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Carb size to small????

12K views 50 replies 19 participants last post by  xntrik 
#1 ·
I have been trying to tune up my motor, and I'm starting to believe my carb size is too small. After about 4,500rpm the motor will still rev up to 6500 but it gets there slowly, and the power maintains but doesnt have that neck breaking pull that it does under 4500. After playing with the timing alot with no effect, I was wondering what you guys thought about my carburator size. Im running an Edlebrock 750. Here are some of the specs on my motor.

408 stroker/windsor
Aluminum 215cc runners 2.08/1.60
10.68:1 compression
Roller Cam 224/.568 /112cl/ 3200-6700rpm range
Victor Jr intake
Edlebrock 750CFM carb
 
#27 ·
Your fuel pressure should not drop like that. As Rick said, that is showing you there is a problem somewhere in the fuels system. It only dropped to 3 psi but, you weren't at full throttle nor were you in high gear flat out. The pressure would have continued to drop if you would have stayed in the throttle. I obviously can't see your fuel system, so that makes it hard to troubleshoot. What pump are you running? Are you running a return line? -8 should be large enough to feed your engine, so I doubt the line is the problem. Do you have any (or many) 90* fittings in the system?

I really wish you were closer or I knew you better. I have an 830 Holley with annular boosters, sitting on the shelf that I would really like you to try. I would bet the house on it that those that "think" 750 is enough cfm for your engine would be shocked when they saw how it ran with this carb on it. While in "theory" (and the formula" may tell you 750 is enough, we all know how throey and reality are VERY often two different things. There is more to car selection than size alone.

I have to agree that since you posted the fuel pressure drop, I don't think a carb change will cure your problem. Once that is fixed I know for a fact and will put my money where my mouth is this carb will out perform the Edelbrock hands down.

I am sure we can work something out if you are interested in a little testing.

Royce
 
#28 ·
Edie 750 don't really flow 750 BTW. Another reason the formula doesn't work. Carbs seldomly flow what they are rated. The word "rated" in it self is a educated guess and manipulated to satisfy known perameters.

3 pounds isn't enough. Get a bigger pump or eliminate any restrictions to maximize what you have.
 
#30 ·
brainsboy said:
Hey guys no High jacking or taking over my thread please, besides at 60$ an hour this is getting expensive for me:) ....

So in simple terms just because I'm reading 3psi doesnt mean I'm getting enough volume? That doesnt make sence because if its under pressure (3psi) then the fuel is still waiting to get into the carb, But either way 3psi is not great, I need a new fuel pump?



Rick is it restriction or G-force pushing back on the fuel? Im using a standard 5 gal fuel cell with #10 going to the pump and after the pump is all #8, the vent on the top I would have to check I forgot about that..
I look at your cam specs and don't see how excessive G Forces could be your problem. The cam is pretty small. If the car weighs under 2000 then I could see it being pretty peppy but in say a mustang I don't see wheel standing low 10 1/4 times, I certainly could be missing something though so feel free to elaborate. What do you think it's making for HP?

It certainly could be a bad pump, pickup problem, clogged filter(s) (you should have a filter prior to the pump and a filter after the pump). Dropping pressure like that says something is too small in the system or restricting flow.

maintaining proper voltage at the pump is also very important and should be verified.

Have you talked with Holley tech on this yet?
 
#31 ·
Rick WI said:
The cam is pretty small. If the car weighs under 2000 then I could see it being pretty peppy but in say a mustang I don't see wheel standing low 10 1/4 times, I certainly could be missing something though so feel free to elaborate. What do you think it's making for HP?
QUOTE]

1987 Suzuki Samurai
last weight taken was 2223 lbs w/ driver

I have done alot of research on these motors and most are making in the 490-510 HP range with less compression and less heads then what I have. Based on everything I have done I should be in that ballpark range. Does that mean im at 500hp, not at all, but Im not worried right now if Im making 485 or 500. Maybe Im doing this backwards but Im trying to get my suspension adjusted, trans, motor to all work together the way it is now before I start going for more horsepower. Once I get everything working how it should, I will goto the dyno and fine tune for max HP.
 
