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Troubleshooting Diagnosing problems done here.

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  #16  
Old May 9th, 08, 8:26 AM
bigsky bigsky is offline
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Default Re: Edelbrock carb problem! 1406

Great info guys, thank you, but you must know, I'm not a high tech carb guy, so with all the info I gave on here about the carb theories was stuff I was told from other people. Right or wrong I have no idea! I think the performance shop guy I talked about told me to get the holley 770 instead of working the 1406 edelbrock, was the best way for a person like me to rectify the stumble problem in my car. But according to Edelbrock, the 1406 is a "economy carb" which I believe in just the name alone, its not the carb for me. My stroker wants fuel at wil So my 770 is here, but it is the MN fishing opener this weekend so I wont be putting it on. When I get it on, I will let you guys know the difference, thank you for all the great comments. By the way, anyone want to buy a great performing 1406 Edelbrock carb, like new, with a performer intake. Jeff
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  #17  
Old May 9th, 08, 8:28 AM
jeff swisher jeff swisher is offline
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Default Re: Edelbrock carb problem! 1406

i ran that 600cfm on a 388 stroker with an auto and rpm intake 2200stall and it would twist to 6700rpm that was the sweet spot..ported 624 castings 1.94-1.60..and it would pull the left front wheel an inch

DO THIS TEST.. car in neutral loosen the metering rod cover screws, maybe 3.5 turns the piston will push up the cover ,but not come out

now start the engine they should suck down. and stay down..
Blip the throttle they should come up and hit the cover..blip it repetitevly quick like, they should come up and down following the throttle...

for best performance RUN THE HEAVIEST SPRING YOU CAN under the power piston.. I usually run the old 60's afb springs with an INK PEN spring to make it even stiffer....

Too stiff and the power piston at idle will not stay down...find that spot and you will need to go softer on the spring

That my friends will make your AFB type carb come into the primary power circuit extremely well....

Make sure the squirters are squirting as you blip.. or you have another problem to fix
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  #18  
Old May 9th, 08, 8:35 AM
bigsky bigsky is offline
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Default Re: Edelbrock carb problem! 1406

Quote:
Originally Posted by Schurkey View Post
Too little CFM does NOT cause hesitation.

Too little CFM may cause a reduction in higher-rpm power--but NOT stumbles, misfires, etc.

The problem is very likely in the calibration of the carb--amount of accelerator pump shot, squirter size, or the secondary air valve opening too abruptly.

Come to think of it--didn't Edelbrock introduce the "Thunder" series carbs which have a more-tuneable air valve?
Isnt a "reduction in higher rpm power" a "stumble"?? Just a thought.
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  #19  
Old May 9th, 08, 8:48 AM
bigsky bigsky is offline
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Default Re: Edelbrock carb problem! 1406

How did that little laughing dude get in my quote? I didnt put him there??????????
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  #20  
Old May 9th, 08, 1:46 PM
Schurkey Schurkey is online now
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Default Re: Edelbrock carb problem! 1406

Quote:
Originally Posted by bigsky View Post
Isnt a "reduction in higher rpm power" a "stumble"?? Just a thought.
NO. ESPECIALLY "NO" since you've said it happens when "cruising" and "in any gear" You aren't at high RPM when the thing is giving you problems.

"Reduction in power" should be taken to mean that the engine runs good--but--does not rev as high; and/or does not make as much power at that high RPM. It's not that the engine runs "bad" in any way, just that the power isn't what it could be. And it's a simple matter of capacity: CFM required from a carburetor is MOSTLY (not completely) related to two factors: Engine size (CID) and maximum engine RPM.

Mathematically, it looks like this:

CID X Max RPM / 3456 = carb CFM needed

Note that 3456 is a constant that takes into account the number of cubic inches in a cubic foot; and the fact that these engines are 4-stroke not 2-stroke.

Of course, the underlying assumption is that the carb has been calibrated to supply a proper amount of fuel to go with that air. So--up to the airflow capacity of the carb--the engine runs good, runs strong, doesn't hesitate, stumble, surge, bog, spit, cough, backfire, etc, etc, etc. Of course, anything that screws up the air/fuel ratio (i.e., wrong jets, wrong metering rods, air valve that flops around like a wounded guppy) will also impact power--and, usually--provide driveability problems as well. So it's the "fuel curve"--the air/fuel ratio provided by the carb under different operating conditions--that is responsible for air/fuel-related driveability problems, not the capacity of the carb to supply air.

Can that mathematical formula be refined to be more accurate? Sure!
Add in a factor to account for the efficiency of the engine to pull in air--which is generally called volumetric efficiency, (VE) but "should" be called mass efficiency. And that is primarily related to camshaft opening and closing events; but secondarily related to inlet and exhaust port shape; ram tuning effects on the intake; exhaust pulse tuning on the exhaust side, and suchlike.
Typical figures would range from .75 to .95; but that's not absolute--there are specialized engines with better than .95 just as there are primitive "dogs" that would be under .75

Now the formula looks like this:

RPM X CID / 3456 X VE = carb CFM needed

Can we get better yet? Yup. Add in a Manifold Correction Factor (MCF) to account for the efficiency of the intake manifold. Two-plane intake manifolds need a bigger carb than a single-plane; and within the two-plane or single-plane design, some manifolds are just plain more efficient. So--to keep is simple--Let's say that intake manifold correction factors range from 1.1 (very efficient) to 1.5 (stock two-plane junk) and that the dividing line between two-plane and single-plane is about 1.3. So a very efficient two plane manifold and a very poor single plane manifold use about the same correction factor.

RPM X CID / 3456 X VE X MCF = carb CFM needed

And that is as refined as I know how to compute it. By the way, this formula came directly off the Edelbrock web site.

