Right this is a bit of an odd one. I was riding the MC21 on a private circuit I have access to earlier today and on a long straight section I was flat out (130mph indicated on clock) when I felt the bike suddenly stutter and hesitate. I immediately pulled the clutch in and coasted to a stop, the bike had stopped running by this point. Now I’m thinking was that a seizure happening but it happened so fast it’s hard to say, the back wheel didn’t lock or squirm or anything else to make me think a seize just this power loss in the engine.
Anyway I stopped, got off,checked around the bike for any fluids etc and let it cool for a few minutes. Then I tried to kick it and it was as free as it always is so I started it up and limped home. It felt fine on the way back but I was riding steady.
I’ve done a compression test on both cylinders mainly because I had one to hand from another bike I’m working on and both cylinders are at 130psi. Now as I was removing the plug from the front cylinder I noticed the insulator was damaged, like smashed to pieces and bits of it are all in the belly pan. Ive never seen one damaged like that in 30 odd years and hundreds of bikes I’ve worked on.
Could the spark plug be the reason for the sudden power loss or do you think it was a seizure? I’ve no idea what happened to the plug unless a stone hit it at that moment?
Engine sounds fine by the way.
Sorry for the long post but I wanted to get as much detail in as possible.
Cheers
Chris.

It does sound like a seizure. Perhaps taking the barrels off and inspecting the cyclinders will show you if it did in fact seize

I think you found your problem, with the plug, I have seen them break like that before.
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Agree with Les. Never had it happen to me, but you wouldn't get 130psi after a seizure.

My initial thought was fuel starvation, until I got to that part of the post.

No harm in pulling the chambers off and having a squint up the exhaust ports, but I doubt you'll see anything out of order up there!
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Thanks everyone for the input, it didn’t feel like fuel starvation but then it all happened in the blink of an eye, I did have a look up the ports and in the plug holes after I posted and it all looks good. It’s only done 1200 miles since it’s rebuild so it should be a fairly healthy engine.
What would cause the plug to do that, is it an impact or a fault within it?
I did hear of some counterfeit Ngk spark plugs being sold last year which made me wonder about these!

Thanks again everyone, much appreciated
Chris.

its a horrible feeling isnt it. at least it didnt happen mid corner when on a lean.

I suppose it's hard to say, I guess the plug was shorting against the head (or rad)... as an aside I think it is less likely to seize on a full open throttle, but not impossible.
What's the worst that can happen by running it again? If it did seize then the damage is already done, I suppose you can't make it any worse...

The good news is you found a possible culprit!

I seized my MC28 at full open throttle

prelude2.2 wrote:its a horrible feeling isnt it. at least it didnt happen mid corner when on a lean.

Yeah it’s not nice, at least Ive still got quick reactions I suppose 😄 and like you say I could of been cranked over mid corner ( been there on an Rgv more than once!).

Max wrote:I suppose it's hard to say, I guess the plug was shorting against the head (or rad)... as an aside I think it is less likely to seize on a full open throttle, but not impossible.
What's the worst that can happen by running it again? If it did seize then the damage is already done, I suppose you can't make it any worse...

The good news is you found a possible culprit!

I’ve got a new set of plugs on order so when they turn up I’ll take it out again and see what happens, hopefully it won’t completely destroy itself if it does let go!

Zeus wrote:I’ve got a new set of plugs on order so when they turn up I’ll take it out again and see what happens, hopefully it won’t completely destroy itself if it does let go!

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