Thanks, at that link I found out the only difference between these two plugs is the "5" is a colder plug. How will this affect operation?
I just found out the filter and plugs were changed 4 years ago which was about 170 hours of mowing and about 20 hours of blowing snow. I wonder if the air filter was plugged?
By "adjust the valves", do you mean the clearance of the rocker arms under the valve covers? I think I did that about 135 hours ago but I'm not sure. Would you hear them "clacking" if they need adjusted?