Question for the dual 4 crowd

[Whaler]

I was just wondering what your combined CFM's are with the 2 carbs

Especially the FE's

Tim

[BigEasy]

I was running Holley 550's, but carbs were old, leaky and choke plates were gone.

You cannot get Holley 550's any more, so went to Edelbrock 600's, with manual choke's. Have been happy with them--easy to work on (change idle springs, etc), and now can manually choke as needed. I was told that these would be better, especially if the car sits for long periods, as they will not leak---so far, no problems.

BigEasy

[Trans-Am Jim]

1100 - 2 550's.
They were old and leaky like Big Easy sez, but I when I had 'em apart to refurb them, I had the bases and bowl mating surfaces milled true. With new BG red gaskets, leaks are gonzo. Also took the chokes off altogether and had the choke horns milled off.

[bmalone]

1500 cfm. (two 750's Holleys)

But, I will be replacing with more modern units in the near future.

[Whaler]

WOW You guys are up there in the CFM department

The reason I ask this question, is that I am run a single Holley 850 with mechanical sec. and manual choke ( wired wide open)

I am thinking I could be overcarburated a tad in that it smells rich. I have just broken in the cam and am attempting to set up the carb. I have not pulled a plug yet to look

Adjusting 4 idle/air mixture screws is touchy using a vac gauge. They should be adjusted equally and I have gone from the std curb setting start pt. of 1.5 turn out on each, to 1.0 turn out for another baseline.
So far my min. vac. is looking like 10 inches at idle. But Idle is not quite where I think it should or could be. 1000-950 RPM

It takes few minutes to warm up too.

FYI I find it hard to believe that I have only put 2 gallons of 94 octane in the tank for that inital break-in of 30 minutes and it ran dry the next day....8 liters to be exact ( Canada hey) 1500- 2500 rpm may do it for the 20-30 minutes I ran it

Since then, I have been thru another 2 gallons just playing around in the driveway over a few different days.

The reason I only want 2 gallons in at a time is that it goes to the paint shop soon and I don't want much gas in it then.

I have a 25 gallon fuel cell. This consumption sound right?

You guys have way more CFM than i do easily. Yet your not too fat?

Tim

[HighPlainsDrifter]

Tim,
You have to know what intake manifold type to know what cfm. Example ; I run 2 - 600 cfm edelbrock carbs on a blue thunder dual plane manifold. The dual plane manifold splits the engine in half and even at wide open throttle any one cylinder will only 'see' the left half or right halves of the 2 carbs. So the dual carbs act like a 600 cfm setup instead of a 1200 cfm setup. The fuel distribution is better with 2 carbs than 1. Now if you have an open plenum dual quad manifold like a tunnel ram, you will see all 1200 cfm at once and have very poor low end driveability and mileage. You are running an 800 cfm carb on an open plane intake , each cylinder will 'see' all 800 cfm at any one time so it could be too big for your application. The intake manifold has more influence on the way power is made in the engine than the carb does so I would think about that first ,then decide what carb cfm you want to run.
Perry.

[Whaler]

Hi perry thanks for the info

I have the following intake and its like the one you say is better for runnability on the street

"Edelbrock Performer RPM FE (1500-6500 RPM)

Designed for high-performance street 390-406-410-427-428 c.i.d. Ford FE V8s with standard 390-428 c.i.d., 427 low/medium-rise or Edelbrock Performer RPM FE heads. The dual-plane constant cross-sectional area design builds low and mid-range acceleration while optimized runner paths make top-end hp. Has provision for adding the PCV or breather flange at rear. "


They also list 750 and 800 cfm carbs as being the optional carb to match. I have an 850....lol

Tim

[bmalone]

In addition to the intake design; what cylinder heads you run will have some impact as well. For instance, some heads will take all the fuel at once from a single plane intake without experiencing drivability problems.

[K.Wilson]

Whaler,
I run two 600 Holleys on mine. The "too much CFM" problem is not really a problem with the vacuum secondaries as long as the carbs are adjusted correctly as they will only open the secondary system as required by the air flow through the card. If they are not adjusted correctly you can have the engine "bog down" from to much carb.
I would not run a mechanical secondary carb on a 2x4 system unless you could find two 400's or 450's and even at that they may not be able to be adjusted to give you a smooth power curve.
Keith

[Steve Dickey]

bmalone,
Do you know what size main jets you are running? I will soon start my carb tuning and would like to establish a starting point.
Thanks, Steve

[Whaler]

Nice article on the idle cct of most carbs

http://www.chevyhiperformance.com/ho...38/index1.html

Hit the link at the bottom of that article to go to the other article too.

Bill, I have thew whole Edlebrock RPM package....that means aluminum heads too.

[KobraKarl]

Keith makes the point I think your question was addressing. You are only running a four barrel carb with vacuum secondaries,until you really start to wind ,then you get only what you need,and stock linkage is progressive allowing only to barrells at a mild acceleration.

The main advantage of the secondaries is improved breathing at higher rpms, and two air filters generally feeding four barrells at mid rpm's.

I run the early 550's with #62 main jets ,yellow spring in the secondaries, but the real cfm in a propely sprung secondary carb is ,whatever is needed !, manual secondary is whole nother deal....


Karl

[ted]

If your carb is running rich at idle, you can try several things.

First, screw the idle mixture screws in equally until the engine falters then adjust for best idle or vacuum reading. If the screws go all the way in and the engine is still running, then you're probably pulling fuel from the main circuit which means too much throttle opening. If it's a four idle circuit carb, then the primary and secondary throttle plates should be open evenly. If it's a carb with only primary idle circuits, then you can try cracking open the secondary plates slightly more so that the primary plates can be closed some more. This will put more demand on the primary idle circuit.

