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Discussion Starter #1
Hi guys, I had a race meeting the other weekend and I could tell the bike wasn't running quite right. Didnt fire up as quick, idled a touch slower and rougher and sounded a bit 'fluffy' until hitting the top third of the rev range. It wasn't running crazy bad but you get pretty in tune with a race bike right and I knew something was amiss..

So I thought I may as well start with the easy/basic stuff first and pulled the plugs. Mines a Gen 2 twin spark plug model. I expected to perhaps discover some dirty/fouled plugs but two of them (center rear cylinder and side front cylinder) had an electrode gap of next to nothing (see pic). Maybe 2 or 3 thou of an inch, no more.

I've never seen this before. Any ideas what may have caused this? Someone suggested detonation but there hasn't been any.

P1010063.jpg
 

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Possible maybe alittle bit of contact with the piston? I have seen this before in the car world with a failing con rod bearing. Not sure if it applies here though. How many miles on the motor?
 

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The ground electrode won't bend all by itself....so something must have made contact with it. Perhaps carbon buildup? The piston should contact the head before making it to the plug (especially the center one...not sure on the side one) so something that's not supposed to be there is the likely culprit. I'd use an inspection camera down the plug hole to see what's up....or pull the heads. Keep running it and you're risking destroying everything without finding the cause.
 

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Maybe a pic of the electrode from the underside would help.
Are there signs of impact/hammering?
 

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Discussion Starter #5
No speedo cable so unsure of miles but was never on the road, raced from new, I'm third owner.
No signs of impact on electrode tips either.
Yeah a friend of mine has a camera we can use to check inside.
The motor sounds really sweet... good thing I had two plugs in each cyclinder to keep it going!
 

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Discussion Starter #7
Did you have the plugs out recently? Perhaps they got dropped or bumped against something during install. Or, perhaps you have a saboteur....
No saboteur Lol. The plugs have been in nearly a year and I'm always real careful installing. If I knocked them on install the bike would have run rough straight away.
I had a suggestion of carbon build up reducing the clearance between the piston and electrode and I guess the camera images will answer that one.
 

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...Perhaps they got dropped or bumped against something during install.
That was my first guess. It is hard to imagine anything else unless there is something very strange going on with that engine. It is possible to be on the hairy edge and not notice until later.
 

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Discussion Starter #9 (Edited)
That was my first guess. It is hard to imagine anything else unless there is something very strange going on with that engine. It is possible to be on the hairy edge and not notice until later.
That is a fair assumption but I'm happy the installation job was good. I'm an engineer and super careful with how I work. Also, if I knocked them on installation then it would have run rough from day one.
I put new plugs in on Sunday and took it for a test run, winding it out to red line 3 or 4 times. I haven't had a chance to pull them out again since but it sounded real good again.
 

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Keep us posted mate. Those have clearly taken a solid whack or several from something that wasn’t supposed to be in the way. The only other thing that came to mind for me was whether the washer was on those plugs when they were installed... but I’m guessing you have that covered...
 

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Discussion Starter #12
Keep us posted mate. Those have clearly taken a solid whack or several from something that wasn’t supposed to be in the way. The only other thing that came to mind for me was whether the washer was on those plugs when they were installed... but I’m guessing you have that covered...
Yep washers were in place. No one has been able to offer a definitive answer so far. Absolutely no sign of contact on the electrodes.
Odd that it was the centre plug on the rear cylinder and the side plug on the front. No pattern there.
Someone suggested my con rods are about to let go but people say a lot of things. I mentioned this to a good friend who rebuilds engines and he said if the rods had stretched/moved that much to cause contact then they would have kept going the whole way Lol. He had a listen to my engine and agreed with myself that it sounds sweet.
New plugs looked good after test ride, just waiting on my buddy to get the camera from his work now.
 

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I saw exactly this happen to a friend with his CBR. It was running fine, but wouldn't make power. Spent a long time chasing things (dyno etc) until he pulled the plugs and saw the gap had closed up. Was the con-rod bearings. So while this may not be your exact problem it is absolutely possible for the piston to kiss the plug long before there is total destruction.
 

