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Old 07-19-2007, 03:19 PM
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Flat spot

I have a Edelbrock 1406 carburetor and it has a flat spot. I am used to Holleys and have no problem figuring them out.
When you first give it the gas it has a flat spot(hesitation). I have increased the base timing, rebuilt the carburetor, changed the vacuum advance from full vacuum to ported vacvuum and back again, adjusted the accelerator pump linkage both ways with no change. When you give it the gas I get a good shot from the squirters, above idle when you nail the gas the engine accelerates clean with no hesitation.
Does anyone have any ideas? I'm thinking that the distributor advance might have something to do with my problem. It is a points distributor converted over to electronic with a Crane conversion The Engine is a .030 over 283 SBC, Edelbrock performer RPM intake manifold. fairly wild camshaft(don't remember the specs), 9 1/2 compression no exotic heads and Lake style headers.
I have checked out the Edelbrock manual on the carburetor and it didn't seem to help any.
Thanks - Jerry

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Old 07-19-2007, 03:55 PM
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Move the throttle very slowly and check the pump shot. they should shoot the very instant you move the throttle. I used to have a holley that had a problem like this, if you moved the throttle quickly no hesitation and a good shot, if you moved it slowly a bad bog and no shot. Another engine had a bad hesitation, yet the carb and everything else was great on it, it had this annoying dead spot at always the same spot that wouldn`t go away. Come to find out the intake was warped and it had a vaccum leak, funny thing was, it never used a drop of oil. and there wouldn`t be any signs of oil in the plenum when the carb was removed. I checked the top of the intake with carb cleaner and found no leaks there either. But the signs were there, certain header tubes were white and higher than normal tempatures but it didn`t overheat. I removed the intake and took it to my machinist, he placed it on a straight sander, and found the warpage, he then milled it .010 and it didn`t clean up, he milled it another .010 to make it .020 and the warpage was still there, I threw the intake on his scrap pile and bought a new one.
I`d recommend you check it very closely for vaccum leaks.
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Old 07-19-2007, 05:18 PM
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How much manifold vacuum at idle? (how big is the cam)
Is this an automatic or manual trans? What is the converter stall speed?
A 283 is going to be more critical to converter stall rpm than a larger motor, especialy with a big cam. (more is better)
Unlike a holley carb, a Edelbrock carb only idles on the primary barrels.
The secondary throttle blades are completely closed and do not contribute to the idle. So secondary fuel flow tends to lag on the secondary side a little as the sec throttles crack open and the air valve starts to react. There are little fuel feed orifices that give a shot of fuel as the sec air door opens. Make sure these small fuel orifices are not plugged. Use compressed air to give them a good blast. Keep your face out of the carb while you're doing this.
I find Edelbrock carbs need a lot of initial advance to idle cleanly and respond to WOT from an idle. if you lil 283 has a big cam with a "rough idle" give er all the initial it will take. You may even have to lock out the distributor and run full timing (32 -36deg) at idle if the cam is a biggie. The carbs metering rod step up spring must keep the step up piston down (lean) at idle.
use a vacuum guage and select a spring rated to close at a vacuum
that is lower than manifold vacuum at idle. eg: if you have 6" manifold vacuum @ idle use a 4.5" step up spring.
Edelbrock carbs are designed and calibrated to work with a functional PCV system. Make sure yours is hooked up to the base of the carb and the other valve cover is vented.
Last you may have to change the size of the acellerator pump squirter for more initial fuel shot.
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Old 07-20-2007, 03:48 PM
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Flat spot

Here is more info - idles at 13" vacuum - don't remember the cam specs(if I remember right it is around .501 lift on flat tappet cam) - 4 spd trans - One thing I just noticed is that if I close up the choke about 1/2 way and than give it the gas the engine revs immediately(no hesitation) does that indicate the need for more pump shot or adjustment to the rods/jets?
Thanks for your help - Jerry
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Old 07-20-2007, 04:50 PM
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Yes, you have a jetting (lean) issue......Or, the primary springs are a little weak for your combo.
Steve
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