400ex backfiring

[foreman 450]

When I fire my ex up when its cold it wants to backfire back threw the carb pushing the plunger up against the metal cover on top of the carb. And its hitting it hard enuff to bend it up.

Is this normal or is there something wrong that I need to adjust?

[Budweiserebel07]

I dont know, just an idea, but could it be your exhaust valves arent opening enough ?

[NMills]

Have you done anything to the machine recently???

[foreman 450]

I put in a stage 1 cam few months back.But it was doing the samething before I did that.