ZXR400 blow up

[10bikekid]

Hi everyone was hoping someone can help me as am new to zxrs and racing .
On my first outing I managed to Grenade my motor and once I repair i want to make sure it does not happen again.
The bike is circuit only and has a pipe and carb tune plus running a 250 rev counter.
I think the problem was the lack of a relistic rev limit due to non standard rev counter , I was reving bike to 16000 in lower gears and holding 15500 in 5th and 14500 in 6th but at times coming out of corners the bike would rev higher and fall on its face ( I thought it was hitting rev limiter but I think it was valve bouncing and the fact that it has a reverse shifter probable didnt help)

Anyways eventually the motor snapped off a valve and went slow and noisy

Will fitting a standard Rev counter remedy problem ( and standard shifter)and 16000rpm to high to rev, eg what is standard rev limiter set at
Would Apreciate any help thanks

[FruitLooPs]

A quick google would reveal a redline of 14,500 so yeah you were giving it a fair thrashing..

I wouldn't rely on a limiter to stop me from blowing something up myself. Surely it would've paid to check out the redline or limiter info from the previous owner before taking it out for a race.

Live and learn though lucky you didn't do any more damage really. If it skipped a tooth or two on the camchain it could've been a lot more costly.

[mashman]

I did the same to my road ZXR400. She would rev and rev and could be wrung mercilessly, but it did seem to take it's toll. A week after changing the 2nd head gasket she went pop on the motorway at 125mph... Blew the top of one of the cylinders as it connected with one of the valves... I suspect that it's a natural thing...

[R6_kid:Me]

Praying might help.

Why were you running the 250 tacho in the first place?

Putting the shifter back to normal might help you for the meantime as you obviously can't get your head around it. It is supposed to be better for racing though, hence why it is called 'race shift'.

[F5 Dave]

In the day they liked to drop valves in racing anyway, only solution was to replace every couple of seasons I believe. Add another 15 years & things haven't gotten better, Apparently FZR400 valves fair better with minor modification but that was cross talk from many years ago.

[quickbuck]

In the day they liked to drop valves in racing anyway, only solution was to replace every couple of seasons I believe. Add another 15 years & things haven't gotten better, Apparently FZR400 valves fair better with minor modification but that was cross talk from many years ago.

Yeah, those were them "Good Ol' Days"

Anyhow, I think the problem was Revs... Well Duh!

Now let me clear it up....
A piston will only travel a certain speed before it grows.... Oh, and there is a chance yours were glowing too...

The Standard engine had a 14500 red line, and this means that the reliable speed you can occasionally rev your engine to is 14500.
Now to get 15500 out of one (under load) you have to have a very good carburetion system.... So good work there.

Trouble is that when you accidentally shift from fifth to forth the motor is all of a sudden getting driven! And yeah... the piston speed will go through the roof! It will stretch, and if a spring is slightly slack on closing a valve... Whamo! Scrap one ZXR.

Now, a user on here by the name of neil CB125T races a ZXR450, he may be able to help you out with a few bits.....
Can't promise anything though, as he most likely will need most of the bits he has.... But a good Tacho will help.

Oh, and make sure your tacho is accurate too.... Just like any gauge they can be out.

Another thing, Dyno your new engine, and find out the best point to change gear. Red Line might not be the most efficient.

[10bikekid]

It would appear from futher investigation and with the help of some brits on the ZXR site that a 250 CDI was fitted thats rev limit is set at 19000rpm the stock 400 unit is Im told set to 14500rpm This may have been a crude way to increase rev capability as the motor tune seems to allow for much higher revs than 14500 and would bang into that while still fully pulling, there is an SP limiter for sale that allows 15500rpm so I will perhaps grab that, also it would apear as some of you have said ZXRs do drop thier valves ( perhaps the 14500rpm may save re acurace) and FZ valves are the mod to do

Unfortunatly I have been bought up to rely on the limiter and most bike stop pulling before they get to them anyway ( Puke didnt help the poor thing much Either)

Better put down my money on a 675 Triumph or something else a bit newer

Thanks for help