Finally have my 71 running after 38 years of storage.. New motor, trans and everything else. First few hundred miles - it ran OK but did not start well when hot and had a slight hesitation upon initial throttle when cruising. I read a lot and tried to read the plugs. I saw dark soot on the 'ring' an figured it was running rich. Also easy cold starts without choke made me think I was running rich also. I purchased a K&N gauge and sensor kit and a bunch of holly jets. I installed the gauge and sensor and went for a test ride.. The gauge barely responded like it was very lean. What?? This made no sense as I thought it was rich. I did not trust it so I attached a voltmeter to the O2 sensor. Now I am getting readings. It did look rich so I went down 2 jet sizes from 67 to 65.. It ran worse. The existing issues were multiplied. After more reading about this I realized that the voltmeter says it was lean as at cruise is sometimes was as low as .2 volts after the jet change. I tried the richer 66 jets and it showed improvement but not a final solution. This confirmed I have been running lean all along. I installed a pair 68 jets.. Yahoo! What a difference. It feels better than it ever has. It is amazing how much difference one size makes!. Once I cleaned off the original cork gaskets and installed the new rubber ones, it takes only minutes to swap jets. I will probably try a pair of #69 jets to see what happens. I have read a lot about using an O2 sensor for carb tuning. .4 volts is 14.7 AFR and is not really the goal with a carb. Shoot for higher than this up to .7 when not at WOT to make sure you are never lean. You want .8-1 volt at WOT. The motor only has 500 miles on it and I have not wound it out yet. Will probably remove the secondary power valve and jet the secondaries next. NOTE: My K&N gauge is junk and does not work properly but the O2 sensor from the kit is good (I tried 2 and they read the same). The voltmeter did the trick for sure using the 2 volt setting. Tips: i purchased a Fitech clamp-on O2 sensor mount from summit for 15 bucks.. Drill a 3/4 hole in collector and clamp it. (make sure it's angled down (wire up) 10 degrees or more to avoid moisture issues. Worked great - really easy install and no leaks! I was driving around with the meter connected and glanced at it often during different throttle openings. I jetted made to be sure I am not lean, ever (.4 volts or higher). My car has a t56 with a .5 overdrive and 4.10's. Tires are 25.7" and it runs about 1700 rpm @ 60 MPH in 6th. Love it. I have been thinking about going with injection but now expect I may not. I just installed # 69 jets. It is even better.. O2 voltmeter readings stay within the .5 to .7 range under part throttle and it's running awesome.. smoother, more power and 0 hesitation... Happy with my Holly! 650 DFDP on a 355 sbc 11:1 medium cam good old 2.02 >plug iron heads.