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Im hoping someone can help me out here. I have a 400 chief with the JLO motor in it. If i have the motor running and connect the header pipe to the muffler the engine dies down. It can be kept running by turning the idle up. I cut the muffler open to see if it was plugged up but i could not see any obstructions. Are the mufflers supposed to provide some restriction or should the be pretty free flowing. Also it seems like its loading up alot.