OzVMX Forum
Clubroom => Bike Talk => Topic started by: zorroz on August 09, 2007, 04:31:24 pm
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Well not alot of talking going on here so, I got some new boyesen reeds and the instuctions say to remove the factory reed stoppers and use the rev plate supplied. The old boyesen reeds in the bike (farked) had the factory stopper. Should I or shouldn't I??
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They last longer with the stoppers than without them. What bike are they going in?
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84 RMtwofitty
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LEave em in, they wont fray and fall in the pot. Stoppers wont hold you up my freind you still be pack :O)
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With or without stoppers they can still fracture and get digested.Some people would cut back the standard stoppers to allow more movement and claimed an improvment in performance.I would leave the standard stoppers in place as is and replace the reeds about every 5 hours of race use or when they pass their gap limit.
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I used to run Boyesen reeds in my RM 250's (86 - 90). Always removed the factory stoppers and used the Boyesen rev plates. Never had a problem. Go for it.
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Best place for B******ens is in the past or in the bin.Under any sort of load the top thin petal flaps around uncontrolably while the bottom petal barely moves.Reed technology has surpassed whatever advantages these crappers ever had a long time ago. Standard with the stops in bent in for high rpm and about 10mm for low/mid rpm will see the same results.
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Best place for B******ens is in the past or in the bin.Under any sort of load the top thin petal flaps around uncontrolably while the bottom petal barely moves.Reed technology has surpassed whatever advantages these crappers ever had a long time ago. Standard with the stops in bent in for high rpm and about 10mm for low/mid rpm will see the same results.
Come on Lozza, tell us what really think. Don't be shy. ;D
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You want the truth? :'(
You can't handle the truth ;D
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Actually, I've heard the no-holds-barred Lozza rave about Boyensens, and I suspect that it would make most sailors would blush.
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That it would ;D,actually their 'Pro' series reeds are pretty good (or on par with everyone elses carbon fibre reeds),their 'Rad' valve can be cheaply replicated at home and is not as good as a V-Force assembly.
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still reacon there is nothing wrong with the steal ones, just get em in tight to the barrell and keep em rough
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;D ;D ;D ;D
Best laugh I had all day Freaky
;D ;D ;D
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Have been watching this with interest and still confused .... sheez guys
Can the Institute recomend where to buy these wizz bang replacements :)
I'm sure this was discussed in the last forum and a member has some new bute material :-X
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Have been watching this with interest and still confused .... sheez guys
Can the Institute recomend where to buy these wizz bang replacements :)
I'm sure this was discussed in the last forum and a member has some new bute material :-X
It all about sucking in as much air/fuel, compressing it and making it go bang. The more air/fuel the bigger the bang.
With a two stroke there are three main barriers to sucking it in; the reeds, the transfers and the crankcase pressure.
To get the reeds to pass more air/fuel you can get bigger reed blocks, get the reeds to flex open more, or you can get more tip surface area so that more air/fuel get passed sooner.
Sometimes you can put bigger reed blocks into the same opening (RZ250 replacing RD250) and you can mechine weld etc etc.
Flexing more is an option (bend or remove reed stops) but normally leads to early failure.
Changing the characteristic of the reeds, Boyesens etc, to weaker/stronger normally has more effect on where in the rev range the power is produced rather than giving noticeably more power.
The multiple reed approach, Vforce reeds blocks and other, gives increase tip surface area which gives both a better responce and passes more air/fuel (= more power better spread of power).
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The pipe helps scavenging too but anyways.................
Yes Graeme is correct tip surface area helps a great deal, but(as what Steve Tassinari from V-Force realised) tip deflection also plays a big role.Old school thought was big deflection = big power but the V-Force cage PROVES that lower tip deflection is the go .Having the petals frimly closed well before BDC sets a better low pressure pulse in the crankcase as the piston starts to ascend.
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you can buy universal carbon fibre reed sheets (carbon tech brand) from Serco in Qld. depending on reed size you can get up to about 4 sets of reeds from the pack. they sell it in 2 thicknesses. it costs about $70-$80 approx for a pack but it actully works out to be quite cheap when you can cut out at least a couple sets of reeds.
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is this what you use in your pe leith... i am about to change my reeds in the PE or check them out at least anyways. seems the only people who make reeds for the old girls are boyesen. i mean they cant be that bad can they. should feel an improvement over the stock reeds, no? i might give them a try anyways. they dont cost much.
