Spark Plug Issues
Spark Plug Issues
I replaced my HD6R12 spark plugs with a pair of iridium NGK DCPR7EIX's. I gapped them at .040 which is within the tolerance range for the HD6R12 plugs. My bike has become harder to start and I'm wondering if I just have the gap wrong. My bike is a 2006 FLHTCUI (ultra classic). The service manual states the HD6R12 is the only spark plug which can be used, but the manual was written back in 2006 and there are now multiple spark plugs that are listed as compatible including the NGK's I installed. I'm no expert when it comes to spark plugs so if anyone could point me in the right direction, I would appreciate it.
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Joined: Aug 2006
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From: Pennsylvania
I replaced my HD6R12 spark plugs with a pair of iridium NGK DCPR7EIX's. I gapped them at .040 which is within the tolerance range for the HD6R12 plugs. My bike has become harder to start and I'm wondering if I just have the gap wrong. My bike is a 2006 FLHTCUI (ultra classic). The service manual states the HD6R12 is the only spark plug which can be used, but the manual was written back in 2006 and there are now multiple spark plugs that are listed as compatible including the NGK's I installed. I'm no expert when it comes to spark plugs so if anyone could point me in the right direction, I would appreciate it.
Edited to correct misinformation.
Last edited by CoolBreeze3646; Jan 23, 2021 at 10:05 PM.
My service manual states the gap is .038 - .043 so I went in the middle with .040. Again, this is for the HD plug rather than the NGK plug so I don't know if the gap should be different for different plugs that are compatible with the same vehicle.
Seasoned HDF Member
Joined: Aug 2006
Posts: 24,496
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From: Pennsylvania
I read that wrong you are correct the gap is .038 - .040. Put HD6R12 back in the bike gaped in that range and all will be well.
Last edited by CoolBreeze3646; Jan 23, 2021 at 10:06 PM.
Ultimate HDF Member
Joined: Feb 2009
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From: Garland, TX
Agreed. I tried a few different plugs and bike always runs better on stock HD or screamin eagle plugs.
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Seasoned HDF Member
Joined: Sep 2017
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From: England
Well not your bike but a Sportster but it uses the same plug.
I changed the plugs for NGK but had a problem that I took it to an indy to sort out.
When he was doing a general check over he saw the plugs and changed them back to the HD ones. He said the HD plugs are the best. He has had the NGK plugs fail.
I changed the plugs for NGK but had a problem that I took it to an indy to sort out.
When he was doing a general check over he saw the plugs and changed them back to the HD ones. He said the HD plugs are the best. He has had the NGK plugs fail.
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From: SW Ohio
Here's a good rule of thumb:
If your bike ran well before you started working on and doesn't after you've "improved it", put it back the way it was. A Harley Davidson motorcycle is akin to a tractor in a field. No spark plug, oil, octane booster or anything else (except for a brief case of money) will make it perform better.
If your bike ran well before you started working on and doesn't after you've "improved it", put it back the way it was. A Harley Davidson motorcycle is akin to a tractor in a field. No spark plug, oil, octane booster or anything else (except for a brief case of money) will make it perform better.
