Fire in the escape pipe

Hello people, I have a problem with my 1600 fusca 79.
When the accelerate put the multiple pipes and start to leave fire through the escape pipe. The engine was rectified from Cigûeñal, bolt rod, oil pump and the piston and hoop cylinder kit, but when it goes out for a while it goes out and does not turn on a spot that spends an hour and something.
I would appreciate any suggestion.

Compa, two points ... or you have the escape tube partially covered or you have gas left over the admission.