gy6 150cc with performance exhaust back fires

[tkrocks2]

I have just got my first gy6 150cc scooter I have had many scooters in the past but this is my first chinese scooter.

When I bought the bike it had a cadillac converter in the exhaust so I removed the stock exhaust and put a free flowing performance exhaust on it along with a perfermance cdi box and coil.

The engine back fires sometimes when I let off the throttle a friend of mine told me it sounds like it is sucking in air form some where.

I removed the exhaust and where the exhaust bolts to the cylinder head used some exhaust sealent I'm waiting to see if that fixes the problem.

I'm not sure what else I can do if that is not the problem.