NC23 racebike engine failure

[Spoony]

Took my NC23 to the track on Sunday for the first time. Was a having a ball getting use to it, corner and exit speeds gps logged almost as quick as my gixxer thou after only 2 sessions.

Anyways, 3rd session the engine let go. On the bike sounded like grinding gears or something slipping in the gear box, i didn't really think it was engine initially, till I turned around to see huge amount of smoke come out the pipe. Engine was still running but definately down on power some. I just upped to 6th gear and cruised around the outer edge of the track back to the pits (maybe i should have shut it off then and there? oh well lol).
I'm guessing maybe some valve to piston action?

Having been warned about over reving, my shift points were at 14,000, I don't think I was downshiftting too hard (all a big adjustment coming off an 06 gixxer 1k)
However, only moments before it shat, was on the front straight and it did pop out of gear. Defiantely wasn't a miss select (maybe just not in hard enuf) but full noise/throttle it got a fault neutral, tho honestly I can't see how such a short incident can kill a motor? (never had such issues with the year & years of suzuki's i've tortured lol).

The motor was like 2 track days old, topend and bottom end, had head and valve work etc (apparently) .

Will drop it off and pull the head to see some more when i get a chance. So annoying, 30min of riding since purchase

[greggo]

False neutral + head work = dropped valve. Hopefully it hasn't done too much damage.

These old dungers don't take kindly to over revving (as has been a long discussed topic on here). Stimpy has a nice story about shifting his NC23 from 6th to 5th at Phillip Island and it doing that very same thing.

[drew70]

Your lucky you got two sessions, my record for busting a 23 motor is three and a half laps

[trickyd400]

Yup, I managed 3 sessions before my original engine went onto 3...



This was due to the big end going, allowing the piston to hit the valve, which bent, then broke off, rattled around in the bore, chopped a hole in the top of the piston, then sat back down in the valve seat, sideways!

Solution was a replacement engine from a race bike being broken by a chap in Sheffield. All is now well.

[drew70]

Spooky looks like the same valve/cyl as mine




Both motors i blew the problem was with the same cyl on the exhaust side .
Over reving a bit on the down change done it i think .

[Spoony]

haha nice pics! Finally got the motor out and appart, I think I can top them some

I've had someone mention to me some engine builders running pretty 'fine' cam timing, looking at this they didn't even use slots, just welded the sprockets on . Any truth to running such cam timing making the engine more suspect to destruction?

I've got a spare rebuilt engine, but as I mentioned in another thread it seems this is from a CB400 or Aero, so I'd have to get the headers modified (angled exhaust ports 1 & 4). Cheapest option though given the head on this one is trashed even the bore isnt' looking good. So much for buying a setup fresh bike for less dramas lol.

Should they really pop that easy? I guess best option is to fit some sort of revlimiter.

[greggo]

Jeepers. I wouldn't be trusting just anyone to alter the cam timing and weld the cam sprocket on. I wouldn't be keen on it at all.

They would have to have some sort of kick arse reputation for building hot reliable motors (Rather than very fast hand grenades).

Nice bit of destruction in the last one. How on earth does it drop 3 valves in one cylinder! Yikes!

[CMSMJ1]

Excellent pics mate. Shame it went pop!

[Mikiboy]

I guess it depends on the tune.
I blew up my original on the practice of my 1st race at darley Moor.
The new one has been refreshed and set up very well and has been in the red for 2 years with hardly using the brakes and just the engine for slowing and it has been fine since. The last 10 years, I moved the peak power down to about 12000 revs and have set it up for mid range so it pulls like stink from 9k and if I get a gear wrong, it will still pull from 5k in a too low gear.
I guess a lot will be down to luck if you actually lunch the engine

[stimpy]

you will also need to get the airbox modified to suit the aero motor,as the carbs mount on a different angle
you will also need to ditch the engine mount spacers from memory
i thought it was more hassle than what it was worth,and only did 1 meet with the aero motor,no airbox
the 23's go well in stock form...raced mine for 3 years complete with stock airbox and exhaust did well.
was only last year i added ram air...and this year was going to use some race down pipes.
best bet is to find another rr motor,
stimpy