, select all, copy...saved! Looks like I have some tuning to do.

BTW, is it normal that the throttle blades drilled on new Holley carbs like you explained? I read it and will read again but I think I understand.

kik,, thanks man,,, on drilling the primary butterfly's,, you can always test that symptom by opening the secondary throttles up with the little screw under the passenger side of the rear throttle shaft. it's accessible from the bottom. it's usually pretty tight to get under there with a small pocket screwdriver.

i made another throttle air/curb idle air opening screw/with bracket plate,, with spring behind the screw,,on the passenger side top,, to kind of resemble the driver side. tapped a couple of the holes behind where the choke and or vacuum secondary pod would mount,, if equipped. my carb has neither,, so,, i did that. then i don't have to remove the carb to do it. it's a pain to get to with it on the intake. if it's really alot of opening,, then it'll even cause some bog/lag on the secondary opening. so,, you can effectively move the problem from the front to the rear,, if it's a REALLY low vacuum situation. if not,, in some cases,, it'll work well enough to not have to drill them.

if you open up the secondarys,, it allows you to close down the primarys and keep the same amount of ilde air comming in. the secondary's aren't as finicy about comming off idle in that way (because airspeed is faster when they try to open after 1/2 throttle opening anyway/engine rpm comes up by then usually),, and it gets the primary butterflys back into the transition slots. . i'd doubt if it's the case in your scenerio if it's pretty much stock as far as cam with that 750 carb size.

usually that's the case with a low vacuum situation where a bigger cam/big single plane intake/or like something with narrow lobe centers or the like,, or a way to small carb on a big cube engine. in that it wants more idle air than the carb butteryfly area at idle can provide without opening up the primary's into the transition slots up front.

in those low vacuum situations,, the carb idle,, won't take the air/fuel in,, cuz of reduced idle vacuum/suction,,, so the auto reaction is to open up the front idle air screw to get more air in to make it idle. then some folks automatically add more pump shot to cover up their opening it up out of the transition slots. so then it puts the primary butterflys out of the transition slots,, making low speed cruise and idle a pain to nail down.

the transition slots are trying to pull fuel hard when they're closed up (squared looking from the bottom),, then gradually weaken as you open up the butterfly past the mid to upper part of the slot. easier to say that if you keep the primary's closed up as much as possible and cover it up with the rear,, till you have to drill. once the idle and primary's are sorted,, the secondary's are cake. the extra air speed covers up most of it. good luck with it. andy

all that is to get the lag/fuel pull delay out of it that you can,, before messing with pumpshot.

also,, being that you have a double pumper carb,, to really nail it down,, you can remove the secondary opening linkage that's connected to the primary,, and wire the secondary shut,, for temporary tuning purposes. it keeps the secondary's out of the picture till you're ready to mess with them. then you can wide open it,, just on the primary's to get wide open throttle nailed down through the front power valve restrictions,, and not have the secondary's getting into action at all. otherwise,, the secondary's will start opening just past 2/3's throttle opening of the primary's

Too bad you live so far away! Haha, lol

I think I've got the clues to be on the way for a decent tune up... I've first got to replace the power valves. I checked idle vacuum on a warm engine in drive and was real close to 20. Accordiding to Holley and what I'm reading, I should take that number and divide by two for proper power valve size, right?
I did try to adjust as you instructed and I'm now pretty sure I have to drill some holes in the throttle blades because the only thing that happens when I try to adjust the idle screws is engine RPM goes up a little and never stumbles...(turned all the way in).
I can slow my engine RPM down and try it but it just starts bellowing out smoke (running too rich with all 4 corners turned in almost all the way).

BTW, these things are harder to tune than a S&S Super G! If you need one of those set I know how to while riding down the road! I wish the Holley was as easy. I guess the Edelbrock is but I don't care for them at all...(They don't cost enough) ha! Holley is like Wendy's, "You don't pay more, you get more"! LOL

I have to work this week so won't have much time to mess with it but I hope the weather is good this coming weekend because I want to settle this.

