I heard that running open airboxes can make the jetting troublesome to tune, is this true?

What sort of filtration could be used? Pod filters/socks?

Would this give an appreciable power boost over a standard airbox with four holes drilled?
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MC21 Honda NSR250. TYGA 300 conversion.

I think Andy runs the HRC airbox with pod filters.
He runs the all foam ramair type filters, not the K & N type filters with the metal at the back. I think he says that the metal backed filters have a detrimental effect.
He's made very good power with his set up.
I'm sure he'll be along to add his comments.

anonymous.shyster wrote:
Would this give an appreciable power boost over a standard airbox with four holes drilled?

In my experience open carbs are worth about 1.5bhp at peak more, but cost about 2 bhp anywhere under 10,000rpm.
Up one size on the main jet is about right if you go to open carbs.

But;

a; open carbs sound cool
b; open carbs look really cool
c; open carbs are the HRC way
d; if you're racing then open carbs mean jetting for the occasion isn't a ton of work
e; carbon fibre...............
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Does going to open carbs require removing the air solenoids and/or blanking off the power jet?

No need to touch the solenoids (is there ever a reason to do anything with them, does anyone know?) and you only blank the power jets if you are converting to premix and will therefore be metering all of your fuel (with mixed in oil) through the main jets.
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Hermit wrote:you only blank the power jets if you are converting to premix and will therefore be metering all of your fuel (with mixed in oil) through the main jets.

Really?
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Vampire?

Damn and i blocked my ones and am running injection hmmmm . lol
I have a hrc air box on my track bike and a standard drilled on on my road bike both running hrc jet kits and the track bike sounds gr8 and i does rev up a lot faster than the roady but it is just open carbs no pods or anything .

Well, that didn't work*.
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anonymous.shyster post that should still be here;

There would be absolutely no reason to remove the power jet when removing the oil injection, or vise versa. Both could be done on their own, however, when removing the oil injection you'd want to go up a size in the mains because of the premix metering. That's how I understand it anyway.

As for the air solenoids? I know one hose goes from the X piece to the airbox... it seems as if without an airbox, it would lose its home! Which may require moving over to the HRC setup (power jet removed, bigger mains)...

So, when are there going to be more of the NSR world HRC style open air boxes up for grabs?
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Neal post that should still be here;

Great response , if my understanding about ram air is correct - as the carbs become pressurized the fuel inside them is too and this equal pressure keeps the jetting the same as before the ramair
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NSR-lizard that should still be here;

As anonymous.shyster says, power jets can be run with the oil pump or premix, if you so choose. The issue with switching to premix is that you effectively weaken the mixture for a given jet size. The power jets are progressive which means (in simplistic terms) that the higher the RPM the richer they fuel, so running pre-mix through them could run the risk of running leaner (read 'running in a leaner bracket') at the top-end.*

Tuning a mechanically progressive system (PWJ's) isn`t easy, and the system isn't particularly efficient, hence why HRC uses only the mains to meter wide open throttle. Power jets are 'safe' on a road bike, or even a trackday bike, where for reliability and longevity are helped by an increasingly rich mixture as the revs rise, but it's far from ideal when racing.

*As noted, the PWJ system gets progressively richer, to a point where it becomes too rich. Running pre-mix through them, and effectively leaning the circuit off, may actually be beneficial in some situations. Using ramair for example, could benefit from the additional enriching at the very top down a long straight.
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Apart from that, children stop sniping at each other, this is not Crash.net.

Yes Hermit's post was inaccurate, yes NSR-lizards "really" was a bit sarcastic, after that it all gets a bit silly there's no need to swear and no need to get personal...any of you.

*a topic split with multiple individual replies moved out of the thread. Replies are never deleted, they're moved to an admin section.
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Last edited by fontyyy on Wed Jul 20, 2011 12:40 am; edited 1 time in total

Cheers fontyyy.

Um, I am not sure if the reason for the PJ is as stated here.

When I was racing my rs125, it had the pj carb and a BPS unit. The power jet would switch off past peak power- this has the effect of extending over rev as otherwise the jetting goes too rich past peak power. This was easily verified on the dyno.

I need to post my dyno runs off my recently completed 300. With air box, peak was 61hp. With air box removed, peak was 66hp, with more power from around 8500 up. The air box is definitely staying off!!! Power drops off quite dramatically past peak. I have an opinion that this is perhaps a design brief of the tyga pipes? The reverse cone looks quite aggressive -perhaps to reduce a tendency to over rev? The same dyno has runs from well tuned production racing mc21s putting out 45, so it shouldn't be too far off..

300 is a hell of a lot more impressive than the 250 was on the road

The nsr's don't use an electronic power jet like the newer rs125's. The 300's like lots of air and maybe that's part of the reason for the readings you got .Would be good to see them though .

Yes, as soon as I get them emailled to me, I will post them up. The MC28 airbox had the 4x 20mm holes drilled, but the bike definitely ran better without it. EGT was reporting 1150 degrees peak, so about right for the road. The airbox was definitely holding it back, of course other people may have had a different experience.

dodgy wrote:
300 is a hell of a lot more impressive than the 250 was on the road

Maybe you should've tried delimiting the 250! If it was anything like those so-called "production racing" bikes and making a stonking 45hp (i.e. completely stock), then it's hardly surprising a 300 with another 20hp feels like night and day, is it?!

I'd put 50p on my MC21/250 with standard port timing making more than your 300 on the same dyno, and if it didn't, you'd be shocked at how close the two were, I guarantee it. The difference between mine and a stock 45hp 21 is too, night and day.
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"well tuned production racing mc21s putting out 45"

Well tuned??? 45???

It's what I was thinking. Maybe a slip of the keyboard?
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