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Plug gap closed up

4K views 27 replies 10 participants last post by  Kiwi Rider 
#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.

Screw Auto part Nut Fastener Automotive engine part
 
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#3 ·
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.
 
#4 ·
Maybe a pic of the electrode from the underside would help.
Are there signs of impact/hammering?
 
#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!
 
#7 ·
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.
 
#11 ·
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...
 
#12 ·
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.
 
#13 ·
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.
 
#16 ·
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
 
#19 ·
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.
 
#20 ·
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?
 
#21 ·
The inspection camera should be able to tell if the pistons are stockers or not. Look at pics of stock pistons....4 valve reliefs with a mild dish in the middle or at most it'll be flat (depending on year), while race pistons will have a dome or raised center section that fits into the combustion chamber to raise the CR. If such pistons ARE installed...then you really have to watch the cam timing for piston/valve clearances and they can get pretty close to the plug ground electrodes even to the point of needing to clock them to avoid being hit. Some motors also run better if the gaps are clocked in a certain direction...but this is getting into pretty serious racing like professional classes.

Just a thought: If you open the throttles you should be able to see the backs of the intake valves with a strong light. Some deposits can accumulate on them due to oil being sucked down the guides and baking on the valves. If this accumulation gets thick enough it can then flake off....perhaps on a mild over-rev? Whatever....if a big chunk comes off it goes into the chamber and while it isn't strong enough to really hurt anything badly it IS strong enough to close up a plug gap. Depending on the fuel you are using and the oil consumption being experienced it's possible that you're getting some deposits and that might be all that went on with the plugs. Worth a look?
 
#25 ·
Update: My buddy wasn't able to get the camera in the end from his work so I just took it to the track for a tentative test day (just before Xmas). I did about 30 laps then pulled all four plugs out. The gaps were perfect.

I've not long returned home from a racing road trip where I competed in a couple of rounds of our National Pro Twins series. Six days hard riding all up. Pulled the plugs today and still all good. So I will write it off as a un-explained one off.
 
#27 · (Edited)
Thanks for the update, Kiwi.

As tovar said, detonation sounds like the only reasonable explanation.

The question now, what caused the detonation? Any problem with the fuel pump lately or bad fuel? Is this a performance engine or stock? Does it have a Power Commander?

Did you happen to remember hearing any "rattle" or "clacking" type sounds especially when applying throttle under heavy load?

Detonation is very hard on an engine so if there is an underlying problem you will want to get that fixed asap.
 
#28 ·
Hi, fuel pump is all good. I pull it out every winter in the off season and strip it down for cleaning. Always good fuel.
The only performance mod is fitting of a power commander as per the rules of the Pro twins racing class.
Oddly I dont remember hearing any untoward noises and my M4 full system is not overly noisy to mask it.
Will keep an ear out though, I've got a big weekend of racing this weekend for the Burt Munro Challenge Three days racing and one of the events is a hill climb. Should be a good work out! Cheers.
 
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