For the 383 engine that was the subject of the first post--I wish you had said what the max RPM is. I'll take a WAG and say 6,500 rpm, and that your VE is somewhere around .80.

383 X 6500 = 2,489,500
2,489,500 / 3456 = 720

720 X .80 = 576

576 X 1.3 = 748 carb CFM needed

Your best "baseline" carb is a 750. But, ANY size carb that has a proper fuel curve will give you an engine with no air/fuel related driveability problems. An air valve on the secondaries that flops open before it should will DEFINITELY screw up the fuel curve--and that's where I'd be looking on your existing carb.

Last edited by Schurkey; May 9th, 08 at 2:04 PM.
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  #21  
Old May 9th, 08, 4:50 PM
vrooom3440 vrooom3440 is offline
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Default Re: Edelbrock carb problem! 1406

I have personally run both the 1406 and 1407 Edelbrock carbs on a decent 402 BBC on top of a RPM Air Gap intake. Between the two for street manners I actually prefer the smaller carb. The issue with the 750 is that vacuum changes too fast as you start to open the throttle. When vacuum drops below 5-7" the idle circuit shuts off and you have bog city. Plus I had serious challenges getting the secondaries to provide enough fuel.

The 1406 is slightly spreadbore and has a much smoother vacuum transition. It also pulls fuel better on the secondaries.

I would make sure you accelerator pump is setup correctly. You need the hole closest to the arm pivot AND you need to adjust it for a full stroke. Every time you move to a different hole you need to readjust the pump stroke on the Ede carbs.
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  #22  
Old May 9th, 08, 11:18 PM
zeke67 zeke67 is offline
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Default Re: Edelbrock carb problem! 1406

Quote:
Originally Posted by Schurkey View Post
Too little CFM does NOT cause hesitation.

Too little CFM may cause a reduction in higher-rpm power--but NOT stumbles, misfires, etc.

The problem is very likely in the calibration of the carb--amount of accelerator pump shot, squirter size, or the secondary air valve opening too abruptly.

Come to think of it--didn't Edelbrock introduce the "Thunder" series carbs which have a more-tuneable air valve?
Yeah Schurley, reading that again I didn't write it very eloquently. I was trying to hit on the calibration of the 1406 not being sufficient and meant that the Holley 770 out of the box would have a better calibration. But it sure didn't come out that way. Even said to put the 1407 calibration in the 1406 later in the post.
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  #23  
Old May 9th, 08, 11:23 PM
zeke67 zeke67 is offline
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Default Re: Edelbrock carb problem! 1406

Quote:
Originally Posted by vrooom3440 View Post
I would make sure you accelerator pump is setup correctly. You need the hole closest to the arm pivot AND you need to adjust it for a full stroke. Every time you move to a different hole you need to readjust the pump stroke on the Ede carbs.
What do you do to do that, bend the rod or the arm itself?
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  #24  
Old May 10th, 08, 12:17 AM
vrooom3440 vrooom3440 is offline
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Default Re: Edelbrock carb problem! 1406

You bend the rod. It has a bend in it that gets either straighter or more bent depending on what you need. The pump plunger and especially the S connector should just about bottom out at full throttle.
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  #25  
Old May 11th, 08, 7:40 PM
bigsky bigsky is offline
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Default Re: Edelbrock carb problem! 1406

WOW, way to complicated for me, like I said from the start, carbs are not my gig. So I will try the swap and hopefully that cures the problem for a carb dummy like me. Thanks for all the great info, I wish I knew this stuff, and I wish I understood some of it! Thanks again, all, Jeff
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  #26  
Old May 11th, 08, 9:39 PM
zeke67 zeke67 is offline
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Default Re: Edelbrock carb problem! 1406

Jeff, you can learn this stuff. The Edelbrock book will have the calibrations for both the 1406 and the 1407. It is also pretty good at telling how to tune your carb step by step.
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  #27  
Old May 13th, 08, 8:10 AM
mezzanine79 mezzanine79 is offline
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Default Re: Edelbrock carb problem! 1406

I just purchased a Speed Demon 650 over the Edelbrock 650 Thunder AVS because I read the Edelbrock carbs were made for camshafts up to a 220 @ 0.50% lift duration and the Speed Demon 650 is for durations of 220 to 250 @ 0.50% my cam shaft is in the 230-236 range anybody have any suggestions if that was the right purchase? Im replacing a worn out Edelbrock 1406 w/ electric choke 600 cfm.
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  #28  
Old May 13th, 08, 8:45 PM
primernovaben primernovaben is offline
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Default Re: Edelbrock carb problem! 1406

the numbers from the rods can be deciphered in the manual on the calibration page. with my eddy i messed with the springs for the primarys and it made a huge difference. have you had the top of the carb off? if so i would make sure the rods are in the jets. i made this mistake, went to drive it and it bogged hard. also there are two vacuum ports on the front and when i had mine on the right hand side (looking straight at it from the front) it didnt like it one bit. other than that i would just play with the adjustment screws and feel the motor out on what it wants
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  #29  
Old May 16th, 08, 8:34 PM
bigsky bigsky is offline
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Default Re: Edelbrock carb problem! 1406

Ok, the verdict is in. The 770 cfm Holley corrected all the problems, no matter which way anyone worded it, stumble, power loss etc................. I bolted it on, the dual feed line leaked took it off, put it on, it leaked again, took it off, looked it over, nothing obvious, put it on, and no more leaks, did all the Holley adjustments they said to do on their DVD, which was great, went and ran it, it is a whole new car, unbelievable difference, very happy now, with no stumble, Jeff
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