I'm currently trying to finish up my truck after a lot of mistakes and disappointments. One of the issues a trying to finlaize is the correct carburetor. I had an Edlebrok 1406 600 cfm carb on the truck. It started running really rich and the gas flow into the carb was just spilling into the carb. My engine is a mild built SB 350, 30 over, 190 valve heads, a 280 comp cam with Holley dominator intake producing about 300 hp. At the recommendation of my loacl speed shop owner, I replace the carb with a Holley 4160 450cfm 4 barrel carb. The theory I was old was the engine is an air pump that can only push soo much air and burn soo much fuel. Any additional air is a waste and fuel will not burn seemed to make sense but since I put the carb on I have a flat spot or hesitation spot in exceleration. I've adjusted the secondaries and fuel pressure but can't get rid of the flat spot. The speed shop is now recommending a distributor upgrade with a coil. I'm not sure if this is needed or if the carb just needs to be upgrade to a 600 cfm again. Any thoughts would be appreciated.
Sounds to me like your acclerator pump either isn't pumping, or isn't pumping enough. a common problem is the float bowl gasket. sometimes it plugs or partially blocks the opening for the accelerator pump. you are correct, an internal combustion engine is just a big air pump. the more air you can move in and out, the more HP it will make, up to a point. it wouldn't hurt to run a 600 CFM carb either. with that 450 CFM carb, I would be concerned about running lean, which is just as bad as running fat. run it for awhile then read your plugs, they will tell you alot about whats happening in the cylinder. sounds to me like the Dude at the Speed Shop works on Comission!! I have always liked and ran Holleys, but the Edelbrock carb is a good carb. when properly tuned. I don't have enough experience with them to be of any value to you, but I would check the needle and seat for debris before shitcanning the Edelbrock. that is a very common cause of flooding. just my non-sensical ramblings from a lunatic mind Vance
600 cfm and a sbc are a perfect match. check for the correct float setting. ive been told that those edelbrocks are notorius for incorrect float settings out of the box...
I concur. Check float levels first. You are probably making less vacuum than stock and just need to swap out your step-up springs. Also, the manual that comes with Edelbrock carbs has all kinds of tuning info in it. I had to buy a book for the Holley's
Jetmek is right! Same for Carter brand! Fuel pressure of 4-5 pounds works good for me. Give that 600 Edelbrock a second chance! Frank
Check the gap in the accelerator pump lever adjust to just touching the pump lever at idle but not pushing down on it that will probably cure it
That 450 cfm sounds a bit small for a 300 horse motor. It's easy to run too much fuel pressure on the Edlebrock and get just the problem you talked about.
Oops, I was thinking Edelbrock advice. If you're going to run a Holley- I'd suggest the tuning kit. It comes with the cams and nozzles to change the timing, duration, and strength of your accelerator pump shot. We had an SBC that would go lean (backfire through carb) with throttle stabs. I put a bigger nozzle and quicker pump cam on it- it cleared right up.
Thanks, consensus seems to be the accelerator pump and fuel pressure. I'm going to try the edbrock again before buying another Holley
Before you open up the Edelbrock to check float levels, check the metering rods to see if they're straight. They're easy to bend and I got a new 500 cfm with a bent metering rod that stuck. Symptoms were same as you're having with your Edelbrock. It was a quick and easy fix. Be careful when you put the metering rods in, don't force them. Push the metering rod/piston down with your finger and then slide the retaining plate over it and then screw it down while you hold the plate in place.
I had an Edelbrock 500 cfm carb that never ran correctly right out of the box. You could hear the fuel spilling over into the motor causeing it to run super rich. First thought was the float levels. So the only to set the float level is to open up the top of the carb, which voids the waranty. Oh how nice. Reset the float level to the lowest recommended setting, still ran like shit. Oh yeah one of the screws on the top of the carb was tightened so tight that the screw head was damaged to trying to open it. All the screws were torqued down differently. The motor wouldn't start if the vehicle was pointed slightly up hill, got sick of the damn thing and waiting on hold for the tech lines for over an hour that I traded it off for $50 worth of shit. just my experience, but I am nowhere close to a professional carb tuner. I figured the top of the carb was warped and wouldn't seal correctly but no warranty since I open it up to reset the float level. Geno
And get the hell away from that speed shop guy--he obviously intends to fix the problem by selling you a whole new car in installments rather than by actually thinking about the problem. He's not your friend, he's your parasite!
450 cfm is way too small for a midly built 350. CARB CFM = so lets say 355 cubic inches times 6500 rpm divided by 3456 equals 667 cfm. Multiply that number by a guess at your volumetric efficiency, lets guess 90%, and you get 600cfm. I have always run a 650 carb on a mild 350 and worked out about right. Edelbrocks are notorius for playing games out of the box. I had one start for the first time and spew fuel straight out of the box. When I contacted the parts store where I bought it they told me to hit it with a hammer to unstick the float. In the end it ran good after some tinkering but like Geno said, break the seal...no warranty. You need a 600 or 650. Really depends on how high you rev it and your VE, but either way 450 is too small. Holley or Edelbrock can both run good, just your preference here. You already have the edelbrock so you are money ahead to strap it back on and tune it. Just my .02
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