Small Engines (Lawn Mowers, etc.): B&S 98902-2210, air filter foam, crankcase breather

Question
Hi,

I have recently acquired a secondhand walk behind mower with a B&S 98902-2210 engine. Turned out crank was bent and engine wasn't in great shape so I took it completely to bits, cleaned everything and replaced the crank, head gasket, crankcase gasket, bottom oil seal, primer bulb, governor springs, carb diaphragm, flywheel key and spark plug.

I've thoroughly cleaned the air filter foam and carb (soaked the carb in brakclean and blew it out). Now the motor is back together it can be started but only when I hold the throttle back at idle. When it runs it is popping out of the exhaust and it only runs for a short while before abruptly stopping with about two pops back out of the carb. I have to press the primer bulb and hold the throttle closed to start again and even then it is not easy to get it to catch.

Can you suggest any ideas about what my problem might be? I seem to be getting a good spark and have no problem pumping fuel into the carb with the primer. Compression appears to be quite good but I am wondering whether the crankcase breather may be partially contributing to my problem. Also, when I hold my hand over the air intake closing off about 2/3rds the motor seems to run for longer even though the exhaust keeps popping.

Very expensive to replace the plastic carb here (I live in NZ) and parts are expensive in general. Any help most appreciated as I'd like to work this out.

Answer
You darn Kiwi's.
You need a new exhaust valve.
The only way the exhaust can "pop" is if the valve isn't sealed when the fuel/air mix explodes.
First thing you need to do is recheck the flywheel key.  If you didn't get the crank tightened enough it will shear the key.  If the key is ok you have an exhaust valve issue.