I have a 1986 honda 250ES big red. The motor idles perfect, but when I hit the throttle it cuts out. I have cleaned and rebuilt the Carb (float and float valve, choke valve). Compression is 180 psi with no leakage. I put a new plug in it and I noticed that after just a short period of run time (5 minutes) it is black on the end and smell like gas. Issue has been progressive. For last couple of years I have had to run with choke slighly on. Appears to have good spark but have not checked the timing. Any idea's?

 

Have you checked it for vacuum leaks? A cracked intake manifold or boot, bad O rings in the flanges, loose bolts, etc?