Not sure on the gap. i bet you could call Horizon and they can give you the answer on that one. I was going to suggest you clean your screens. If I am not mistaken that particular carb has 3 screens. 2 of them you can see and the other one is hidden (very small). The carb overhaul kit comes with the screens I believe. However if you take some clean fuel and some compressed air when you have the carb apart you can blow on the backside of the screens to remove the debris. The small one you must back-flush with fresh gas. It is a chore to remove and re-install the screens so if you can clean them and blow them out from the backside it might be better for you.

Also on a side note. If you are running in-line filters on your fuel hoses they need to be cleaned as well.

I was thinking as well, If your fuel tank has a clunk/filter make sure it did not come off as this will allow the fuel line to "float". The fuel tubing has a tendacy to work harden on the neck of the clunk and can break off leaving the line free to float above the fuel level.

Looking at the picture of your spark plug the engine appears to be in good health. I would, after cleaning your carb make sure the needles are set properly using a good tach.

Glenn