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Thread: R90/6 with dual plugs

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  1. #1
    Sheep Dog
    Join Date
    Apr 2007
    South Central Indiana

    R90/6 with dual plugs

    I'm restoring a 1974 R90/6. It's had a couple of upgrades over the years before I became the owner. Ignition, rear shocks, and dual plugs. Somewhere along the way, it was tapped for dual plugs, but nothing else was done. No coil, etc. Right now the bike runs just fine with spark plugs in the bottom of each cylinder hooked up to nothing. Any kind of plug I can put in there and remove the bottom spark plugs. Don't think I'll ever consider finishing the dual plug thing. Seems to be a debate as to if it improves anything.

  2. #2
    Monza Blue 1974 R90/6
    Join Date
    Dec 2006
    I'll bet if you offered to swap there is somebody out there that would like to have the dual plugged heads and would give you their unmodified heads in return. Some risk, I know, but an option to consider.


  3. #3
    Registered User stanley83's Avatar
    Join Date
    Oct 2009
    Somerville, MA
    PPO of my '76 R75/6 dual plugged it, PO didn't like how it ran and went back to running single plugs. I continued with the single plugs until the Dyna III electronic ignition went flaky. I then decided to go back to points with an amplifier as if the amplifier dies, it can be pulled from the circuit and run as originally designed. During this period, there was no problem with the idle spark plugs acting as just a "plug" in the head. I see no reason not to continue as things are.

    I later decided to try out dual plugging, as the expensive work had already been done, the cost being two new coils, fresh plugs, a second set of spark plug wires and a set of Ted Porter's ATU shims. Before adding the shims, starting was a bit rough, but now it starts more easily than it ever had for me. I now run regular gas without pinging.
    Justin in Somerville, MA
    76 R75/6, 78 P200E, 63 VBB
    Lots of bicycles

  4. #4
    Registered User toooldtocare's Avatar
    Join Date
    Mar 2008
    DeLand Florida
    My R90S (sold) had dual plugs when I got it, but the second set was not hooked up. It ran fine with points, but a bit hard to start and would ping from time to time. I just left the second plugs in as finding something to plug the hole would have been harder. I also found out that the modification could use two type of plugs, those with coarse or fine threads. My R90S had the coarse threads, my 78 Motorsport (also modified) had the fine threads. I sold the bike to a person that installed a Dyna III with new coils and he said the pinging went away and it started easier. I think the modification may make the bike run better, but if you are happy I would just leave well enough alone. My next airhead (soon to own a 77 R100S) will not be modified.


  5. #5
    Registered User
    Join Date
    May 2008
    B.P., MN
    A dead plug in the lower hole will eventually get crudded up with carbon. Which is not good. A special shorty bolt of proper thread size should be in that lower hole if you run one plug. My R90 had a dead plug in the bottom for about 5K. when removed a couple years back it was heavily carboned up.

    When an old dual system was installed the bike woke up. There's a reason somebody thought about adding a lower plug to the old hemi-head design, and it's called flame front or flame propagation or gas that is no longer available. Tons of commentary and advice has been written on the topic which should be read.

    The lower hole many times was bored at 12mm because of less metal between a 14mm hole and the valve seats. Two cents only IMO.

  6. #6
    Minnesota Nice! braddog's Avatar
    Join Date
    Jun 2003
    Twin Cities - MN
    I had a dual plug setup on my '77 R100S. It was a very hard starter. I converted it back to a single plug setup, and frankly it ran awesome, and started flawlessly with little effort. I just left the plugs in the lower sockets.

    I hated how hard the bike started, so set the valves, balanced the carbs, made sure the plugs were all good, etc. Then I took it to my airhead mechanic. His first recommendation was to put it back to a single plug per head.
    Brad D. - Member #105766
    '77 R100RS - Black Beauty (big pipe, baby!)
    '94 R1100RS - Sylvia

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