#32 ·
A Suzuki, that's awesome. If you list all your specs in detail I'll model in Engine Analyzer Pro for you to give you an "idea" what the progam thinks your making. Seat to seat cam timing numbers are best for input into the program.
 
#33 ·
Ben,
You have a PM. I will send you the carb. Just get your fuel supply problem fixed first. You pay shipping on the carb to and from. If you REALLY like it we may be able to work a deal. This carb is brand spanking new and never been on an engine other than dyno tested. I really think you need or should have a solid 6-7 psi of fuel pressure at all times (I have run even more without a problem).

I will see if I have an extra fuel rail/line for the carb as well. That way you can use your existing fuel line, I think I have a hard tube dual feed line out there.

Royce
 
#34 · (Edited)
Wow, thanks Royce. Get the works on the shipping, insurance, certified, I'll pay for it all, rather be safe when it comes to shipping something like that.

I have been looking at some fuel pumps. I found a used mallory 250 for 200$ and I found a mallory 500 for 300$.

Anyone know the deal on uploading pics here. I was trying to upload my photos but cant for some reason

Ben

 
#35 ·
Ben, what kind of pump do you have now? I think I also have a Holley Pro Series "black" pump out there too. What regulator are you using? If it is the holley regulator that comes with the blue pump (the cheap Holley regulator) I would ditch that like a bad habit, they are terrible. I know they came out with a new one/style, but I haven't tried one yet or talked to anyone that has.

Let me know what you want to do and how fast you want it shipped. I will also need your address and full name.

Royce
 
#36 ·
camaroman7d said:
Ben, what kind of pump do you have now? I think I also have a Holley Pro Series "black" pump out there too. What regulator are you using? If it is the holley regulator that comes with the blue pump (the cheap Holley regulator) I would ditch that like a bad habit, they are terrible. I know they came out with a new one/style, but I haven't tried one yet or talked to anyone that has.

Let me know what you want to do and how fast you want it shipped. I will also need your address and full name.

Royce
The one that comes with the blue pump is not strong enough. It will eventually start flooding the carb. I don't know what Holley was thinking. I tried three of them before giving up.
 
#38 ·
cuda66273 said:
Quote:
" About the only thing I can tell from this far away is that (If clean) the 750 cfm carb is not his problem "

I would like to debate this statement but I have already wasted too much of my short life expectancy trying to tune the chronic mid range lean out of these carbs, they're a $289.00 piece, why spend 10 hours at $60.00 and hour trying to tune it and fix it when a real carb that is capable of handling the task is $400.00?

If you find the problem and a way to fix it please let me know, I have 4 or 5 near new ones sitting here that could be fixed and sold.
I'll give ya a shout next time I am down that way and we'll take a look at those carbs. I think if you're suffering from a lean condition in the mid, a two step metering rod instead of a three step would fix that in about 5 minutes.
 
#39 ·
Great thread! Right on target for me too since I'm rebuilding the fuel system on a car I recently bought. I hope, however, that some of the stuff I've read is wrong about needing a -10 line from the pump and -8 to the carbs. I already installed -8 from the existing Holley blue pump and -6 to the carbs (dual Holley 600's on a Weiand 6-71 blower) with a -6 return line. I did mount my Aeromotive bypass regulator on the passenger side radiator support and not on the motor. Yay for me! Figured that one out by myself. Also purchased a second fuel pressure gauge for the cowl to watch while I'm running. 2 attaboys!

Any comments on whether my motor will get enough fuel with this setup? I don't have any dyno data yet, but it's a:

'85 S10 Blazer 3420#
383, 7:9.1
dual Holley 600 4160s converted to 4150s
Weiand 6-71 10 lbs. boos
Scat 9000 crank
Comp 226 duration/460 lift/114 ls
Sportsman II heads 200cc runners/72cc chambers
hyd 1.5:1 rollers, 2.02/1.60 valves
Sanderson ceramic big tube headers
TH350/TCI valve body w-shift kit, Quick Silver shifter
B&M 2400 converter
4.56 Richmond gears
12 bolt truck w-Moser 30 spline axles, Auburn posi (I think)
c-clip eliminators
32x17.5x15 MT on Weld Aluma Stars
Strange coilovers
4 link
 
#41 ·
Carb Size.................

