I recently acquired a 2004 model Murray rider with the afore mentioned engine.The problem is this, when idleing it runs snooth, advance the power and it backfires through the carb. I checked and changed the flywheel key( had the wrong key installed and was partially sheared), cleaned the carb, changed the plug, re-set coil to flywheel gap. I have been reading here and suspect the valves need adjusting. Anyone with other suggestions?

Hi,
You certainly could set the valves to 0.006" per the YouTube videoYouTube - ‪Lawn Mower Repair Valve Adjustment‬‏ (http://www.youtube.com/watch?v=NjzX-PUjKmo)
The wise thing is to rotate the engine and watch the rocker arms. They should move about the same. There were a bad batch of camshafts about that time.
Peace,
Clarke

Thank you Clarke. So if I understand you correctly, excess valve lash can cause backfire through the carb?

Hi,
No the bad cam is more likely to do it because the exhaust lobe wears down and you get normal intake and reduced exhaust so that air/gas mix enters and but cannot leave well.
Peace,
Clarke