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Thread: KHK rear cylinder running rich

  1. #1
    Join Date
    Sep 2015
    Location
    Central Illinois
    Posts
    161

    KHK rear cylinder running rich

    My '56 KHK rear cylinder is running significantly richer than the front. The front spark plug is clean as a whistle; whereas, the rear plug is black with soot. There is soot on the inside of the tail pipe.
    1. Does this sound like a air leak in the intake for the front cylinder? If that is the case and if the bike is tuned looking only at the front cylinder, that might explain what I am finding.
    2. If there is no leak, should I tune for the richest cylinder and let the other one run lean?
    3. Should I leave tuning "as is" and run a hotter plug in the rear? This might burn off the material that would otherwise accumulate on the rear plug.
    George Tinkham
    Vintage Iron Riders, AMCA, AMA, ABATE of Illinois, MRF, Springfield Milers BMW MC
    www.virmc.com
    '56 H-D KHK
    '60 H-D CH
    '42 Indian 841
    '48 Chief
    1 each of German (R69S), Italian (V65Sp), & Japanese (150 Touring Benly)

  2. Re: KHK rear cylinder running rich

    Strange to see such a difference between 2 cylinders fed from the same carb!!!

    An intake manifold leak would lean both cylinders...

    I would suggest investigating the front head.

    It might have a crack, not unusual on a K head.

    That would only lean the front...

    Once that's sorted, if that's the problem, the whole plot will need leaning a bit...

    Patrick

  3. #3
    Join Date
    Sep 2015
    Location
    Central Illinois
    Posts
    161

    Re: KHK rear cylinder running rich

    After Bonneville next month, we shall check the compression and head for evidence of leaks. Tom Cotten suggests doing a manifold pressure test. I have switched the front & rear spark plugs, and the rear still gets soot. The front now is clean. This tells me the plugs are fine. The only problem with the bike's running is that it may be a bit harder to start -- now, it takes 3 kicks! Also, the engine sort of "stumbles" when cruising at a constant speed in town (30-40 mph). This stumbling phenomenon is why I checked the spark plugs in the 1st place.
    George Tinkham
    Vintage Iron Riders, AMCA, AMA, ABATE of Illinois, MRF, Springfield Milers BMW MC
    www.virmc.com
    '56 H-D KHK
    '60 H-D CH
    '42 Indian 841
    '48 Chief
    1 each of German (R69S), Italian (V65Sp), & Japanese (150 Touring Benly)

  4. #4

    Re: KHK rear cylinder running rich

    Weak coil lead to the sparkplug ???

  5. #5
    Join Date
    Sep 2015
    Location
    Central Illinois
    Posts
    161

    Re: KHK rear cylinder running rich

    Quote Originally Posted by Paps View Post
    Weak coil lead to the sparkplug ???
    I'll check that and everything else that influences how much energy a particular spark plug gets. I have never done this; so, I suppose I just check the Ohm readings for each plug wire. Does that sound right?
    George Tinkham
    Vintage Iron Riders, AMCA, AMA, ABATE of Illinois, MRF, Springfield Milers BMW MC
    www.virmc.com
    '56 H-D KHK
    '60 H-D CH
    '42 Indian 841
    '48 Chief
    1 each of German (R69S), Italian (V65Sp), & Japanese (150 Touring Benly)

  6. #6

    Re: KHK rear cylinder running rich

    I would check the spark using a spark plug tester under the dim of night on both cylinders to use as comparisons. If you need an example of a good sparkplug tester, I will post one. A good sparkplug will suffice but a sparkplug tester will prove the point.

  7. #7
    Join Date
    Dec 2009
    Location
    Salt Lake City, Utah
    Posts
    5,240

    Re: KHK rear cylinder running rich

    I have to ask this question to the masses. If an intake manifold was leaking at the point where the intake manifold joins with the cylinder, wouldn't it only lean out that one cylinder?
    Fabe
    '41 FL
    '50 Chief
    '64 FL
    '42 WLA

  8. #8

    Re: KHK rear cylinder running rich

    Good question Fabe ! Makes sense to some extent since the leak is closest to that cylinder and should suck more air in than the opposite cylinder, but any leak will affect both cylinders in one way or another. I think the answer may be the leak could be more serve for the closet cylinder, however, bad for both.

  9. #9
    Join Date
    Mar 2008
    Location
    Beautiful Northern NM
    Posts
    12,202

    Re: KHK rear cylinder running rich

    The manifold is basically a plenum chamber , and each negative pressure pulse will fill it , either through the carb as it should, or also through any other point air can enter. So despite a leak at one nipple, it can affect both cylinders to some extent and lean out the mixture.
    Friends help friends move,
    Real friends help friends move bodies!

  10. #10
    Join Date
    Sep 2015
    Location
    Central Illinois
    Posts
    161

    Re: KHK rear cylinder running rich

    We think we found the problem: Bad ring on rear cylinder. It was not running rich, just getting too much oil. Looking to replace iron ring with one with moly in it. Who makes such rings?
    George Tinkham
    Vintage Iron Riders, AMCA, AMA, ABATE of Illinois, MRF, Springfield Milers BMW MC
    www.virmc.com
    '56 H-D KHK
    '60 H-D CH
    '42 Indian 841
    '48 Chief
    1 each of German (R69S), Italian (V65Sp), & Japanese (150 Touring Benly)

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