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Old 02-21-2013, 07:55 AM   #1 (permalink)
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Spark plug gap

Ok, went to change my plugs the other day, and checked the owners manual for my 06 FLHTCUI that says the gap should be (.038-.043). When I pulled the plugs I checked them. They were gapped at .043. I realize that is to spec, and my bike was dyno'd that way, but would that not be at the top end? would (.038-.043) not suggest 0.04 would be the best gap? Has the decades of riding Honda warped my thought process? Isn't (.038-.043) a wide range, can't the specifications be a little more specific?
Just wanting to do the right thing.
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Old 02-21-2013, 08:25 AM   #2 (permalink)
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Originally Posted by Rendell View Post
Ok, went to change my plugs the other day, and checked the owners manual for my 06 FLHTCUI that says the gap should be (.038-.043). When I pulled the plugs I checked them. They were gapped at .043. I realize that is to spec, and my bike was dyno'd that way, but would that not be at the top end? would (.038-.043) not suggest 0.04 would be the best gap? Has the decades of riding Honda warped my thought process? Isn't (.038-.043) a wide range, can't the specifications be a little more specific?
Just wanting to do the right thing.
Gap to .038, they will wear to .043, then replace them.





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Old 02-21-2013, 08:31 AM   #3 (permalink)
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ok. Will do. Thanks for the response. So the ones I pulled were well worn..
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Old 02-21-2013, 10:44 AM   #4 (permalink)
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Has the decades of riding Honda warped my thought process?
Yes it has, but that has nothing to do with your spark plug gap.
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Old 02-21-2013, 11:30 AM   #5 (permalink)
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Yes it has, but that has nothing to do with your spark plug gap.
Well Played, Sir
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Old 02-21-2013, 02:03 PM   #6 (permalink)
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Gap to .038, they will wear to .043, then replace them.





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ok. Will do. Thanks for the response. So the ones I pulled were well worn..
having a hard time with that concept...

Were your plugs really that old? How did they look?

Just gap to .040 and don't worry. I put in new champions every year whether they need swapping or not.

rkc

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Old 02-21-2013, 02:30 PM   #7 (permalink)
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RKC,
I never heard of plugs wearing before but hey I'm no expert..
Gaping at .040 is what I figured. I'll do that.

They were still good, both right at a good dry brown. That's where they should be.. One in the front was a little lighter. I kept them as a spare set. I've just always kept a spare set on the bike. It has saved me before, especially with my Dad's 52' Harley back in the 80's.
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Last edited by Rendell; 02-21-2013 at 02:33 PM.
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Old 02-21-2013, 03:49 PM   #8 (permalink)
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There's a tolerance range with plug gap settings. In other words, the gap doesn't have to be at an exact setting to get the spark you need. Like others, i set my gap in the middle of the range. The gap setting will get wider as the plugs wear but most of us replace them way before any real wear is noticable.
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Old 02-21-2013, 08:27 PM   #9 (permalink)
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Gap to .038, they will wear to .043, then replace them.





That is what I do and I agree.
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Old 02-22-2013, 06:07 AM   #10 (permalink)
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I replace mine every 20k miles...or is it 10k...whatever the book says...
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Old 02-22-2013, 06:25 AM   #11 (permalink)
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I have used .040 for 40 years. Is it time foor me to change? Hell no.
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Old 02-22-2013, 10:08 AM   #12 (permalink)
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Quote:
Originally Posted by Rendell View Post
Ok, went to change my plugs the other day, and checked the owners manual for my 06 FLHTCUI that says the gap should be (.038-.043). When I pulled the plugs I checked them. They were gapped at .043. I realize that is to spec, and my bike was dyno'd that way, but would that not be at the top end? would (.038-.043) not suggest 0.04 would be the best gap? Has the decades of riding Honda warped my thought process? Isn't (.038-.043) a wide range, can't the specifications be a little more specific?
Just wanting to do the right thing.
No, .040 would not be the suggested "best" gap. As with other things, there is not a one size fits all answer. Remember, the manual is printed for everyone to read and is not customized to one person's application.

Larger gaps require larger voltages from the coil but make a bigger spark. For "normal" riding, you could actually benefit from something more like a .050 gap with more efficient combustion and slightly better fuel economy. If your coil is getting weak or you let your plug wires or plugs go too long, cold starts could become more difficult over time and firing fouled plugs gets harder.

Smaller gaps require smaller voltages from the coil and make a smaller spark. For "high speed" riding, you could benefit from something more like a .030 gap to reduce heat build-up and allow older (or slightly fouled) plugs to fire more easily. Starting is made easier also.

Using the correct heat range for different conditions is also a consideration for similar reasons.

The range printed in the manual is designed to give the "best" results in the widest range of conditions for the most people.

All that being said, the differences from one extreme to the other may be negligible in real life, but they do exist. It is similar to the argument about HP. Will I really feel the difference between 85 and 87 HP? Some think it is important, others not.

If you just want it to work, do exactly what has already been suggested. If you want the "best", do some research. The difference from good to best in this case is probably not worth the effort.
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Old 02-22-2013, 11:08 AM   #13 (permalink)
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G-man, I would guess that using the same plugs as were in there, the same gap that was in there would be the best medicine then. The Honda owners manuals always give a a single gap, and they make the manuals for the bike. For instance, the manual for the 03 VTX1800R3 Spec 1 says that the gap for the NGK (I forget the actual number of the plugs) gap is .030, if the rest of my memory is correct. I guess that is another notch on the differences chart. I believe that I will pull the ones I put in the 06 ultra classic, and gap them at .043-.044 like the ones I took out are gapped. I used the same plugs HD stock plugs. I know that it did not have that decel pop before and does have a decel pop after changing the plugs. I normally wouldn't guess a .003 gap difference would make it pop, but then I have been very clear that Harley and Honda are two different species..

Thanks everyone for the help.
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Old 02-22-2013, 12:51 PM   #14 (permalink)
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Quote:
Originally Posted by Rendell View Post
G-man, I would guess that using the same plugs as were in there, the same gap that was in there would be the best medicine then. The Honda owners manuals always give a a single gap, and they make the manuals for the bike. For instance, the manual for the 03 VTX1800R3 Spec 1 says that the gap for the NGK (I forget the actual number of the plugs) gap is .030, if the rest of my memory is correct. I guess that is another notch on the differences chart. I believe that I will pull the ones I put in the 06 ultra classic, and gap them at .043-.044 like the ones I took out are gapped. I used the same plugs HD stock plugs. I know that it did not have that decel pop before and does have a decel pop after changing the plugs. I normally wouldn't guess a .003 gap difference would make it pop, but then I have been very clear that Harley and Honda are two different species..

Thanks everyone for the help.
I didn't catch the part about the decel pop before. Not sure how a plug gap would cause that. If it did, I think it would be an indicator you want the gap to be a little smaller or that you should be running a cooler plug, but I suspect the popping has a different cause.

As far as a manual being written for a specific bike, yes. But different owners with that same bike will live in different climates, use different gas, have different riding habits, make different modifications to the bike, and when some have 1000 miles and others have 100,000 miles, they will all still be looking at the same manual.
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Old 02-22-2013, 11:03 PM   #15 (permalink)
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G-man, The bike did not have decel pop before I swapped the plugs, then I swapped in new HD Plug the same as the ones in there. and it started popping on decel. Both pair are the same plugs as the ones that were listed as what is needed by the book. Anyway, I gapped them at .043. It's less popping, then when gapped at .040. Anyway. I'll do some checking on the plugs and see how it goes.
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