I always read these threads with great interest and amusement, the carb wars and fuel systems theories are always good but you can quickly see who knows what they are talking about and who doesn't. Don the Mopar guy is giving the benefit of 40 years of knowlege and advice basically for free here and some people are giving out inaccurate and obvious stupid advice-- but also for free! When he says get rid of the "Edel--Broke" carb for that guys application he is right on--that carb can not be tuned to do what the correct Holley or BG can do---period. Now everybody, lets THINK--brainsboy is DRAG RACING, he needs a carb that will work for his application. The WOT cfm rating of any carb is meaningless unless you relate it to a SPECFIC carb.....Are you still with me, I hope so. He needs to get a 750 cfm DRAG RACE type carb. Somebody said try an annular booster Holley 830---WRONG, that is a ROAD RACE carb (9381), it is square jetted, pri and sec PV, no sec jet ext and calibrated for road racing......Do you think road race guys use drag race style carbs? He needs a 750, maybe bigger drag race carb tuned to his engine and chassis . Hey ty for reading--have a great day.
 
#42 · (Edited)
Walt B said:
I always read these threads with great interest and amusement, the carb wars and fuel systems theories are always good but you can quickly see who knows what they are talking about and who doesn't. Don the Mopar guy is giving the benefit of 40 years of knowlege and advice basically for free here and some people are giving out inaccurate and obvious stupid advice-- but also for free! When he says get rid of the "Edel--Broke" carb for that guys application he is right on--that carb can not be tuned to do what the correct Holley or BG can do---period. Now everybody, lets THINK--brainsboy is DRAG RACING, he needs a carb that will work for his application. The WOT cfm rating of any carb is meaningless unless you relate it to a SPECFIC carb.....Are you still with me, I hope so. He needs to get a 750 cfm DRAG RACE type carb. Somebody said try an annular booster Holley 830---WRONG, that is a ROAD RACE carb (9381), it is square jetted, pri and sec PV, no sec jet ext and calibrated for road racing......Do you think road race guys use drag race style carbs? He needs a 750, maybe bigger drag race carb tuned to his engine and chassis . Hey ty for reading--have a great day.
They make powervalve plugs. With a powervalve plug the jetting would no longer be square. You jet up 8 to 10 sizes to compensate. If you are talking about a carb that will stage on the primaries and leave on the secondaries, I think that is more than he is looking for.

Jet extensions are cheap. So are powervalve plugs. DRAG RACE carbs are expensive. 830s are are $150 to $200 on Ebay. Or, he can spend top dollar and ask the BG techs what they recommend.

If he was going to have a "drag race" carb tuned to his engine and chassis, do you think he would be asking us for general carb recommendations? Do you realize how much money it would cost to have what you are saying done? He is asking for real world advice, not an idealistic line of bull. Unless of course you are planning on sponsoring his efforts... Then, preach on.
 
#43 ·
alf1096 said:
It seems like everyone here knows carbs has anyone ran the holley street avenger I want to put it on a 383. What Size thanks
A Holley Street Avenger is a very nice street carb. We buy dozens a year for motors. I have no idea what size you'd be looking for but it would lay between the 670 and the 770 depending on your HP. If you are truly making over 400 HP than I'd go with the 770, if not the 670.

Jetting thou on these are not good to go out of the box. We installed a 570 yesterday on a 283 and rejetted from 53's to 66 square. So, as with any other carb it will require tuning out of the box.
 
