• Re: rough mid-range and carb questions

    by » 12 months ago


    I set up the dual AFR gauge on temporary exhaust pipe extensions, and did some testing.  I made short videos of the gauge at various RPMs to roughly average the readings.  While interesting, I don’t believe these numbers are accurate.  They’re higher than reasonable to the point of being nearly impossible, particularly for the full throttle readings.  The gauge instructions warn that any air leak in the exhaust will cause high readings, and I think that’s what’s happening here.  The exhaust stubs are slip joints that aren’t clamped, and the extensions I put beyond that are butt joints attached with a pretty ill fitting clamp.  With the sensors only about 7” from the end of the pipes, that may be an issue as well. 

    A= 1-3

    B= 2-4


    RPM   2070   3050   3520   4040   5280

    A         13.8    12.3    14.4    14.9    15.7

    B         15.4    13.8    14.6    14.7    15.6

    Around idle, there seems to be more of a difference, and I do see that in the smuttiness of the plugs, and even the O2 sensors I used.  Above about 3500 they’re pretty consistent, though higher than expected, particularly for the full throttle readings.  EGTs will tell more in flight, and the carb rebuild did leave me with slightly leaner needle and main jets. 

    I'll fly it again and see what the EGTs are in flight, then decide if I need to make any changes.  I'm still leaning toward believing there's nothing really wrong with the engine.  Many people report roughness around mid-range, but I think my mount stiffness and airframe are amplifying this.  The only thing I can do it try to improve the smoothness, which is easier than starting over on the mount.  Next step will be the larger balance tube. I may go back to the Ignitech ignitions and taper the spark advance to a more proper level for mid-range.  Winter will dampen my enthusiasm for testing though. 




  • Re: rough mid-range and carb questions

    by » 11 months ago

    FWIW, I increased the size of the balance tube to AN12, so the hose is 3/4" ID, but the fittings are only 5/8" ID.  I got really excited while it was warming up on the first run.  Idle and anything below 3000 is just spooky smooth now, compared to the stock (about 3/8" ID) balance tube.  Unfortunately, the 3500-4000 vibration is still there, though at least somewhat improved.  For now, I'm declaring this as good as it gets, and I'll attempt to ignore the vibration. 



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