Small Engines (Lawn Mowers, etc.): briggs&stratton engine

Question
QUESTION: This is a b&s 12.5hp I/C 2003 engine it will not start we have stripped it down could not find anything wrong with it rebuilt it still the same it blows the fuel back through the carb we have squirted fuel through the carb and it does try to run can not see why it blows it out instead of sucking it in

ANSWER: When you say you "rebuilt" it what parts did you replace?  Did you remove the cam shaft, valves and crankshaft?

Eric

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

QUESTION: This engine was blowing fuel back through carb that is why we stripped it down we built it back up grinding valve in and replacing gaskets and oil seal we set valve clearance when we came to start it again it was the same blowing fuel out of carb . We removed valves camshaft and piston but not the crank. The engine would not fire before or after the work on the engine .The engine is in a Stiga ride on mowewr and had been stood in a building for 3 months over winter I started it up and drove it into workshop to work on cutting deck I did the bed come to start engine and it would not start on inspection it was blowing fuel out of carb that is why we stripped it down. I have MS and use the tractor to get about as I cant walk without crutches and this enables me to get about and do small jobs in my workshop thank you Garry

ANSWER: Were there any dots or lines on the camshaft and crankshaft for setting the cam/crank timing?

What are the valves set at?

Is the flywheel key sheared?

Eric

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

QUESTION: There are dots setting timing they are spot on valves were set as spec sheet and there is a key way in place
Garry

Answer
Fuel coming back through the carb can only be caused by mechanical timing, the intake valve is open when the piston is coming up the cylinder or possibly ignition timing, sheared flywheel key.

If the mechanical timing is correct (timing dots and valve clearance) and the flywheel key is not sheared (ignition timing) then no gas should come back through the intake valve.

Did you lap the valves before installing the head?  Is the intake seat bad?

Eric