May I please receive help in solving an engine problem? I have a Briggs & Stratton 33R877 engine (ride mower), and it is blowing air out of the carburetor. Here is what I've done so far:

  1. Piston rod breaks/shatters and the engine dies

  2. Took the engine apart, replaced piston rod, cleaned out the engine case

  3. Put engine back together, camshaft and crankshaft are aligned (lined up dots on both gears), oil splasher/governor installed, oil pump inspected, new gaskets installed. Installed engine head back on, checked valve faces for any possible damage (none seen).

  4. Installed rocker arm pusher rods back into place (steel rod with exhaust valve, aluminum rod with the inlet valve)

  5. Installed engine back into ride mower, charged battery

I've noticed the following:

  1. Electric motor can't turn the crankshaft, there is too much resistance

  2. Manually rotating the crankshaft seems OK at first, but as the piston begins compression, it becomes difficult to turn. At the start of compression I can feel air getting sucked into the carburetor as I would expect. It is at the end of this compression that I feel air coming out of the carburetor. The crankshaft freely rotates for about 75% of the time, and is difficult for about 25%.

This seems like a timing issue with the camshaft or rocker arms, as air is coming out of the intake valve and back out of the carburetor (it seems like the inlet valve and not the exhaust valve is opening as the piston comes back to TDC). Adjusting the rocker arms doesn't fix it, and a second engine tear down and inspection reveals the camshaft and crankshaft are aligned.

My question is, with all of the above, what is going on? I'm at my wits end, something is misaligned or broken and I'm not seeing it. If you need me to inspect anything to provide more data, please let me know.

Thank you!

  • 2
    At the start of compression, the valves should be closed. You should not detect air entering the carb .
    – fred_dot_u
    Dec 31, 2022 at 9:52
  • It sounds as though you have the engine out of time. Go back and check your work.
    – jwh20
    Dec 31, 2022 at 11:32
  • Welcome to Motor Vehicle Maintenance & Repair! Dec 31, 2022 at 12:40

1 Answer 1


It sounds to me as though the intake valve is sticking open. If the cam timing is set right, the only way air would be coming back out of the carb is if the intake valve isn't closing all the way or closing quick enough. Several things to check:

  • Intake valve is not bent.
  • Ensure the intake valve is on the valve seat correctly when closed.
  • Spring on the intake valve is up to the task.

I'd still go back and double check cam timing.

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