performance issues

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4

416duster

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I have a 1971 plymouth duster with a 416 stroker motor. At low speeds and normal driving it performs as expected, as soon as it is put under a load that is a different story. Under heavy acceleration the motor begins to pop and surge. I have tried the basics, plugs, wires, timing. Any ideas would be greatly appreciated.
 
demon 750, had 76 jets in primary and 80 in the secondary. went to 80 in the primary and 86 in the secondary. Got a lot more acceleration at the low end but still the same result and higher rpms.
 
That thought crossed my mind, its electric fuel pump holding at 7 psi. Is that too low for this type of motor?
 
7 PSI under load is plenty for any carb. So how are the float levels set? I'd suggest a "wet thread" setting on the secondaries. That is, fuel should be just wetting the bottom of the sight plug threads and dripping down the bowl face. Not a gusher, you should be able to contain it with a folded blue towel. What size needle/seat assemblies are you using? .110 (Holley 6-504) would be reasonable. Once you have verified both of these, and if you still suspect you are lean, increase secondary jets by 4 numbers. This is a massive change. You'll either see a massive difference, or this wasn't your problem.

Make sure you are getting full system voltage to the ignition system.
 
Was this a previously used and known good carb? What are your tune up specs? (timing curve, idle speed, etc) What are the specs on the engine?
 
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