Is it a new or used carb you put on it? Sounds like it may be a bad carb. Is gas leaking out if you just leave it sitting overnight not running or only when running? If always, float needle is sticking in carb. Otherwise it might be a bad or missing o-ring on the main jet as a possible guess. Just curious - where are you located? Not likely, but if close by I could look around garage. I think I have a 6.5 Tec. You cannot have it for keeps, but you could try the carb to see if yours is bad.
Couple other thoughts - If no one here knows you might check out this forum. One of their moderators worked with Tecumseh for years and is a real expert on those engines
http://www.perr.com/phpBB3/
(hope giving you a link is not a forum rules violation - if it is - sorry)
Lastly if too expensive or time consuming to fix, a Honda clone 6.5 is pretty cheap and depending on clearances on your cart, should be a mostly drop in replacement. You would need a longer Tav bolt to bolt it to the engine if you have a torque converter and a bicycle brake cable and a little hardware to juryrig a new throttle set up.