From your symptoms it seems you probably have a blockage in the carburetor. If it had been too rich you should have a wet spark plug. You can verify that it is lean, not rich, by removing the spark plug and putting a spoonful of petrol into the plug-hole, then reinstalling the plug, reconnecting the lead, setting the controls in the start position, and pulling the cord. If it starts but will not continue to run, you have a carburetor fault, and if it isn't the diaphragm its most likely a blockage. The next step seems to be to take a look at it. Navigate to this page:
http://www.tpub.com/content/recoveryvehicles/TM-5-4240-501-14P/css/TM-5-4240-501-14P_103.htm
You can see earlier and later pages by using the "next" and "back" buttons near the top of the page. If you go back to 99 (back 4 pages) on the following few pages you can see how you should have gone about replacing the diaphragm.
If you have questions as you go through the process for cleaning and setting up your carburetor, just post them here. However, if you are able to replace a diaphragm, you probably won't have any problems following the manual. Please let us know how you go, even if you find a huge clod of dirt in the main jet and everything is fine after you remove it - we all like to know what happens, and we may be able to help.