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since 1980 the first mod i have done to every 2 stroke is to fit a set of boyesen reeds.(prob 50 bikes) EVERY single bike ran way better right throughout the rev range. if the reeds were stuffed that i replaced, then that would would explain some of the improvements. statistically unlikely for all. my motto is 'theory means stuff all as compared to actually trying it!!'
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A dyno will prove that all wrong.
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I know that pe400's run better on stock metal reeds over boyseens. ;)
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i found the 175 was better on boyeson's than stock?
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dynos are only one part of the process. when a dyno can do laps on the back of a bike it will gain more credibility. for eons magazines have tested items on mx bikes on dynos. but then when actually on the track with a rider a different outcome occurs.
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I agree K20, while Boyesens may not offer up any more actual horsepower they do make the engine more responsive which has the effect of seemingly more power. Everything I've ever fitted them to definately ran crisper and felt better. For the price, I believe they are worth trying as everyone has different ideals. If you don't like them simply pull them, I'm sure someone will take them off your hands ;)
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dynos are only one part of the process. when a dyno can do laps on the back of a bike it will gain more credibility. for eons magazines have tested items on mx bikes on dynos. but then when actually on the track with a rider a different outcome occurs.
That is a silly statement, you can tell everything by the shape of the curve.The shape of the curve will give a instant snapshot of how the bike performs on the track. Magazines test bikes in usualy stock form, they only report what they see they do not tune the engine. Which is why the dyno king usualy has not got the fastest lap time.
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Have been watching this with interest and still confused .... sheez guys
Can the Institute recomend where to buy these wizz bang replacements :)
I'm sure this was discussed in the last forum and a member has some new bute material :-X
It all about sucking in as much air/fuel, compressing it and making it go bang. The more air/fuel the bigger the bang.
With a two stroke there are three main barriers to sucking it in; the reeds, the transfers and the crankcase pressure.
This one's got me wondering. If the fuel mixture is drawn into the crank as the piston heads upwards, wouldn't the volume be limited by the volume of air being displaced above it?
Ie, if you have a piston at bdc on a 250cc machine wouldn't the volume of air drawn into the crank be 250cc as the piston ascends, since it's not a force fed system, 250cc is about all you could acheive.
Worded badly I know, but it's the best I could do on a Monday morn.
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This one's got me wondering. If the fuel mixture is drawn into the crank as the piston heads upwards, wouldn't the volume be limited by the volume of air being displaced above it?
Ie, if you have a piston at bdc on a 250cc machine wouldn't the volume of air drawn into the crank be 250cc as the piston ascends, since it's not a force fed system, 250cc is about all you could acheive.
Worded badly I know, but it's the best I could do on a Monday morn.
Gas compresses. Gas has momentum.
As the fuel air mixture is drawn into the crankcase, there is momentum involved from the previous cycles. Basically, the fuel air mix is still flowing into the crankcase well after the piston hits TDC.
Then there is also the scavenging effect of the pipe which draws the mixture through the combustion chamber, including drawing the mixture through the transfers which in turn draws more mixture into the crankcase.
It actually draws fresh fuel air mix right through into the exhaust pipe but the returning shockwave from the expansion chamber then forces it back into the combustion chamber before the exhaust port closes. So you have fuel air mix coming from both directions helping the charge the combustion chamber (at least when it's on pipe anyway).
That's why expansion chambers are so critical to get right.
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This one's got me wondering. If the fuel mixture is drawn into the crank as the piston heads upwards, wouldn't the volume be limited by the volume of air being displaced above it?
Ie, if you have a piston at bdc on a 250cc machine wouldn't the volume of air drawn into the crank be 250cc as the piston ascends, since it's not a force fed system, 250cc is about all you could acheive.
Worded badly I know, but it's the best I could do on a Monday morn.
At BDC the exhaust port is wide open so there is a bloody big volume above the piston. Case volume is normaly bigger than the cylinder volume with transfer ducts included.
As Chris said a 2T engine is force fed in scavenging and plugging. generaly a 250 would displace 325-330cc per stroke. A fuel particle takes 20 cycles to go through the engine.
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well i just ordered some boyesens. so see how it feels once they are on. whats the worst can happen... for 45 dollars ill just hang them up in the shed as i lesson learnt if they are no good an look around for the carbon sheets as leith was saying. id say trying to find a new pair of metal ones would be hard
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This one's got me wondering. If the fuel mixture is drawn into the crank as the piston heads upwards, wouldn't the volume be limited by the volume of air being displaced above it?
