My stock carb runs great when cold for about 2-3 miles and then it falls on it face from idle to 2500 rpms. Idles perfect and full throttle is good just a bad stumble after warm up. Any ideas?

I was wondering about the exhaust myself... Would a collapsed inner pipe affect the emmissions at all?  The guy i bought it from had just taken it through emmissions and it passed with flying colors.  he said the guy hadnt ever seen anything that old running that clean.