I know I'm being lazy here...forgive me?
Motor cold revs clean from 6K
R7 (First couple of laps). Motor hot....Bhwuuuuuu...until 8k, then it hits the fan. So rich or lean?

do a plug chop and have a look here: http://www.theultralightplace.com/sparkplugs.htm

Did the plug chops. Good colour showed, thought I was clever!
Then in the races, same again first couple of laps, clean from 7K.
Lap 3, motor really hot, below 8K, Bawuuuuuuuu, slip clutch, wang, gone.
Meaning I could take some turns in second for the first couple of laps, then either slip the clutch or rev it's rings off in first for the next 5 laPS..
No time in a race to fiddle with the choke lever. And in practice I can't ride as hard as I race. There is 'rule of thumb' with strokers, bawuuuu means
lean, or rich? It's the needles me thinks, but up or down?

sound like i mite be your crank seals mate

Seals and crank are new. Perhaps 200 Km (?)
No sign of weapage. But, could be I suppose.
Don't want to split motor if not required.
But I have another motor, just rebuilt for the roadie bike, just needing a cylinder (hint hint!) and another coming from a wreckers yard.
Can't mix'nmatch too much, the racers an MC18, the roadie and 'new' motor MC21. Hex Cyls, and round cyls.
But between the lines, I'm reading it's going lean from sucking air through crank seals once everything expands?
R7

it mite be a crank case leak mate if it been freshley rebuilt

Could also be leaking from the RC valve seal or exaust gasket. I know it doesn't sound right, but I went through 3 cylinders on my RZ before I figured it out.
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If the motor is good when cold, but maybe starts to gurgle when it warms up then it's possibly rich. Also, if it starts easily from cold, but is harder to start when warm then it's probably rich.

If it's tricky to start and requires choke for a while before it runs properly then it's probably lean.

If, when it's running it goes....Bwaaaaaaaaaaaaaah... when you whack open the throttle and nothing happens then it's probably lean.

Any chance of some engine and jetting specs? Might make the job of solving the problem easier.

Mains 185
Pilots 42
Needles J2BB Middle clip position.
Mixture Screws 1.5 Out

Engine specs? Airbox? pipes? etc etc?

Hi Guys
Thanks for interest.
No Airbox. TYGA pipes and Silencers. (Stickers too!)
K&N 'Oval' Filters.
I think I found the "Rule of Thumb" thingy.......
"A RICH problem gets worse as an engine heats up"
"A LEAN problem gets better as the engine heats up"
(Think it's from Eric Gorr?)
Crank seals, if the right, leaks gearbox oil into crankcase, and RHS smoky.
Plus funny smell. (Amazing how we forget RS stuff when playing with NSR's!)
If Left, oily and stuff around Magneto.
Nope to both on my motor.
Anyone got an MC18 Airbox they don't need/ want?
R7

If the plug colour looks ripe on the plug chop then chances are you're good on the main, but you might need to look to the slow jet. If you're running pre-mix then you might be ok, but if you're on straight fuel and injecting the slime, then I'd say go to a #40 or even back to a #38.

Oh, first thing to do though is try it without those silly K&N's. Do this before you make any changes to the jetting. It'll probably feel better right off the bat. This is your race bike right? If so I wouldn't worry too much about running the filters, especially if you're after all the mojo you can get.

I'd go with Matt on that... the K&N's are crap on 2-strokes, despite what anyone with an LC will tell you! It's the metal end-plate, see?! Negative waves man!

#42 slows and B's in the middle clip sounds rich, especially if you are running the oil pump. All the NSR's we've played with here so far have liked the needles on clip #2. (Not that it's a cert' for everything, and naturally atmospheric conditions will vary wildly from region to region!)

No mention of the power jets in your spec's. Are they blanked?
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When they are lean off the bottom they pick up slow, when its right it will go Bing Bing.

When running premix on both my 18 and 21 they both needed 42's to get them clean off the bottom. With the oil pump though, as Matt says, they ran great on 38's.
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If I have to take the carbs off once more...

Hi Guys
Very interesting.
It's my race bike, blanked off power jets. Run Premix 30:1 747 with 95 Octane unleaded. Full out plug chop showed nice tan biscuit colour, dry exhausts with grey circle about 10 mm in from stinger end. This was the carb set up when I bought it, and haven't changed a thing other then gearing.
I was distracted by buying an RVF at the track, at a bargain price compared to my home town VFR race bike prices!
We have a practice day in a month. I'll bin the filters (like I said-we forget RS stuff so easily! ) and spend quality time on the carbs, starting with the needles.
I bought an MC21 Roadie, with the motor in bits, buggered, but the rest is excellent. I removed the pipes to fit the new motor. Pipes? Compared to the TYGA's on the racer, what were Honda thinking? Look like bricks, and feel like bricks!
R7