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Exup-less YB8/10

 
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Jonny B Bad



Joined: 05 Dec 2009
Posts: 555
Location: NE London

PostPosted: Mon Aug 30, 2021 12:11 pm    Post subject: Exup-less YB8/10 Reply with quote

Anybody tried running a YB without the exup valve?
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2bims



Joined: 03 Apr 2010
Posts: 7289

PostPosted: Tue Aug 31, 2021 6:31 am    Post subject: Reply with quote

Yeh....a yb11....shouldn't be an issue...it just makes the bike quitter around time at low revs
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Jonny B Bad



Joined: 05 Dec 2009
Posts: 555
Location: NE London

PostPosted: Tue Aug 31, 2021 10:46 am    Post subject: Exup-less YB8/10/11 Reply with quote

“quitter around time at low revs” eh 2bims!
Is it also outstandifold in the wetty grippers? Laughing
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2bims



Joined: 03 Apr 2010
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PostPosted: Tue Aug 31, 2021 11:29 am    Post subject: Reply with quote

Friggin autocorrect...quieter around town
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Jonny B Bad



Joined: 05 Dec 2009
Posts: 555
Location: NE London

PostPosted: Tue Aug 31, 2021 12:31 pm    Post subject: Exup-less YB Reply with quote

Thanks 2bims - where would we be without the technology thinking it knows better than us at every turn? Driverless cars anybody?
I’m particularly interested in what adjustments to jetting might have been applied, to make it run properly, after ditching the exup.
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coops



Joined: 19 Apr 2021
Posts: 12
Location: New Zealand

PostPosted: Thu Sep 02, 2021 5:49 am    Post subject: Reply with quote

You can, but you must leave the exup servo on and running.
Ive done it on my race bike and you actually loose quite a lot of power below 5K
It cannot be tuned back. I would highly recommend leaving the exup on your bike.
You will be dissapointed if you remove it.
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Jonny B Bad



Joined: 05 Dec 2009
Posts: 555
Location: NE London

PostPosted: Thu Sep 02, 2021 7:19 pm    Post subject: Exup-less YB Reply with quote

Hello Coops, thanks for responding - doesn’t look like too many folk have delved into this. I know that the conventional wisdom, as far as an FZR is concerned, is that losing the exup valve also means the loss of bottom end & mid-range and I kind of wish I’d saved myself a lot of heartache by accepting that years ago, but something persuaded me to keep experimenting.

A YB is a bit different to most FZRs in that it uses the 1989 Yamaha set-up, where the slides have over-size transfer ports, making them flutter when revving the engine to pull away, massively accelerating the needle jet wear that FZR carbs suffer from, courtesy of the step at the bottom of the inside of the slide, which causes the needle to rub against the engine side of the needle jet. I’m in course of trying to do something about that, because trying to tune, whilst the needle jet is being ground to dust every time you ride the bike is not great!
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Jonny B Bad



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Location: NE London

PostPosted: Thu Sep 02, 2021 7:44 pm    Post subject: Exup-less YB Reply with quote

The next difference is that the YB takes in air into the air box at the front of the bike, rather than the tortuous route taken by the FZR. The 1989 FZR, with the over-sized transfer ports in the slides (in 1990, the FZR changed to smaller ports, mitigated by an extra half turn on the mixture screws) also used an ecu which did not take a signal from the exup servo, so the latter can be disconnected.

Anyway, following many years of experimentation, I’ve got a YB working with a much bigger and freer-flowing air filter and the exup gone. The slides with the large transfer ports, that I thought were an impediment to getting it fuelling properly, I now find are essential. The way I think of it is the FZR was configured to run “slow in, slow out” but what I’m running now is “fast in, fast out”. I’ve had to increase fuelling on the pilots, needle jets and mains and I needed to change from the original fairly rich needle to a much leaner one. But the end result, as you’d expect, is a motor that feels much stronger everywhere - yet to be verified on a dyno.

I don’t know whether the FZR could be made to flow air as well as the YB, to take advantage of these mods. Are you using the air box or pods on your race bike and do you happen to know the size of your mains? I’m running pretty well with 137.5 mains across the board - no differential between inner & outer cylinders because of the configuration of the YB air box.
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Bud977



Joined: 03 Mar 2013
Posts: 525
Location: Sydney

PostPosted: Thu Sep 02, 2021 10:03 pm    Post subject: Reply with quote

I don't have EXUP on my YB8, but it's a race bike so not much help to you. Low down power is not as important on a race bike as a road bike. I also have FCR flatslides so carb set up is not relevant either.

However, I have read on the FZR1000 forum that it is not recommended that the EXUP is removed on a road bike. It puts a big hole in the power curve.
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coops



Joined: 19 Apr 2021
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Location: New Zealand

PostPosted: Thu Sep 02, 2021 10:32 pm    Post subject: Reply with quote

Thanks for the reply. Yes i didnt think about the slight difference FZR/Bimota induction. I have read much over the years about exup removal on FZR and consensus is very anti exup removal. The diaphram carbs (really on any bike) definately work best with air box. That is a known. They work very well on the FZR, but there is always a trade off with diaphram carbs, very complicated.
But my own race bike.... Has definately developed over seven years.
Final spec is, slightly ported and combustion chamber work done 12.5:1 comp
Yam 4 race cams, thunderace crank, 6 speed box, carrilo rods, pistons, race valves, FCR39 carbs with foam pods. And of coarse exup removed. The bike is stock plus 20RWHP. had it on the dyno a few times. The very last thing i did was remove the exup (only to remove weight) and it was remarkably gutless below 5k. I have played with different cans and its better ,but on track i have to keep it above 5k which i dont really like. Oh yea, and a slipper clutch which is amazing and makes it easier to keep above 5K. I actually thought about putting exup back on. Its a 1989.
It really should go back on the Dyno. That may improve bottom end. Or just keep it pinned Very Happy
But i am now concentrating and spending my cash on my recently aquired Furano. Which is definately not plying the game. But that is another story.
Steve.
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