Iron Eagle 200 cc intake runners, straight plug I don't remember the chamber volume. The cam was bought as one that would be suited for a RV or for towing. This will be my work horse pulling my heavy a$$ BB 75 to the track.

i'll be arround this weekend if you have a need to ask anything. be glad to help.

yeah,, if your fuel idle mixture screws won't kill the engine when turning them all the way in,, you're idling with the butterfly's opened up into the transition slots and it's pulling the fuel from there and continuing to run off the transition circuit,, and not the idle discharge circuit.

the actual idle discharge circuit/holes/idle mixture screws in the metering block,, are on the down stream side of the transition slots/circuit so to speak,, in the same fuel flow path,, internally. both circuits are fed by the same idle fuel restriction jet in the metering block though. it's set up by holley so that it allows one to close up the idle fuel circuit with the mixture screws in the sides of the metering blocks and still have transition fuel available upstream. as you tip into the throttle,, the idle circuit becomes unactive as it's transitioning into the transition slot circuit,, then it draws the air through/arround that slot to meter the amount of fuel through the same jet/idle feed restriction in the metering block,, just that the idle circuit becomes inactive when you tip into the throttle on light acceleration,, or open it up at idle,, past it. but both are fed by the same ifr jet.

you might try opening up the secondary's bigtime,, so that you can close down the primarys where the butterfly's are just above the "touching closed" point,, but not touching the bore walls. they can stick a bit if touching. see if that brings the front idle mixture screws active again where they'll kill the engine when turned all the way in,, or almost all the way in. make sure the rear's are out about 1 1/4 - 1 1/2 from lightly seated. you'll effectively make the rear idle mixture screws not active when you open up the rear throttle opening anyway,, from what it sounds like. they'll be in/or above the transition slots back there at the butterfly's.

20 is considerable vacuum!! you shouldn't be opening the power valve till you're a bit into the throttle under a pretty good load if it's a stock power vave. don't think that's your problem just yet though,, UNLESS it backfired and popped it......does your carb have the anti-backfire valves? my older carbs don't have them. most of the newer ones come with them. is there a secondary power valve in yours? if you get a known good power valve in it,, i'd get the idle nailed down first where the idle mixture screws are active,, and where it'll kill the engine when turned all the way in. then you're butterfly's will below the transition slots and in the idle circuit where it should be.

think they come with a 6.5 power valve. yeah,, 1/2 of that 20 your pulling,, like a 10 or a 12 will work,, maybe even take a 14. once you sort the idle,, you can get an acurate cruise vacuum to judge that sizing. just below the lowest cruise vacuum,, so it's not opening at those ranges till you tip into the throttle. a lower number for more economy may call for more pumpshot though.

a higher number will start richening it sooner (if it seems lean or sluggish) in the throttle opening with a decent load on the engine,, as vacuum drops off. like not just stabbing the throttle,, but progressively getting into it fully and firmly. with that much vacuum,, sounds like a fairly small cam with wide lobe centers. good milage setup potential.

thanks Greezer. kik's combo sounds real similar to yours. sounds like his has alot of idle air demand with the cubes and short cam timing/high vacuum.

only time kik. it'll get there. we'll lend a helping hand,, from afar!!

Rat, here is my cam spec.



http://i160.photobucket.com/albums/t193/kik_start/P9170144.jpg


I hope you have a great week!

Thanks,

John

yep,, 114 lobe centers,, short duration,, and a quite a few cubes with some compression. all tend to say high vacuum/air demand. will work well with any low numerical gear ratio. once sorted,, that beast ought to idle at 450-500rpm in gear!!! might be able to read the letters/numbers on the v-belts as they go spinning by!! ha ha!!

yep,, 114 lobe centers,, short duration,, and a quite a few cubes with some compression. all tend to say high vacuum/air demand. will work well with any low numerical gear ratio. once sorted,, that beast ought to idle at 450-500rpm in gear!!! might be able to read the letters/numbers on the v-belts as they go spinning by!! ha ha!!


I had it idling lower than that at one time.

That would be 112* lobe separation on that cam, not 114*, just FYI. Tighter than stock, but not really all that tight by performance standards. It's the Edlebrock Performer cam which is resold by many companies under many different part numbers.

That would be 112* lobe separation on that cam, not 114*, just FYI. Tighter than stock, but not really all that tight by performance standards. It's the Edlebrock Performer cam which is resold by many companies under many different part numbers.

right,, my fault. it is 112.

How do you come up wit 112 center? The box says 107 on the intake and 117 on the exhaust????

add together and divide by 2,,, and that's the lobe separation angles of the camshaft. 107 + 117 = 224 / 2 = 112. the numbers on the box are the exact recommended install positions. for when you degree the intake lobe at max opening centerline point of the intake to 107 crank degrees after tdc,, the exaust should be at a max opening centerline of 117 degrees after bdc.

