First, think of the Air/Fuel mixture ratio as the extra gasoline (running rich) cools the combustion chamber down. Running lean offers no cooling effect.
On the mower deck, it "sounds" that something didn't go back together quite right. If you could post a picture(s) of your deck, that would be a tremendous help for us to answer your question(s).
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Roland Bedell
CC Models: 100, 105, 1450, 782, (2) 784, & 2072
Buy: Made in the USA