Try a new set of plugs. While doing that check the contacts in the plug boots for corosion. Plug wires are also suspect. I have seen several cases of misfiring that leads to the exhaust filling with unburned fuel air mix. Then when firing resumes..pow!
Accelerating puts a higher requirement on the ignition so misfiring may occur then.
Tell us what the color and condition of the plugs are.
I don't mean to rule out a lean condition that peteyz24 suggests.
Did this problem occur coming out of storage or after some period of clean running this season? Gum/ varnish formation during storage can partly close jets causing lean conditon. This is serious but usualy does not develop while riding. It can quickly lead to enging failure. Another lean problem that can occur while riding is cracking of the carb boots.
Anyway first try new plugs and verify the wires and boots are good.