you may not beleave this story. but it is true. my friend had a 1969 full size van. with a slant 6 225. it had a one bbl holley carb. it would not idle. it keept dieing. and then pump the throttle, and black smoke would roll. i took the carb a part. and i drilled out the idle jet about .003. and made a fuel well plug. put it back together. it ran so good we got it to idle at 250 RPM. very smooth. we could count the fan blades. course that wasnt the curb idle RPM. true story.