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mikuni 36mm big bang

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Hope someone can advise.

just fitted a mikuni 36mm pumper carb to my xr400. start up fine, tick over fine, slight throttle inputs fine, but as soon as twist the throtlle over about a 1/4 turn when backing off massive popping (explosion) accompanied by big blue flame exiting the exhaust. not good. thought it may have been the pumper unit, but isolated that by holding the nylon cam so no pump action, same result. now at a loss what to do.

all help appreciated

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What kind of intake/exhaust mods have been done?

Do you know what jetting is in the carb currently?

What's your elevation?

Sounds like too lean OR an air leak. Be sure the front and rear carb boots are sealed good around the carb inlets. Be sure the oring between the intake manifold and head is there and in good shape.

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