#44 ·
Hey Lluc

Nobody said hotrodding and drag racing was cheap, someone else made the comment that the guy has spent close to 10 g on an engine and puts a $250 carb on it--you do the cost analysis. In your "real world" what do you think it will cost if he blows a head gasket or melts some pistons because the fuel system was inadequate? Jegs has BG GC carbs for about $500, he's already spent half that for a worthless carb for his engine. I'd rather change the carb and upgrade the fuel system than replace pistons, rings a head gasket or worse--thats just me. As for E-Bay you might get a good deal on what you need and you might get a piece of junk. How many pieces of junk are you willing to buy. Finally I was assuming he would get the right carb and tune it himself--whether new or used.
 
#46 ·
0-9381 830

The 830 carburetors weere designed as a restrictor carburetor for Trans-Am, and NASCAR racing. They use an 850 main body with a 750 Baseplate. This does not allow for the best air velocity (as a smaller venturi, with a larger baseplate would) hense they really should only be used when they are mandated by the rules.
 
#47 ·
Tech @ BG said:
The 830 carburetors weere designed as a restrictor carburetor for Trans-Am, and NASCAR racing. They use an 850 main body with a 750 Baseplate. This does not allow for the best air velocity (as a smaller venturi, with a larger baseplate would) hense they really should only be used when they are mandated by the rules.
I've heard of using a larger baseplate to improve air speed, but how do you overcome the larger butterflies hitting the smaller carb's venturis?
 
#49 · (Edited)
Cuda, Thanks for the great info! I copied and saved it.
I'm having a 750 Speed Demon installed tomorrow.
I'm up in the Seattle area...



cuda66273 said:
Fuel Pressure is only a measurement of restriction in the fuel system and has very little to do with fuel VOLUME.

Sounds like you running a Holley Blue, thay actually will flow at about 75GPH at pressure, combine that with the Blue Reg which flows maybe 50GPH and you'll see that a good mechanical will out flow it. Holley Blue Systems, IMO are good for about 300 HP max.

There lot's of thing's that determine fuel requirements, let's analyze a few.
1. You have a 1/2 line, that's very good, but what size are the inlets and outlets on your pump? The answer of course is 3/8" so your pump will only flow at the rate allowed by the smallest orafice in the system, remember electric pumps are gravity fed so although the outlet side will flow more volume due to the pressure the inlet which is gravity fed is the limiting factor.

2. Let's use a 12 second flat car for an example. I would think we can all agree that this car would burn about 1/2 gallon of fuel to make a 1/4 mile pass. So if we do the simple math to determine GPH (Gallons Per Hour) 1/2 gallon x 5 (60 secs. / 12 sec pass = 5)= 2.5 gallons per minute x 60 minutes = 150 Gallons per hour, far beyond the capacity of a Holley Blue or any mechanical pump.

3. So we've determined how much fuel we need to make a pass and the capacity of the pump required. But what about the gear ratio in 1st of about 2.50? Does this have any affect? Of course, we all know from experience that a car will use way more fuel at 30 MPH in 1st than it will in 2 or high gear so we need to make allowances for the huge consumption in low and 2nd gear. Some of this is built into a Demon with the larger float bowls creating a built in reserve, but if the pump can't keep up the floats will drop and the carb will not be able to maintain a nice flat fuel curve as demand increases. Do you think that maybe 20% more fuel would be a fair number to use?...so we're now up to 150 + 20%= 180 GPH

4. Let's not forget G forces, does your fuel line run from the cell straight up the carb taking the sortest route possible? By running the fuel line in this manner you further complicate the G forces working against the pump, all the fuel in the line is pushing back against that pump and will actually stall the pump (Remember the main feed line from the tank to the pump it's going in the opposite direction also starving the pump of adaquate supply)

The correct way to plumb a car for optimum fuel delivery would be to have the main line come all the way to the front of the car, we run them through the radiator core support, a big loop and back to the regulator.. Now when the car launchs the fuel is driven by G forces to the regulator giving it a supply while the pump recovers from the shock. So if your running your lines in what is probably not the best location then I think a 10% increase in pump capacity should be built into the equation....180 + 10%= 200 GPH

(How's your fuel system holding up so far?)

