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Thread: Firing on one cyclinder

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    Registered User DrGeo's Avatar
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    Firing on one cyclinder

    1995 R100RT... 45K miles...Cruising along and one cylinder decides to quit...Checked plugs for spark = right side yes, left side no...where do I go from here?

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    I would start with the wiring going to the coil, maybe one broke.
    1994 R1100RS--,1974 R90/6 built 9/73,--1964 Triumph T100--1986 Concours

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    Day Dreaming ... happy wanderer's Avatar
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    Start at the bottom, work your way up.
    - swap plugs. Does the non firing cylinder follow the plug to the other side? It's the plug. Replace both sides with new Bosch W7DC or equivalent.
    - swap the wire leads. Does the non firing cylinder follow the wire to the other side? It's the wire and cap assembly. Check resistance. Should be around 5K Ohms.
    - If the first two tests did not change anything you may have the rarest of problems and it might be the coil but I highly doubt it.

    Let us know what you find out!
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
    '81 R80G/S, '82 R100RS, '00 R1100RT

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    Liaison 20774's Avatar
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    DrGeo -

    Usually what people do at this point is start by swapping parts left to right until you see a change. Plugs, then plug wires, then coils (you can do this by swapping the high tension leads at the coils). When the problem shifts sides, you've found what caused it.

    MJM types faster!!
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    MonoRT MonoRT's Avatar
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    Dead secondary windings in the coil would kill both plugs (been there). One dead plug is either a loose plug wire connection at the coil (been there too), a bum plug wire or a bad plug. If the plug wires are firmly attached at the coil, you can swap plug wires from side to side to see if the problem moves. If not, try swapping the plugs.

    If you've got one bad plug wire, the other one is probably thinking about going bad also - unless there is some obvious trauma to the bad wire.

  6. #6
    Day Dreaming ... happy wanderer's Avatar
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    Quote Originally Posted by MonoRT View Post
    Dead secondary windings in the coil would kill both plugs (been there). One dead plug is either a loose plug wire connection at the coil (been there too), a bum plug wire or a bad plug. If the plug wires are firmly attached at the coil, you can swap plug wires from side to side to see if the problem moves. If not, try swapping the plugs.

    If you've got one bad plug wire, the other one is probably thinking about going bad also - unless there is some obvious trauma to the bad wire.
    Absolutely. As I said, highly unlikely it's the coil however I have seen one cracked tower coil go flaky on one side and misfire so since then I don't just rule them out. Now to be fair it was a '81 early tower coil, one of the infamous black plastic Crack O'Matic coils... Snowbum set me straight on that thing right quick.

    But whenever you find a bad plug, wire or cap changing both sides is the way to go for sure.
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
    '81 R80G/S, '82 R100RS, '00 R1100RT

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    #4869 DennisDarrow's Avatar
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    Is anyone gonna bring up how the OP can ruin his ignition, electronic, system by testing plugs without grounding them?????????...........One can surely do serious damage with the system if you don't someway ground each plug as you test them for spark............The above advice as to starting to switch parts, starting with the plugs is the way to go...........Good luck..........

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    Registered User DrGeo's Avatar
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    Firing on one cylinder

    Quote Originally Posted by DrGeo View Post
    1995 R100RT... 45K miles...Cruising along and one cylinder decides to quit...Checked plugs for spark = right side yes, left side no...where do I go from here?
    WOW !! Thanks for all the come-backs...Now I have to get to work...Will let you know what happens.

  9. #9
    Day Dreaming ... happy wanderer's Avatar
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    Quote Originally Posted by DennisDarrow View Post
    Is anyone gonna bring up how the OP can ruin his ignition, electronic, system by testing plugs without grounding them?????????...........One can surely do serious damage with the system if you don't someway ground each plug as you test them for spark............The above advice as to starting to switch parts, starting with the plugs is the way to go...........Good luck..........
    That's a really good point Dennis! Much like a BMW factory manual we're (read I am) assuming mechanical skills. In their case, fair enough; the audience is trained mechanics. In our case, it could go either way!

    Might as well post these up again. Good Airhead solution requiring a paper clamp, a known working plug and some pliers for bending one handle of the clamp so the plug is held captive. Found in _any_ office type desk top drawer! How handy (read free..) is that? :-)
    Scroll down for how to use.





    I never leave home without one.
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
    '81 R80G/S, '82 R100RS, '00 R1100RT

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    Registered User DrGeo's Avatar
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    firing on one cylinder

    Quote Originally Posted by DrGeo View Post
    1995 R100RT... 45K miles...Cruising along and one cylinder decides to quit...Checked plugs for spark = right side yes, left side no...where do I go from here?
    I switched plug wires and the good side became the bad side so I will order a set of wires and start off fresh...I even remembered to disconnect the battery while working ...Thanks to all who helped with advice...I'll come back when all is well and running.

  11. #11
    Registered User PAS's Avatar
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    If your bored, carefully slice open the big rubber plug. Most likely you will find the resistor burned in half. Happened to my 81 R100RT years ago. I cut that end off and put on an NGK LB05F cap and now have a spare set of wires. The wire itself is SS and of high quality.
    LB05F.jpg

  12. #12
    Day Dreaming ... happy wanderer's Avatar
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    Quote Originally Posted by DrGeo View Post
    I switched plug wires and the good side became the bad side so I will order a set of wires and start off fresh...I even remembered to disconnect the battery while working ...Thanks to all who helped with advice...I'll come back when all is well and running.
    When you have the old wires off it would be interesting to measure their resistance using a meter on OHM setting. Inquiring minds like to know!

    I recently replaced some wire sets with NAPA stuff. You have to ask for it to avoid wire "kits" but they still sell solid wire (don't use silicone wire) black spark plug wire by the foot along with the brass bits and rubber boots for the coil side. They also sell the NGK 5K plug caps so you can build your own set for pretty cheap. These will work as well as anything off the shelf. Solder the coil side brass connector to the solid core wire and put a dab of RTV silicone under the rubber boot on the NGK plug cap to keep water out for best results.

    NAPA Part numbers (well.. in Canada anyway) :
    8030 NGK Spark Plug Boot $5.19 each qty 2 $10.38
    734803 BMX 7MM HT Plug Wire BLK $0.72 ft qty 4 ft $ 2.88
    727008 BMX Distributor TERM $0.46 qty 2 $0.92
    727302 BMX Powerpath Prod $0.52 qty 2 $1.04
    Total $15.22

    PS: All this stuff is even cheaper in the USA!
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
    '81 R80G/S, '82 R100RS, '00 R1100RT

  13. #13
    Registered User DrGeo's Avatar
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    Firig on one cylinder

    Thanks to all who shared advice...Installed new plug wires and we're back in business...God is Good...Richard <><

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