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Discussion Starter #1
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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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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Discussion Starter #5
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/BwkghawnFQn/?utm_source=ig_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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Discussion Starter #6
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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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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Discussion Starter #9
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...

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.

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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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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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.
I'll take a guess that it may be the cap vibrating, in a slight CCW-to-CW manner as the engine runs, ultimately ~unscrewing~ the threaded nut (let's say) 1 or 2 degrees per oscillation.

...If the nut will come loose again I probably gonna get plug caps that snap on the thread, not on the nut.
This seems like a viable and smart solution.

:smile2:
 

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The other thing to check (for random misfires, esp. at low revs/idle) is the connection of the plug cap to the HT wire. It's a screw together connection, and with age, the wires inside the cable break and disappear, and the end of the cable expands a bit, so the screw doesn't contact things too well electrically.
Easy fix - unscrew the plug cap, snip off about 12mm (1/2") from the end of the cable, and screw the plug cap back into nice fresh wires.

This solved a misfire problem that I had for far too long :eek: - because I kept thinking it was the carbs. It wasn't until I got some second hand coils to change them out, and thought to check how well the plug cap was attached (not very well) that I thought to check the same thing with my plug caps.
 
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