Small Engines (Lawn Mowers, etc.): backfire, backfire

Question
QUESTION: I have a 16 hp vanguard B&S 303447 type 1068 engine that backfires at high speed, low speed and sometimes under power.When I first push the throttle for more power it will hesitate and some times under full power it will hesitate. Could it be the needle valve under the float? Or What else could it be.The breather tube from engine to carburetor had cracks in it so installed a new one,but it still backfires.. Thanks for any information.

ANSWER: Is it backfiring through the carb or the exhaust?

---------- FOLLOW-UP ----------

QUESTION: Through the exhaust.

Answer
The three most common causes would be;  A carburetor that's loading up and raw fuel is firing in the muffler.  An exhaust valve or valve seat that's leaking or a seat that has come loose.  And a flywheel key that's sheared.

Probably I'd start with checking the flywheel key, then rebuilding the carb and lastly checking the valve motions and removing the head.

In the shop, I would do a leakdown test before pulling the head but you need special tools for that.  I think one of these three will be the problem.  Other things might be a blown head gasket or cracked head, loose head bolts, and a bad valve spring.

Let me know if you have any questions.  Thanks, PK.