Tuning Holly 4150 with O2 sensor

Discussion in 'Troubleshooting & Diagnosis' started by 71RS6SPD, Oct 2, 2017.

  1. 71RS6SPD

    71RS6SPD Veteran Member

    Nov 14, 2016
    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.
    Last edited: Oct 2, 2017
    Goodwrench1965 and PalmbchZ28 like this.
  2. Joekool

    Joekool Veteran Member

    Feb 4, 2002
    Hollister CA USA
    Now try tuning your timing curve, you'll be surprised how the O2 sensor responds from timing tweaks.
  3. SRGN

    SRGN Veteran Member

    Feb 20, 2009
    Central NJ
    If you really want to get the tune right, use a wideband O2 sensor. The narrow band units are most accurate at stoichometric, and accuracy is highly dependent on sensor temperature as well. We tuned by narrow band O2 in GNs for years, but once you use the wideband it is a noticeable difference.
    Goodwrench1965 likes this.
  4. 71RS6SPD

    71RS6SPD Veteran Member

    Nov 14, 2016
    I will take that advice.. from both of you! Need a new O2 gauge anyway - been driving around with a voltmeter connected. Geez.. just checked costs. Wide band sensors are high! Need to wait for dash plans - building a new oak panel for gauges this winter. The only remaining issue is a slight hesitation on throttle at cruise. I ordered larger tube type squirt nozzles. it has 28's in it now. I still have a fitech injection system in my head but will give the holly till next year to make me happy.
  5. wiseryder

    wiseryder Veteran Member Lifetime Gold Member

    Sep 11, 2004
    belding,mi USA
  6. 71RS6SPD

    71RS6SPD Veteran Member

    Nov 14, 2016
    So I got it pretty close - just a final tune on the pump cams when I get them in a day or so. I also ordered a new wide band gauge and sensor to see how much better I can get it. Plus I have not really done WOT tuning yet although it feels pretty good. I ordered an RPM air gap and will install in a week or so. I know more tuning will prob be required as I am switching from a single plane. BTW - the first new manifold I received is junk. Plenum not in the center of the carb mount holes. It's off centered over 3/16"! Sort of shocked at the lack of proper inspection by Edelbrock. RPM Air Gap.

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