Seems like the more I know the less I know!

Thanks, that's a whole nother subject...I better solve one thing at a time...now back to the carb,,, can't wait till the weekend.

Seems like the more I know the less I know!

Thanks, that's a whole nother subject...I better solve one thing at a time...now back to the carb,,, can't wait till the weekend.

always something else to learn!! when i quit learning,, i guess i'm ready to take the leap of faith.

OK Rat, I hope your out there...

I tried to do some tuning on the carb and here is what we did.

Started drilling 1/16th holes in front throttle blades (1 each side) and it did help some but still running rich...drilled another in each of the front, problem still there but seemed better because each time we were able to turn the idle screw out some... but could not lean it out enough (idle mixture screws) to quit smoking.
To make a long story short...we ended up drilling more and then could not get RPM's down low enough so....we got to buy some throttle blades. I found some local for 8.00 with new screws. I trashed the old ones and installed the new ones and that's where we're at.
I can't get this thing to run right when at idle??? It smokes and really stinks like un burnt fuel. It runs good going down the road but when I jump into the pedal smokes pretty bad. Jetting is stock with 71 front and 75 in the rear with both power valves being 6.5...

There is a very small set screw (if you turn the carb over that you can see) for adjustment on secondaries, I turned this to see if that would help with getting more air in and it did but still not right.

I think I've given up for a while but will start back next weekend.

kik,, pm sent. that's the screw to open up the secondary's,, to allow you to close down the primarys some. sent a pm with pic link to show the home made bracket/idle air screw i did for the secondary's on mine. check it out. comes in way handy for tuning. don't have to remove the carb to get to it. you'll like it. andy

kik,, close the secondary idle mixture screws all the way for now,, so you can take them out of the loop/picture. disconnect the secondary butterfly opening linkage and remove it,, set it in the toolbox for now. take the secondary's out of the picture completely. if you can get BOTH primary and secondary butterfly's closed up where they are in the idle discharge circuit,, you'll be way better off with the high vacuum. the secondary opening you described might actually be pulling fuel from the transition slots,, so if you can do the idle drilling on the primary butterfly's like before,, but not a bunch of holes,, just one hole in each butterfly that you keep enlarging a 1/32" at a time,, the rear butterfly won't be opened up into the transition slot pulling any fuel. just thoughts. andy

closing the secondary's below to the bottom of the transition slot should keep them from pulling idle fuel,, and shutting off the idle mixture screws should make the secondary idle/transition circuit inactive for now.

Made it to page 4 and I'm spinning...I'm not sure if I can get anything from that post because it's way over my head!

I'll try what you said about taking the secondaries out of the picture for now and keep working on one thing at a time. I'll get back...

kik,, pm sent. that's the screw to open up the secondary's,, to allow you to close down the primarys some. sent a pm with pic link to show the home made bracket/idle air screw i did for the secondary's on mine. check it out. comes in way handy for tuning. don't have to remove the carb to get to it. you'll like it. andy


I didn't see the link for the bracket?

I didn't see the link for the bracket?

http://www.putfile.com/pic.php?img=5665688
http://www.putfile.com/pic.php?img=5665680

bracket links ^^^ if you can get both closed up by drilling that old set of butterfly's,, that would be best to "test" with. then the little bracket above isn't really needed. jbweld the holes up and start with a single hole in each butterfly that gives the same "area" as all your holes you have in them now,, guage/measure/find "area" before jbwelding them up. you were on the idle track with that. drill them on the rear of the primary throttle shaft.