Ie, if you have a piston at bdc on a 250cc machine wouldn't the volume of air drawn into the crank be 250cc as the piston ascends, since it's not a force fed system, 250cc is about all you could acheive.
Worded badly I know, but it's the best I could do on a Monday morn.
Gas compresses. Gas has momentum.
OK not argueing, just asking. My understanding of vacum is that its a case of atmospheric equalization, as the piston travels, up a vacum (negative air pressure) is created in the crank, the area in the crank then tries to equalize itself with the air pressure outside the crank, by drawing air in, once equalized the vacum no longer exists, air stops flowing in.
As the fuel air mixture is drawn into the crankcase, there is momentum involved from the previous cycles. Basically, the fuel air mix is still flowing into the crankcase well after the piston hits TDC.
At 8000rpm, there are 133 revolutions a second, so if my maths are any good the fuel has 266th of a second, before piston goes from tdc to bdc
Then there is also the scavenging effect of the pipe which draws the mixture through the combustion chamber, including drawing the mixture through the transfers which in turn draws more mixture into the crankcase.
I thought the fuel mixture was forced up through the transfers through positive atmosphere pressure (compresion) created by the piston traveling down.
It actually draws fresh fuel air mix right through into the exhaust pipe but the returning shockwave from the expansion chamber then forces it back into the combustion chamber before the exhaust port closes. So you have fuel air mix coming from both directions helping the charge the combustion chamber (at least when it's on pipe anyway).I thought the returning shockwaves where timed to travel back up the pipe to reach the exhaust port just as new fuel charge potentially made it's way out, before the piston closed off the exhaust port.
That's why expansion chambers are so critical to get right.
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At BDC the exhaust port is wide open so there is a bloody big volume above the piston. Case volume is normaly bigger than the cylinder volume with transfer ducts included.
As above, not argueing, just asking. Can you include exhaust system and transfer ports as part of volume, An engines "displacement" is measured through stroke and bore size. If they helped to increase volume of fuel to the combustion chamber, why not just make the ports bigger?
As Chris said a 2T engine is force fed in scavenging and plugging. generaly a 250 would displace 325-330cc per stroke. A fuel particle takes 20 cycles to go through the engine. Is their any research published on this, might be an interesting read.
If the ultimate aim is to get more fuel into the combustion chamber, why do people stuff cranks, therefore decreasing the crank volume?
........now back to Boyseen reeds ::)
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gotta love the boyesen discussion! comes around often. many magazines over years and years have tested items on a dyno then on the track. a popular quote used for years from one mag when they test engine components "a dyno produces cold, hard, statistical facts - not always the same as seat of the pants facts". i do agree with you lozza that examining boyesen reeds certainly does make me wonder why i have success with them, as the smaller set are always so stiff and the upper set too flexible. this seems so for every set i have seen?
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Gas compresses. Gas has momentum.
OK not argueing, just asking. My understanding of vacum is that its a case of atmospheric equalization, as the piston travels, up a vacum (negative air pressure) is created in the crank, the area in the crank then tries to equalize itself with the air pressure outside the crank, by drawing air in, once equalized the vacum no longer exists, air stops flowing in.
If you were talking about a liquid then yes. But the fuel air mix is not a liquid, it is a gas. The piston stopping at TDC does not immediately stop the inlet flow to the crankcase. The momentum of the inlet charge flowing into the crankcase means that it wants to keep flowing into the crankcase even after the piston has hit TDC. It is not until the pressure it is flowing against is high enough to reverse the gas flow that it is no longer charging the crankcase. That's when the reed valve closes to reduce the inlet charge being forced back through the carby. Although that takes time too and so inevitably, some of it does make it back past the reed valve, which affects air/fuel ratios on the next inlet stroke.
As the fuel air mixture is drawn into the crankcase, there is momentum involved from the previous cycles. Basically, the fuel air mix is still flowing into the crankcase well after the piston hits TDC.
At 8000rpm, there are 133 revolutions a second, so if my maths are any good the fuel has 266th of a second, before piston goes from tdc to bdc
Yep, it happens real fast.
Then there is also the scavenging effect of the pipe which draws the mixture through the combustion chamber, including drawing the mixture through the transfers which in turn draws more mixture into the crankcase.
