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Guys can anyone give me some insight on adjusting my motorcraft 4bbl carb? I have cleaned it and installed a new kit. I have preset the idle mixture screws to 2 turns out from seated. Starts good with the choke and high idle, then when kicked down dies. Idle speed adjusted, idle then oscillates between 650 and 850 rpm, but gradually trends lower and lower. when I adjust the idle mixture screws out 1/2 turn at a time, the idle speed increases, but never gets lower no matter how far out I turn them, idle just gets higher and higher but still oscillates +/- 200 rpm. test drove and with accelerator pedal held constant, you can feel the car surging that same 200 rpm. I'm about ready to throw the carb in the trash. should I send it to a place like pony carbs and pay the $350 and have them recondition it? I want to run original if I can. Its on my 1971 Boss 351. Thanks in advance for any advice!
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Did you replace the accelerator pump diaphragm when you installed a new kit? If the diaphragm hasn't been replaced it may be leaking.
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Yes I did change the accelerator pump rubber cup, it was green in color and I cleaned the bore it rides against. Could float level cause the unevenness at idle or that surging at 30mph? I have almost the same carb on the car now which works perfectly. Only problem with it is the PO in his wisdom took a die grinder to the choke coil housing structure removing it clean off the carb and taking the butterfly out as well. Essentially no choke, which makes it hard to start, grinding away in the mornings. I bought this identical carb with the choke setup intact off ebay. Came off a 1972 Mach 1, 351c HO/CJ. Just cant get it sorted. I'm thinking about moving all the parts from the old carb over to this carb I'm working on, but i'm afraid I will end up with neither carb working good.
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Hey mlp40. While reading your first post I was thinking float level. Every time I've tried to fix a surge on a carb from the 60's and 70's 2 and 4 barrel carbs, it has been the float level or vacuum leak.(Usually the base gasket.) Often you will get a paper measuring cut out in the carb rebuild kit. On the original carbs you must bend the float to height. I think it's 2 inches but I'm not sure. Its been a while. Hope this helps.
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Hey I really appreciate your relating the same problem you have experienced to float levels. I will take the carb apart in the morning and really take extra care in setting the float height. This 4300 has that twin pontoon float and its pretty easy to get the two sides uneven.
Thanks!
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I took the 4300D off my car years ago. I had it rebuilt twice and came to the conclusion it was a piece of junk. I replaced it with a Holly and have never looked back. The tech that did the rebuild told me back in the day most of the state cops had these carbs taken off their cars for performance issues also. I know many purest will think I sinned but hey I like to drive my car and if changing the carb helps with drivability well so be it.