here's the abbreviation lingo you'll need to know,, sort of translated.
afr: air to fuel ratio.
stichiometric (sp.?): supposedly the perfect temp/ratio of the burn of fuel.
tps: throttle position sensor. measures voltage through a riostat to determine.
lamda sensor: form of measuring exhaust content,, wide band o2 sensor.
main well: the inner 2 verticle passages in the metering blocks with cup plugs/or solid pressed plugs up top to seal them. these are full of fuel at float bowl level. the float bowl is the resioviour for feeding this well,, keeping it full. main jet controls how much fuel gets through to it. so it's level can drop below the float bowl level under hard suction from the booster venturi.
idle well: the outter 2 verticle passages in the metering blocks with plugs up top to seal them. same function as above,, but for the idle/transition circuits.
pmab: is the primary main air bleed,, the front barrel end of the carb,, the 2 inner jets/air bleed orrifices up top next to the float bowl vent tube. just down inside the air horn. increasing size introduces more air,, leaning wot but richening part throttle. also delays time/rpm for the main jets to become active. like a controlled vacuum leak to emulsifying fuel.
piab: is the primary idle air bleed,, the front barrel end of the carb,, the 2 outter jets/air bleed orrifices up top next to the pmab's. bleeds air into the idle well to start breaking up the fuel/to start atomizing it in the well before sending it through to the engine.
smab: same as above,, but secondary side of carb/rear
siab: """""^^^
ifr: idle feed restriction jets. in the metering blocks at 3 o'clock and 9 o'clock of the power valve,, when it's installed. on the main body side of the metering block. little brass jets pressed into the metering block,, if your metering blocks have cup plugs vs solid plugs,, up top of the main and idle wells.
pvcr: power valve restriction channels. the 2 little holes/jets right behind the power valve,, can see them with the power valve removed only. not brass or anything,, just holes drilled into the verticle main wells at an angle.
also idle and main well emulsifying air bleeds in the metering block to add additional air into the main wells at the metering block.

yep,, can be overload following that goofy dude. he makes alot of changes together. it flips back and forth like that,,, which is why i mentioned about reading between the lines. crazy. that one dude has a clue though. old school from back when....

no problem,, don't rush it,, i'd still give that thread a read in your spare time. more will sink in each time you read it. such a long read though. it did say something about NOT using copper wire for a permanant fix,, about it causing a gum buildup because of the reaction with fuel. maybe just scroll through looking for the pics of the ifr channels to know where to install some wire to lean the transition circuit,, after the idle is situated. you may not even need the secondary idle mixture screw circuit with your setup as it is. just the transition part back there.

cool,, start with that,, and let me know when the idle in gear is good with the secondary's out of the picture..... no need to drive it till we downsize the ifr's. andy

OK, Rat...

I've done some more adjusting on this and still have not fixed the problem. I drove it about 400 miles (very poor gas mileage maybe 10mpg) to get it broke in, it runs good but still rich, and black smoke at idle. After break in the idle smoothed out tremendously though.
I really appreciate all the help and information you've provided but I think I'm at a dead end. I am talking to a quality shop here called "CARB CO. and I think I'm going to let them tune it for me. The guy says he can hook it to a machine (air/fuel meter) that will tell him exactly what it needs...This company restores old Holley carb for high end cars...so hopfully they will be able to help.

The guy says he can hook it to a machine (air/fuel meter) that will tell him exactly what it needs...


that would be ideal. a days work for them and you're on the road.
andy

This being said, if it won't hurt anything being a little over sized can the 750 be jetted to properly feed the correct amount of fuel?

Carbs work this way.
Air passes through venturi, fuel is sucked into air stream at certain ratio according to jet, air bleed, etc parts in the carb.
Radically changing jets won't convert a 750 to act like a 600.
Its going to have a 12:1 -16:1 A/F mixture, according to the jets.

Can a 750 be used? yes,, if its a vacuum secondary carb.
A QJ flows 750cfm, 3310 Holley, 750 AFB does too.
Stomp the throttle and the primaries are WOT, the secondaries open slowly by the pull of the air flow through the engine.

I've been thinking all along if I should just go buy a 650/750 Holley street or truck avenger carburator and see if that would solve anything.???

I've been thinking all along if I should just go buy a 650/750 Holley street or truck avenger carburator and see if that would solve anything.???

might not be a bad idea either to get to a vacuum secondary carb like hhott71 is saying. maybe you can borrow one somehow to try before you plunk down a chunk of change. like just to see if the idle into transition circuit is any better with another carb.

a smaller carb may be leaner in the idle feed restrictions and give you what you're looking for. might can borrow/swap in just some smaller carb metering blocks and see. they'll most likely have smaller idle feed restrictions. or stick some .010 then .015 then .020 wire in in your current blocks/ifr's to lean them down some. the 20" high vacuum levels you're pulling wants to pull lots of part throttle fuel. about any performance based holley carb you put on there may be rich at part throttle because of the way holley jets the ifr's for slightly lower than stock vacuum levels. but a smaller carb/metering blocks may limit the fuel flow down like you're looking for with smaller ifr's in it.
andy