Spark Plug Blown Out?

Guys, while trying to sort out a "misfire" in my 2012 Roadster, I came across something I've never seen before. The number 6 plug was out of its thread completely, laying at an angle in the tunnel/hole/shaft - I fished it out with long nose pliers. The plug gap is completely closed. The HT lead looks like it has been repaired using a resin, I don't think the damage is from burning although there are scorch marks on the plug ceramic, and there's no debris in the hole which I'd expect from the missing rubber sheath(unless it all blew out while I was limping the bike home). The working theory is that a previous owner didn't seat the plug correctly after "repairing" the damaged HT lead and this worked loose, leading to my "misfire" which was also accompanied by noise like a blown exhaust gasket I couldn't track down.

The thread seems ok, I screwed in a compression tester fitting and it seemed to seat correctly. Next steps are another attempt to improve my borescope skills to check the piston crown (I couldn't get a focused view of the threads), and a compression test once I get enough juice in the battery to crank it over more than one rev. Assuming these work out OK, fit new HT leads and plugs then give it a go.

I'm handy with a spanner, but NOT a skilled tech/mech so my questions are:

Has anybody ever seen this before?
Is there anything I'm missing?

You found your missing cylinder maybe if the plug was out of the hole (no compression on that cylinder). All new plugs and HT leads would seem to be in order. The gap closing up was likely due to the plug bouncing in the hole and pounding the electrode shut. The plugs look like they were exposed to water more than once from washing the bike. See if you can torque a new spark plug to 18Nm in the affected hole. If so you should be good, if not you have some choices to make on how much money you're willing to spend to fix the problem.

Thanks Atomsplitter. Interestingly the plugs all showed corrosion around the faces, like you spotted, except the affected one. I wonder how long it's been sitting there, working loose. Washing is a twice-weekly activity here with all the dust.

That is a surprising way to find the "miss". Never even considered this when I read your post about the "miss" problem. I can't remeber of ever hearing of this on the forums.

Idaho Red Rocket 3 said:

That is a surprising way to find the "miss". Never even considered this when I read your post about the "miss" problem. I can't remeber of ever hearing of this on the forums.

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That's kinda why I was asking, nobody I spoke to has come across something like this, but I'm actually a bit relieved. The noise had me worried - I couldn't track down anything blowing with the tank down, so weird thoughts around blown head gaskets or sticky valves without the obvious other symptoms were running in my head. If the new plug can sit correctly it's an easy fix. If not, as Atomsplitter implied, a new head is beyond anything worthwhile out here.

Might be worth converting over to a coil over plug ignition setup. I'm in the process of pricing this myself, I need new leads and am suspect on a coil. The price of a conversion is looking to be less than the price of a new round of leads and coils, and if you have a coil failure in the future you only need to replace a single coil which is cheap. It is a fairly straight forward conversion, noting to crazy. There are links below explaining it in more detail.




Other than that, it's very unlikely that the piston will have been damaged but it is worth having a quick look. The threads are more likely to have an issue, it's not uncommon for the plug to pull the last bit of thread on its way out. Have a look if you can, if it's not too bad torque the plug and go for it, a bit of damage to a small segment of the thread isn't going to cause an issue. If the thread is damaged to a point where it won't hold a plug you can get the head respired, it won't be cheap but I imagine it will be less than a replacement head. Good luck.

Thanks guys, but another question, and sorry if I'm drifting off thread.... I'm sorting out the compression test now, but I can't get the engine to crank more than one revolution. Battery's charged & plugs are all out, but when I hit the starter it turns then cuts out. Hit it again and same thing happens. Is there something in the ECU that stops it cranking with the plugs out and coils disconnected? I'm not getting any warning light.

Heidhurtin said:

Thanks guys, but another question, and sorry if I'm drifting off thread.... I'm sorting out the compression test now, but I can't get the engine to crank more than one revolution. Battery's charged & plugs are all out, but when I hit the starter it turns then cuts out. Hit it again and same thing happens. Is there something in the ECU that stops it cranking with the plugs out and coils disconnected? I'm not getting any warning light.

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No MIL light would seem to indicate the ECU isn't the problem. Try reconnecting the coils low voltage wires and see if that gets it spinning.

Idaho Red Rocket 3 said:

That is a surprising way to find the "miss". Never even considered this when I read your post about the "miss" problem. I can't remeber of ever hearing of this on the forums.

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I’ve not head of it on a R111, but a numpty mate managed to do it on his Hinkley 900 Trident! Put new plugs in on a service, just wound them in hand tight. Half way to a Bike rally one of them popped out!

Twenty years on we still take the piss!

Bunny Burrell said:

I’ve not head of it on a R111, but a numpty mate managed to do it on his Hinkley 900 Trident! Put new plugs in on a service, just wound them in hand tight. Half way to a Bike rally one of them popped out!

Twenty years on we still take the piss!

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Was there any lasting damage? I used the borescope and the threads look a little rough but intact. As my abilities improve I'll see if I can take some useful pics tomorrow. I'm reluctant to run a thread cleaner down them in case I drop swarf into the chamber though, so after all this talk I'll probably end up doing what Atomsplitter suggested - fit new plugs - if they hold the torque then run it and hope. The pressure gauge fitting screwed in easily and held the pressure, so there is hope......