Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

Help!
Last week a  friend gave me his 2011 Ariens snow blower.He told me it backfired a few times and he couldn't start it after that.  It's in real good shape but the engine would not start and was hard to turn over.

I removed the valve cover and found the aluminum  push rod on the exhaust side bent and out of place.After removing the head I then removed the valves and found that the valve guide on the exhaust side
was almost completely out  it's sleeve.The intake side push rod was also bent but the guide was fine. .I pressed the guide back in place replaced the two push rods and installed the head with new gasket.
After adjusting the valves at top dead center, I put everything back together and started the engine.It fired up right away and ran fine.After blowing a bit of snow I turned down the throttle to idle and the engine backfired through the muffler.
It started easy again at full throttle and I backed it off slowly and turned it off.That was yesterday. Today I took the valve cover off to check and the valve guide was out again and the push rod on the exhaust  side was bent again.
Any ideas on what is happening would be appreciated.