Hey Guys,

I got my MC21 out on the track on Friday and got 3 good sessions under my belt! I got my knee down a couple of times too!!!

Had a bit of trouble with the carbs flooding themselves and a bit of a flat spot (read gurgle) because the valve seats are damaged and don't close right off when the floats come up!

The question today is in regards to the airbox.

The bike is only a race bike so does it stay or does it go??

If it goes what jet sizes should be run and what are the ups and downs of doing this??

The rest of the bike is standard.

Today I've done both the wire splices and the carbs are off the bike (for about the 20th time) so what would you guys recommend

For top end power it's got to go.

Jetting? There's no way anyone can say for sure what will work for your bike, wire spliced and running open carbs I'd start about 160 on the mains if running the power jets (which it is as it's all stock) and come down from there.

Up's are;
more power.

Down's are;
lost mid range
increased ring wear (due to dirt and dust entering the engine)
potential for stones etc. (from gravel traps) to enter the engine and destroy it.
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I ran mine on track with the airbox fitted, but no filter. The sound with just the lid removed is mad, and I found it distracting. Fontyyy loved it

I suppose the question is, would your results improve for the extra 1 or 2 horsepower it'd give you up top, as opposed to the couple you'll loose in the midrange. No point have it all top end if you're not good enough to make use of them, yet find yourself in the mid range once or twice per lap.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

Nothing wrong with my midrange... except that I have too much!

I think only Steve's F3, Bill Limb's ram-aired MC21, and a handful of tested 300's make more than my '21 with no airbox!
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Andy.
NSR-WORLD.COM

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And possibly my 28 which also has no air box!

Is that with AVGAS, an 010 HRC ignition, and a Frank Wrathall port job leigh?

MC21 on standard PGM-III and porting!
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Andy.
NSR-WORLD.COM

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Yes, but.......... it is also a slow MC28 , and you was also comparing against full F3 and a ram aired 21.

I have a Pentax SP now anyway (but sadly not the original bodywork) . Completely standard at the moment but I will be doing wire splice and maybe air box mod.

Timely topic, I am not far off getting the bike rego'd (MOT for those who aren't convicts), and I am trying to decide whether to run pods or a modified airbox, open carbs is just a little too extreme for me.... Guess I'll try both and see. Can't believe I am seeing jet sizes discussed.... I think it's good actually...

watfordhorn wrote:
Yes, but.......... it is also a slow MC28 , and you was also comparing against full F3 and a ram aired 21.

Yeah, good call! I haven't seen any "real" graphs of your '28 still though!

shmokica wrote:
Can't believe I am seeing jet sizes discussed....

You won't be seeing any jet sizes recommended by me!
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Andy.
NSR-WORLD.COM

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OK.....So if I take out the airbox I'll lose a bit of mid range but gain a bit of top end.

If I take the lid off the air box will it have much of an affect?? Would I need to change jet sizes?? If so any ideas??

This air box mod that people talk about......does it work and where do I find how to do it??

I have the same question.
My MC21 has the HRC Delimiter (= wire splice), the air box is fitted and there are no holes drilled into the lid. The bike pulls strongly to 12,000 RPM in top gear (and beyond, I chickened out around 12,200 on a very gradual downhill 'aerodrome'). I weigh 80 kg. It doesn't feel at all strangled up there. Hence wondering what the gains would be by drilling holes in the lid?
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30

If you look at the size of the inlet holes to the airbox - and bear in mind there is a web seperating the two halves of the box - you'll note that they are smaller than the diameter of the carbs. Effectively, the carbs can flow more air than the airbox...
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball



Last edited by Dave Ett on Wed May 28, 2008 2:58 am; edited 1 time in total

I'd say Leigh's 28 will be making a sound 68ish now with the new rings etc. Not bad for an old slug of a PGM IV!!
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NSR300R - Why did i ever have a 250...

I drilled 4 x 25mm holes into my lid. Jetting was factory, only cans and wire splice done. It definatly gained top end but made the slight flat spot it had mid range worse. I taped up 2 holes and it felt much better mid range, and still just as good up top.

I notice a very distinct differnce in induction noise from going from 4 to 2 holes. 4 was very raspy. Simular sound to open carbs. Bit annoying on light throtle, but tough as on full gas. With only 2 holes, all that went away and it was just left with a nice growl through the rev range and the nasty 2-stroke flutter at light throtle was gone
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I might give the 2 holes a try then!

Did you just put them in the top of the lid??