carb question

i have a Holley 650 dual feed carb on my 66 with a 289 motor. My exhaust tips always look black and I was wondering if it is too big of a carb for this motor. I also only get about 12 mpg. Any help would be appreciated...  

its probably running too rich and a 650 dp is too big for a stock 289. a 600 cfm vacuum secondary would be the biggest i would put on a stock 289 and it would be a holley 1850 with the plain secondary spring. sounds like the carbs jets are too big i would replace with the 1850 or even better a 550 cfm.  

Is your engine stock, cam , distributor etc? If stock was the engine a C code or A code, k code, the 650 carb can be rejetted to calibrate it for your engine.