I thought the fuel mixture was forced up through the transfers through positive atmosphere pressure (compresion) created by the piston traveling down.
No, it is sucked up through the transfers by the scavenging effect of the expansion chamber. Think how fast that exhaust gas is leaving the chamber. Much faster than the piston is travelling. The combustion chamber volume is about a third of the crankcase volume. The scavenging effect is much bigger than the compression in the crankcase.
It actually draws fresh fuel air mix right through into the exhaust pipe but the returning shockwave from the expansion chamber then forces it back into the combustion chamber before the exhaust port closes. So you have fuel air mix coming from both directions helping the charge the combustion chamber (at least when it's on pipe anyway).
I thought the returning shockwaves where timed to travel back up the pipe to reach the exhaust port just as new fuel charge potentially made it's way out, before the piston closed off the exhaust port.
Yes but it only does that perfectly at one particular RPM depending on the pipe. The rest of the time it is off to some degree. If the shockwave reaches the exhaust port before the fuel air mix does then the fuel air mix won't enter the pipe much. If it reaches the port after the fuel air mix does then some will enter the pipe and then be forced back into the combustion chamber. It's all in the timing.
That's why expansion chambers are so critical to get right.
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I have recently found out that Boyesen and Carbon Tech are the same company! who would have thought that. Anyway the Carbon Tech people said they actually have the patterns for single stage PE400 carbon reeds and can do them for you on special order. I cant remember the price but from memory it was $30-40 US i think. You can order directly from Carbon Tech USA and saves a lot of time hand cutting them out of a sheet which can be time consuming and tricky especially if you want to do a professional quality job.
Plenty of people have used Boyesens in PE400's with no probs, but you cant expect to drop them in and away you go. You should expect to do some jetting adjustments. Each bike is different.
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Here's a readout of what is happening to the crankcase pressure in a DT3 engine at 9,000RPM (yes, I know they don't normally rev that hard but bear with me). You will notice that the change in pressure doesn't exactly correspond to the change in direction of the piston. 1.0 represents 1 atmostphere. Below one is a vacuum. Above one is compression. The issue is, what is causing the vacuum and compression in the crankcase.
(http://grapevine.com.au/~cjbrooks/images/crankcase.gif)
Now this is a readout of exactly the same engine at the same RPM showing the velocity of flow in the main transfer port. Look at what the gas flow is doing between the exhaust port opening and closing. The gas is still being drawn into the chamber well after the piston starts it's upward stroke. That's the scavenging effect of the expansion chamber.
(http://grapevine.com.au/~cjbrooks/images/tx1.gif)
And here is the exhaust flow. The dark blue line is the flow at the port. See how it flows back into the combustion chamber just before the port closes (EPC).
(http://grapevine.com.au/~cjbrooks/images/exhaust.gif)
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The spinning crank is a big source of intake inertia.Crank rotation is not constant nor is piston velocity, as at TDC and BDC the piston even comes to a complete stop.Crank accelerates faster after TDC and is slower from BDC to TDC.The piston has to also work against combustion pressure.
There are 100's of articles on this type of subject only they are very dry technical reading in the form of SAE papers.
An exhaust or transfer ducts are not included on engine displacement, ports don't do the work the pipe does the work. What would be easier to move fluid with a straw or a 100mm pipe?
The crankcase volume plays no part in the scavenging/plugging process. Case stuffing is a old school idea, as soon as pipes became efficient larger volumes were needed .The only engine that would benefit from this would be a SX only engine trials engines would have low case volumes to begin with.
There a no such thing as a 'shockwave' in a 2T engine it is an acoustic wave that has a particle and a pressure component. Particles travel at half the speed of the pressure.
For the umteeth time there is NO WAY pressure in the crankcase can equal residual pressure from combustion, so the decending piston wil never generate enough pressure to force anything anywhere.
Lozza says dyno's don't lie and people's senses are easily fooled ;D Yes the top petal flaps uncontrolably as rpm rises(Ron Chinoy from RD Dreams strobed and filmed them) how will the bottom petal ever lift? Maybe you have yet to try other reeds?
You obvously paid attention at school Chris ? ;D ;D ;D
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Yeah, this crazy dude explained it to me and eventually it made sense. ;)
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My (limited) understanding of Boyesen Reeds is:
The thin reed helps bottom end power, like it says on the box.
