Spark plug terminal nut. Check 'em! - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 12 (permalink) Old May 22nd, 2019, 12:39 pm Thread Starter
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Spark plug terminal nut. Check 'em!

When adjusting the air/mixture screw (while tuning the carbs) I noticed that one of the cilinders kept backfiring no matter how far in or out I turned the air/mixture screw.

I removed the spark plugs to check the color and then I noticed one of the terminal nuts was loose. It was nearly falling off.

Altough I tightened them with a set of pliers when I installed them, one had come loose in less then 50 miles.

I noticed the same problem when I bought the bike and renewed the spark plugs. Back then, both terminal nut were loose.

Apparently these things come loose very easy, no matter how tight they were during install.

Anyway, once I tightened the nut the engine wasn't backfiring anymore with the air/mixture screw set to 3 3/4 out (K&N, stock airbox, Dynojet 1, open Bos mufflers).

So have any of you guys experienced this same problem with the terminal nut coming loose? And if so; how did you fix it?

I winded a single strand of battery cable wire around the thread prior to installing the nut. Now the nut needed a lot more force to go on so I hope it stays in place.
I was also thinking about Loctite but I think Loctite isn't the best electricity conductor so I went with the fine copper wire.

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post #2 of 12 (permalink) Old May 22nd, 2019, 12:57 pm
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Check the plug number not the grade but the part number, NGK often do two types of the same plug, one with a removable nut and one with a fixed nut, usually if you buy Ducati parts they will be the correct version made by either NGK or Champion
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post #3 of 12 (permalink) Old May 22nd, 2019, 1:18 pm
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I don't doubt you had this happen only that a loose nut would make that big a difference, I have had plug caps designed for nut use show up with no nuts on them and the owner never noticed a problem.

and then there was this one https://www.instagram.com/p/Bwkghawn..._web_copy_link
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post #4 of 12 (permalink) Old May 22nd, 2019, 4:03 pm
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If they come loose just mash the threads a little with pliers and crank the top on.
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post #5 of 12 (permalink) Old May 23rd, 2019, 12:39 am Thread Starter
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Quote:
Originally Posted by ducvet View Post
I don't doubt you had this happen only that a loose nut would make that big a difference, I have had plug caps designed for nut use show up with no nuts on them and the owner never noticed a problem.

and then there was this one https://www.instagram.com/p/Bwkghawn..._web_copy_link
I think I expressed myself in a wrong way.
It's not a big difference.
With the nut loose the bike had a mis- or backfire every 10 to 15 seconds at 0-1/4 throttle.

It's because I was testriding it to find the exact setting for the air/mixture screw that I was listening more carefully. And if you start listening while testing you'll hear and feel every blip, crack, pop,...
If I was riding on a day trip I would probably not even noticed or cared. But I did now, with my ears in test-mode.

So no, it didn't made a big difference but it helped solving the last few mis- or backfires.
It's not that the bike ran like crap before...

Another thought; I compared the nut from the spark plugs in the bike (Champion) with some from the old spark plugs (NGK) and I noticed that the ones on the Champion-plugs have much more "thread-play" (if that is even a word, hope you know what I mean).
The NGK nuts seems to fit way better on the thread.

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post #6 of 12 (permalink) Old May 23rd, 2019, 12:40 am Thread Starter
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Originally Posted by TonyL View Post
Check the plug number not the grade but the part number, NGK often do two types of the same plug, one with a removable nut and one with a fixed nut, usually if you buy Ducati parts they will be the correct version made by either NGK or Champion
I didn't even know that plugs with a fixed nut existed. I'll keep that in mind!
Thanks!

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post #7 of 12 (permalink) Old May 23rd, 2019, 12:41 am Thread Starter
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Originally Posted by duc96cr View Post
If they come loose just mash the threads a little with pliers and crank the top on.
I'll try that if my copper wires don't do the job!

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post #8 of 12 (permalink) Old May 23rd, 2019, 7:41 am
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If you think about it, this means the plug top nut is rotating inside the plug cap. Maybe you can find a way to snug that up a bit ? A good fit would be if the plug caps snapped on with a little pressure. I like the type of cap that contacts the plug threads directly.
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post #9 of 12 (permalink) Old May 24th, 2019, 9:07 am Thread Starter
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Originally Posted by duc96cr View Post
If you think about it, this means the plug top nut is rotating inside the plug cap.
I was thinking the same way. The plug cap snap on the nut fairly tight and on top of that I tightened the nut with a set of pliers to the spark plug. And still it is rotating...

Quote:
A good fit would be if the plug caps snapped on with a little pressure
They do. But I assume I need to apply quite some pressure to snap it on but once it's on I don't think the cap holds the nut with that same amount of pressure.

Quote:
I like the type of cap that contacts the plug threads directly.
That is probably the best solution but I didn't think of that when I bought the plug caps.
I the nut will come loose again I probably gonna get plug caps that snap on the thread, not on the nut.
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post #10 of 12 (permalink) Old May 26th, 2019, 1:11 pm
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Thanks for the tip. Just found this identical issue on my Aprilia RXV450. One plug had a fixed terminal nut and the other was removable and loose. I slightly crushed it and threaded it back on and it seems much tighter.

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