Carb Wiz need your help

I'm running the Edelbrock 1405 600cfm carb on my 351C 4v engine. Floats are properly adjusted and fuel pressure set at 5.5psi, actually needle fluctuates 5-5.5psi. The problem: When @ idle and either left or right side idle mixture screw is lightly seated, the engine continues to run. Engine will bog down, but will not shut off. I was told fuel pressure could be high, but verified with a fuel gauge and all is good. Also float level not properly adjusted, but checked with 7/16 drill bit and all is good. What else should I check for? The engine should shut down right if any of the idle mixture screws is lighly seated?

Installed a like new 1407 750cfm carb and same thing, so what could it be?

Thanks,

DJ

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Something is allowing air into the engine, vacuum leak, improper throttle linkage setup/adjustment, make sure the prim. and secondary throttle plates are seated/closed completely.Also it is possible that it is pulling some air from the PCV assuming you are running one plus around the throttle butterflies.Just a couple of things to consider.

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How much manifold vacuum do you have at idle? If it very low (less than 9 in/hg, large overlap cam) the step up springs may be too stiff, allowing the metering rod to move off of the lean step. A step up spring assortment is available from Edelbrock, PN 1464. Does the engine perform OK and idle OK? If so, it may just be an Edelbrock quirk. Chuck

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Something is allowing air into the engine, vacuum leak, improper throttle linkage setup/adjustment, make sure the prim. and secondary throttle plates are seated/closed completely.Also it is possible that it is pulling some air from the PCV assuming you are running one plus around the throttle butterflies.Just a couple of things to consider.

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Called a mechanic and he said idle mixture screws on edelbrocks will cut out fuel to carb and that's why engines die when lightly seated. So it's getting fuel from somewhere, but without seeing it, he couldn't tell me. What do you think?

How much manifold vacuum do you have at idle? If it very low (less than 9 in/hg, large overlap cam) the step up springs may be too stiff, allowing the metering rod to move off of the lean step. A step up spring assortment is available from Edelbrock, PN 1464. Does the engine perform OK and idle OK? If so, it may just be an Edelbrock quirk. Chuck

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Chuck,

I got steady 12hg @ 900rpm in park. Other than very very poor mileage, the engine performs and idles ok, but still think it could be better. I ordered a leakdown tester for the low vacuum reading and that should come in soon. Could low vac have anything to do with the idle mixture screw problem? Shouldn't the engine still cut out when either screw is lightly seated?

Thanks,

DJ

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this is primarily caused by the throttle being opened too far at idle,exposing the transfer slot to engine vacuum,thereby overriding the idle mixture screws. usually caused by vacuum leak or other tuning issue causing the need for an excessive amount of throttle to achieve idling.Try dropping your idle to 650 may help. Then adjust mixture and re-adjust idle speed.

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this is primarily caused by the throttle being opened too far at idle,exposing the transfer slot to engine vacuum,thereby overriding the idle mixture screws. usually caused by vacuum leak or other tuning issue causing the need for an excessive amount of throttle to achieve idling.Try dropping your idle to 650 may help. Then adjust mixture and re-adjust idle speed.

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I think I understand what you're saying so I'll try it and see what happens. Thanks,

DJ

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this is primarily caused by the throttle being opened too far at idle,exposing the transfer slot to engine vacuum,thereby overriding the idle mixture screws. usually caused by vacuum leak or other tuning issue causing the need for an excessive amount of throttle to achieve idling.Try dropping your idle to 650 may help. Then adjust mixture and re-adjust idle speed.

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I think I understand what you're saying so I'll try it and see what happens. Thanks,

DJ

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I would check your Carb. base gasket first if you changed carbs and it did the same thing. http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

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I think I understand what you're saying so I'll try it and see what happens. Thanks,

DJ

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I would check your Carb. base gasket first if you changed carbs and it did the same thing. http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

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Thundertc64,

You were right! I brought the idle down to 650rpms and then lightly seated both idle mixture screws one at a time and the engine bogged out, thanks! You got me reading up on the carbs fuel circuits and found out that at 900rpms, the air/fuel circuits are bypassed and moved on to the next circuit and thus the reason why the engine wouldn't bog out/shut down when the mixture screws were turned all the way in. I retimed the engine from 8* to 18* BTDC w/ 2 and 1/4 turns out on both idle mixture screws and got 16hG steady vacuum. I don't know if 18* initial with around 38* at 3000 is too much, but the engine idles better and no pings. Starts up easy even when hot and fuel gauge now reads 5psi steady and the needle doesn't bounce around like before. Too late to take her out for a spin, but will do it first thing tomorrow morning. Do you think 18* is too much for my almost stock motor?

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I would check your Carb. base gasket first if you changed carbs and it did the same thing. http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

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Thundertc64,

You were right! I brought the idle down to 650rpms and then lightly seated both idle mixture screws one at a time and the engine bogged out, thanks! You got me reading up on the carbs fuel circuits and found out that at 900rpms, the air/fuel circuits are bypassed and moved on to the next circuit and thus the reason why the engine wouldn't bog out/shut down when the mixture screws were turned all the way in. I retimed the engine from 8* to 18* BTDC w/ 2 and 1/4 turns out on both idle mixture screws and got 16hG steady vacuum. I don't know if 18* initial with around 38* at 3000 is too much, but the engine idles better and no pings. Starts up easy even when hot and fuel gauge now reads 5psi steady and the needle doesn't bounce around like before. Too late to take her out for a spin, but will do it first thing tomorrow morning. Do you think 18* is too much for my almost stock motor?

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Glad it worked out, It would depend on what type cam you are running in your Motor, A engine with a high performance camshaft designed to create power above 3000 rpm will respond well to 18 degrees of initial timing because the air/fuel mixture is not uniformly mixed at lower engine rpm, so the additional initial timing allows more time for this air/fuel mixture to burn in the cylinder.here are a couple of starting points 10 to 12 degrees of initial timing when the camshaft duration is less than 220 deg. @ .050 valve lift, 14 to 16 degrees of initial timing with less than 240 deg @ .050 and 18 to 20 degrees of initial timing with a cam with less than 260 @ .050 valve lift. The total ignition mechanical advance from the distributor must be reduced when you increase the initial timing. but you can adjust to suit your needs and what works best for your engine

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