The first time that I get a running cub, I put some Marvel Mystery Oil in my fuel mix along with some HEET (just a little HEET.....maybe 1 oz. per gallon of fuel) and add that to the tank. Then work on firing it up. I have found that this cleans the gum left over from old fuel out of the lines and carb. The Marvel Mystery Oil will also clean a little and help lubricate the upper engine. This probably isn't as effective as Merkle fuel, but it has always worked well for me and given great results. Besides, the Merkle fuel formula is a well guarded secret!!!
Cub Cadet 123