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Discussion Starter #14
I saw exactly this happen to a friend with his CBR. It was running fine, but wouldn't make power. Spent a long time chasing things (dyno etc) until he pulled the plugs and saw the gap had closed up. Was the con-rod bearings. So while this may not be your exact problem it is absolutely possible for the piston to kiss the plug long before there is total destruction.
OK, good to know. Thanks.
 

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Discussion Starter #16
Question because it's not clear in the photo...where's the spark plug crush washer?

Edit: Never mind, I see this was answered above.
All good, I'm running the Denso U24ESR-N plug which has a washer that cant be removed. Good design feature!
I will be riding with one hand covering the clutch for a while until I'm happy that nothing is about to let go. ;D
 

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I saw exactly this happen to a friend with his CBR. It was running fine, but wouldn't make power. Spent a long time chasing things (dyno etc) until he pulled the plugs and saw the gap had closed up. Was the con-rod bearings. So while this may not be your exact problem it is absolutely possible for the piston to kiss the plug long before there is total destruction.
That may be possible on some motors, but that doesn't mean it is or isn't possible on an SV motor.

I'm not convinced the pistons could contact a plug without smacking some other part of the head, or the valvetrain, first. This all depends on the piston and head design, as well as the valve shape, lift and timing, and how far the plugs protrude and at what angle, etc.

I think the op mentioned one plug on each head was closed...and not even the same one.

I would also suggest checking the part #s on the old ones, and perhaps taking some measurements. Perhaps someone grabbed the wrong length plugs at some point, or they were manufactured out of tolerance.

My bet is still that they got hung up on the seats during install. It doesn't take much force at all to bend those electrodes.

OP didn't mention if the motor's internals are stock, e.g
Stock pistons, stock head gaskets, etc.?
 

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Discussion Starter #18 (Edited)
My bet is still that they got hung up on the seats during install. It doesn't take much force at all to bend those electrodes.
True, but if this was the case then the bike would have run rough right from the time the plugs were fitted. Those plugs had been in the bike for over a year.

OP didn't mention if the motor's internals are stock, e.g
Stock pistons, stock head gaskets, etc.?
Yep, all stock. I run it in a class called Pro twins where engine modes are pretty much limited to running a piggy back fuel controller, slotting the cam sprockets to alter valve timing and re-cutting the valve seats.
 

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The stock pistons are flat topped or even a mild dish...so there's no way they can contact the center plug even if the rod bearings allow the piston to hit the head. The side plug looks to be closer to the piston so there's a chance that it could be hit...particularly if the clocking of the electrode isn't correct should the rod stretch...but even then it's unlikely from the pictures I've seen.

Did you buy this bike new from the dealer? If not....there are pistons with lumps that could get really close to the plugs and could kiss them under the right/wrong circumstances. Maybe someone built a 'cheater' motor? The inspection camera will tell the tale if the pistons are stock or not but if so there looks to be no way they could hit the center plug so that would leave foreign object as the likely cause.
 

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Discussion Starter #20
The stock pistons are flat topped or even a mild dish...so there's no way they can contact the center plug even if the rod bearings allow the piston to hit the head. The side plug looks to be closer to the piston so there's a chance that it could be hit...particularly if the clocking of the electrode isn't correct should the rod stretch...but even then it's unlikely from the pictures I've seen.

Did you buy this bike new from the dealer? If not....there are pistons with lumps that could get really close to the plugs and could kiss them under the right/wrong circumstances. Maybe someone built a 'cheater' motor? The inspection camera will tell the tale if the pistons are stock or not but if so there looks to be no way they could hit the center plug so that would leave foreign object as the likely cause.
That's reassuring to hear. I'm the third owner. The original owner got a professional race bike builder to strip it straight after running it in and it's been a track bike ever since. It spent a long period sitting at one stage, but has been in regular use since 2012. What would be the tell tale signs of a cheater motor?
 
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