The trouble is that the top reed flutters at high revs (like thin reeds always will), and there's never enough pressure differential exerted on the bottom reed for it to operate properly - so the promise of the best of both worlds cannot be fulfilled.
Ultimately they're basically the same as fitting conventional style reeds made from a thin material. Sometimes this will be a good thing, usually it won't.
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Thanks Chris,
The graphs paint a better picture of what's going on in there.
On the graph that shows flow through transfer DUCT what creates that sharp drop (maybe reverse flow) between epo and bdc, if I'm reading it right the piston is on it's way down, shouldn't gases be on the up and up at this stage?
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Thanks Chris,
The graphs paint a better picture of what's going on in there.
On the graph that shows flow through transfer DUCT what creates that sharp drop (maybe reverse flow) between epo and bdc, if I'm reading it right the piston is on it's way down, shouldn't gases be on the up and up at this stage?
CORRECT. What happens roughly between EPO and BDC and what happens before EPO? This for the eleventy millionth time is why decending pistons have no real effect on case pressures.
CORRECT Nathan how can the bottom petal lift if the top petal blows open with a slight breeze.
As a experiment try hitting a reed petal(any type) with your compressed air gun nozzle and see how far it lifts. ;)
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Thanks Chris,
The graphs paint a better picture of what's going on in there.
On the graph that shows flow through transfer DUCT what creates that sharp drop (maybe reverse flow) between epo and bdc, if I'm reading it right the piston is on it's way down, shouldn't gases be on the up and up at this stage?
That's the transfer port opening. The pressure in the combustion chamber is still high enough to briefly force exhaust gasses into the transfer port. But as the expansion chamber starts to scavenge the remainder of the exhaust gasses out of the chamber, it then also draws the inlet mix through the transfer port. As you can see, the piston is already well and truly on the way down but the residual pressure in the combustion chamber is still much higher than the crankcase pressure. In fact you can see on the first graph that the crankcase pressure never exceeds 1.5 atmostpheres. The peak combustion chamber pressure on this engine is nearly 66 atmostpheres.
I suppose I could run the transfer porting a little later on this engine to get longer blowdown (the part of the cycle in between when the exhaust port opens and the transfer port opens is called blowdown) and take advantage of this remaining pressure but it would mean smaller transfer ports so it may not actually make for HP gains. The other option is to make the exhaust port higher which would start the blowdown earlier. But this one is already wild enough.
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lozza can you recommend a brand of reed and reed thickness(as some brands offer a choice) for a standard 77 rm125b. i will try a set. i prefer low end response. thanks.
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Is it a power reed ? If yes then a 0.38-0.42mm carbon mono petal is the go. I can supply one for $20 or you can get the carbon tech which you will get 2 for how ever much Serco charge or what you can get online somewhere.
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Plenty of people have used Boyesens in PE400's with no probs, but you cant expect to drop them in and away you go. You should expect to do some jetting adjustments. Each bike is different.
yea plan on fiddeling around with the jets, got the pipe on it now. fiddled around with the needle setting last week with a few of the old boys at pt. gawler. it improved the top end but bottom end was running rich. an was fowling plugs if idled for too long. boys down there said best thing to do before changing jets in the carb was to make sure reeds were in good cond. as it could affect it. which is all good. i planned on checking them out anyways an wanted to fit something aftermarket in. im mechanically minded so pulling apart an re fitting isnt no drama. it just the tuning that i have to get around. there theory was to go down one on the pilot an one up on the main. sounded pretty good to me so will see how it will go next time i go out.
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PE man, i advise you to hop onto the Yahoo PE forum and read about the guys digging deep into PE400 jetting. The guys you want to speak to specifically are Bob in UK, Franois in Canada and Alex in Victoria. Also maybe oldfart on here could give you a few pointers too. I would be speaking to other owners of 400's who already have them set up right before listening to just any one who doesn't specifically know the 400's inside out unless they were a professional 2T tuner. I also would highly advise you speak to Ian Williams www.iwt.com.au in Adelaide. He's a 2T nut from way back and also is a Mikuni dealer/distributor. Yes hsi site looks all liek karting stuff, but he used to and probably still does a lot of bike stuff, its just that the karts have always been his main thing as he races them
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no worries leith will look into that, yea they told me to head down there an have a word with him. an to also go there to buy the jets.