5. I'll bet you have your regulator mounted to the engine or chassis right? Another No-No....as the engine starts to wind up and the car builds speed that regulator is getting the ever loving ***** rattled out of it. In allot of cases so violently that the spring can't hold the ball on the seat so it releases fuel and can overflow the float bowls. You've all seen this at the track...a car gets about 300' out and it starts leaving a black haze behind it.

6. Feeding the carb from the regulator is another consideration, if you running a dual feed line that uses a T and splits off to feed the bowls your probably OK to about 350 HP aafter that we reccommend to use a 2 port regulator and supply each bowl with a -6 line from seperate ports on the regulator. Thjese lines need to be as short as possible we use a 7.25" line with a 45* on one end and a straight on the other with the BG 2 port it works perfect. Too large of lines or too long of lines will cause the regulator to respond slowly as there's more fuel volume to drop in pressure and more volume to fill to get back to full pressure.

7. Weight consideration and engine size are another factor to look at. If this 12 second car is a 4000# Hemi Super Bee or a 350 SBC in a 2800# 67 Camaro? Again I think we can all agree that even though both of these cars may run 12's their fuel consumption will vary drastically so the rule of 12 second car requires a fuel system that can deliver 1 gallon in 30 seconds becomes a rule of thumb and not an absolute number.

In this scenario the Hemi Bee would probably need a BG 280 whereas the Camaro would have plenty of fuel with a BG 220.

Your Fuel System is just as important as choosing the correct cam, the right Carb, tuning the Ignition, selecting the correct tork convertor, compression ratio etc. it's all part of building a good dependable and consistant car whether it's a Street Muscle Car, Sunday Bracket Racer or a Top Comp/Pro Stocker you must have an adaquate fuel supply to feed the power you've created, if you don't want to or can't afford to build an adaquate fuel system then don't build the power. Piston manufacturers love you guy's it keeps them in business.

I've been in this business for most of my life, been tuning carbs and ignition systems since a was a young kid, worked with my Dad who was probably one of the best tuners ever, he started teaching me this stuff when a was about 6-7 years old....that was about 45 years ago. But if I need a cam I call Jim at Racer Brown a real cam grinder, if I need a Convertor I call on Frank Lupo at Dynamic, when I need pistons I call Ronnie at Diamond, I use the experts available to me, I draw on their experience in their specific fields to obtain the best results. It works for us.....our All Iron, All Motor 318 Mopar in a 3100# car will crack off a 11.60 pass with 6200 RPM shift points anytime I hammer on it.

So in closing I can only strongly reccommend that if your not a expert on fuel systems then consult with someone who is and stop relying on a salesman at a mail order house or some rediculous magazine article to sway your decisions on your fuel system or ffor that matter any other component. Deal with that special individual who has risen to the top of his field and forget the mass producers of fancy catalogs and generic one size fits all components.

If you follow what everyone else does then you will follow everyone else, I prefer to lead, the view is much better up front :D...Mush.....if you ain't the lead dog the view sucks..
 
#51 ·
carb size?

First let me say, this is an interesting thread.

I never cease to be amazed that people who don't actually know how to tune a carb get into the brand name argument. Seems as though most people consider an Edel. hard to tune because they don't have a clue how, and don't want to know.

First thing anyone should do is tune what you have. Most people find it easier to whine about it and then guessing and start spending money hoping to grab something at the swap meet or out of a box that "happens" to work better. And if it does or doesn't, that affects their opinion of a product.

Each and every engine combination is unique and requires it own tuning set up. Ignition, fuel, carb, etc. Find a chassis dyno.

If you ARE having carb problems, and are inept in tuning, call and order a Demon made to your specs. It will be cheaper in the long run.

Oh, and that manifold change thing... don't forget the bigger manifolds have bigger runner ports matching up to the heads. Some heads can't be opened enough to match. They are not necessarily just a slap-it-on idea.

THINK BALANCED PARTS COMBINATION.
 
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