Worst case scenario when all else fails, pull the carb and check out how much idle transfer circuit is exposed at the throttle plates. If it's more than 0.040", then additional holes may have to be drilled in the plates so that the throttle plates can be closed off some more thereby allowing idle circuit adjustment.

And before I forget, the simplest and easiest "rich at idle" problem to fix is the power valve. Either its bad or not seated well against its gasket. Usually pulling off the metering plate and looking at the cavity behind the power valve to see if its wet back in there is a good indicator of a problem in this area. If you've got a cam with lots of overlap, this tuning tip probably will not work as it may be wet anyway.

And while we're on the subject of power valves, the power valve could be wrong for your application. If the manifold vacuum is too low, then the power valve is not closing at idle and causing a rich scenario. A vacuum gauge will help out here. You'll want a power valve that is rated at least 1 inch Hg above whatever your manifold vacuum is at idle (and if an automatic trans, in gear).

And....... the list goes on and on.

[Whaler]

Thanks Ted for the help. I have dug out my Holley book and am reading thru that. You have nailed most tips they speak of.

I have at least 9-10 inches idle vacuum on my gauge.
My power valve is 6.5 OEM on the Holley 850 mech sec.

I only just last night cracked a fuel bowl sight plug screw and witnessed gas bleeding off the cracked screw. This was with the car sitting and not even running. I assume that had I taken the screw right off the gas would have dribbled out considerably.

The adjustment for the float levels seems a tad confusing. One book describes the screw as the adjustment with the 5/8 nut being the locker. Another book said the opposite, that the nut is the adjuster and the center screw a locker???
I haven't made the attempt to lower the float level yet until I have this nailed down. Obviously the high level in the fuel bowel will contibute to richness or flooding.

I do know i want to take the adjustment clockwise to lower the float. It's deceiving to look at too, because the screw and the nut almost look bottomed now?? This carb is brand new & out of the box.

http://www.shockley.net/holley-jets.asp
I think this guy describes the adjustment of the level correctly.


Another piece i found useful is this tip

"Idle Circuit

The idle circuit controls idle and off idle fuel delivery. It is not commonly understood, but the idle circuit can affect the fuel ratio at cruising speeds as high at 50 mph or even more. The idle circuit needs high vacuum to operate, so if vacuum is high enough at cruising speeds, the idle circuit is flowing fuel. I have found that an off idle hesitation that is not a result of an accelerator pump usually is a poorly tuned idle circuit. The number 1 cause is when radical cams force the tuner to open the throttle blades too much and they uncover too much of the transfer slots. When this happens, there is insufficient transition fuel to cover the gap before the main system is activated. More pump shot or richer main jets cam cover this problem, but it also wastes fuel and could hurt power. The better solution is to open the secondary plates some, which will allow the primaries to be closed and cover more of the transfer slots. In extreme cases, as with radical cams, you can drill two 1/16 holes in the primary throttle plates to let more air in rather than open the throttle plates."

http://www.grapeaperacing.com/GrapeA.../carbterms.cfm

The adjuster screw stop is darn had to get at when the carb is on or in a turkey pan..........lol

http://66.96.130.106/archives/1999/1...y/index2.shtml



Tim

[SCOBRAC]

900 cfm.. No chokes.. no worries..

[SCOBRAC]

Don't worry about too much cfm.. A vacuum secondary carb will only flow what it is asked to flow.. If you have done the math and discovered you only "need" 800 cfm, guess what? That's all your engine will draw.. and you guessed it, all your carbs will flow..

So what if you have 1200 or 1300 cfm you only need 800? You are about 32% over carburated. The only way to really know is to measure vacuum above the carbs on a dyno to measure any restriction optimum would be .5-1.0 hg of restriction, getting good flow without stagnating. If you have too much restriction you are losing power. If you have no restriction you are losing drivability and will feel a "bog". The reality is you are probably only opening your secondaries 65-70%..

Some really smart people who run lots of cam and big valves (reducing restriction) and potentially killing bottom end opt for smaller carbs in order to increase restriction within the intake circut for better drivability, fuel mileage, lower emissions and no bog.. Mid range top end HP suffers, at 5500-6500 but drivability and idle are crisp. It might cost you 30 hp at 7000 rpm but might be worth it..

Done well a pair of 450 cfm carbs with #62 main jets will flow better 1500-5500 than a pair of 600 vacuum secondary units will.. You will have to be willing to live without a choke.. (just one more thing to mess with) Run them progerssive for really impressive fuel milage (try 15 mpg) or run them non progressive for a ride like you wouldn't believe..

[427 S/O]

SCOBRAC is right, after pulling, changing, re-install and driving about three times.
I found the best performance with my high overlap cam (7 inches @ idle) and dual 600's was to..... change jets to 64's, drill a 3/32 hole in each throttle plate for better adjustment, install the short yellow springs, remove choke assy., recurved dist., initial timing 18 and 38 total. Acceleration is unbelievable.

[Steve Dickey]

Whaler,
To adjust your float level, get out your slot screwdriver and unscrew the top screw first. You will then instantly see how it works. It will always look "bottomed out".
Good luck, Steve

[Dominik]

I ran a dual intake (dual plane with balancer port) in my FE with two vaccuum 600s, primary jets 70s and no joke. that's a FoMoCo intake. at sea level. the came with 66 jtes at 5000 ft.

on a single plane I was very happy with a 850 cfm 4150 competition double pumper mechanical sec. the "competition" meant big annular boosters. I took the power valve out and increased jets by 6 or 8 numbers (sorry, can't remember).
but after I took the valve out the car was smooth like a kitten from 1000 to 2000 where it bogged around before.

dom