Question about NSR models and carbs?

[Leviathan]

well basically im led to believe older models were supposed to be faster and i believe i was told this was achieved from larger ports on the barrel, and possibly a larger carb?

i was just wondering if anyone would like to put some light on the truth, and how exactly a wider carb helps performance where as the jets stay the same? and more importantly why using larger carbs are so risky.

not to sound like a fool, but to be honest its always interested me as to how we know where the limits of what combustion an engine can take, how an engine can combust for so long with a standard carb, but with a bigger input of more fuel can just make it seize?

[2006 _p1]

Well wider carb lets more air and fuel in and some goes for the ports!

[Welshd1k]

lies ..........


older nsr's or ns's as they where called are slower than nsr's

they have a 26mm card where as all the newer nsr's have 28mm delorto carbs

and if i am correct all the heads' barrels are teh same on the nsr's 91 onwards?
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Current >Cb600fx hornet, ns125rk90 21.7bhp@wheel<
Sold >,zxr400 h2 (33bhp kit) rd350 n1 rd350 lc2<

[Thom]

ns125r is different to an nsr125 just so you know...
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Current Bike: 1996 Yamaha TRX850 (The Japanese-Ducati One).
Previous Bikes: 1990 Honda NSR125 'Rothmans' (The smoking one), 1990 Suzuki GS500e 'Caf? fighter' (The loud one), 1987 Kawasaki GPX400r (The quick one), 1997 XJ600s Diversion (The reliable one), 2000 Kawasaki ER-5 (The spontaneously combustive one)

[Welshd1k]

[Leviathan]

yea i was looking for information based on the progress of the ns/nsr generally because im confused as to how an RS125 say can have a 34mm carb and still not explode....well...when i say wont explode you kno what i mean lol

cheers G.

[Kickstart]

Hi

RS is in a higher state of tune. A carb alone doesn't so much give you performance, more supply fuel and air adequate for the performance. If the rest of the engine can produce more power than that size of carb can flow the mixture for then you loose power.

The RS125 34mm carb runs far larger jets than the 28mm carb fitted to other RS125s.

Larger carb is a bit of a double edged sword. While they does supply mixture for the higher power output they will also tend to have more problems at lower engine speed.

All the best

Keith
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[Leviathan]

i see, but what confuses my small brain is that how does the RS pull it in wheras the nsr obviously cant but the cc is still the same, the only thing i can assume this is down to parts with less friction and maybe the higher compression to suck it all in, i was led to believe the higher cc meant the more ability to suck in more fuel and compress it further and then bash it down faster with more power? please correct me if im wrong kickstart you seem to have a much better understanding of this than i do lol

cheers fellas for your help,
G

[Leviathan]

and so in theory putting a huge carb into a little engine would give better performance (i assume this from a mate of mine who put a 35mm carb on a mito, it ran like shit off a shovel but blew up shortly after) but then how is this possible without flooding? or a painfull de crease in performance as you pack on the accelerator?

and this is why i get confused lol,

so many terms and so many possibilitys,
it is a fine science but learning how something works gives me a better understanding of how to take care of it better lol

cheers,
G

[Kickstart]

Hi

No, a huge carb alone will do nothing. A larger carb will support an engine that is capable of producing more power, but will likely loose lower down power.

The larger the carb for an engine size then the lower the air speed through it, so less pressure drop which means it doesn't atomise the fuel as well. At higher engine speeds the gas speed is high enough. Hence you can land up with an engine that may make a bit more power but the loss of low down power makes it feel like it is making quite a lot more power.

Anyway, as such the size of carb on an engine is a compromise. And depending on the maker they may have already gone for as larger a carb as will work effectively on the engine. Or they might have used a smaller carb and lost nothing at the top end, or smaller again and lost some top end.

With fitting a larger carb yourself you then have all the fun of trying to get the jetting at least close to correct. And the makers have a lot more experience and equipment to get this right than most people do.

All the best

Keith
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[Leviathan]

i see so am i right in thinking the pressure from the downwards piston action has to really suck the fuel past the jets or it wont enter the cylinder in the correct er... consistancy?

this is beggining to make sense, and i can see what your saying, so bogging down will happen in lower revs because there is less of a force present?

the only thing left to clear up is how my mates mito managed to pull it off, would you say the mito is just capable of doing more with its compression? and does that have something to do with the mito having a piston size slightly bigger than the nsr? or am i on a totally different page lol,

cheers kickstart, hows it going by the way mate?

G

[Kickstart]

[Leviathan]

exellent thank you kickstart, thats some useful information, so ramming a huge carb isnt ever such a good idea, from what i had gathered before this post was that if you rammed as much fuel and air into the cylinder you get better performance which isnt really the case, althou maybe the case in superchargers lol. doesnt quite work for a 125 2 stroke!

cheers,
G

[Kickstart]

Hi

Larger carb might be necessary to take advantage of other engine mods. It is all a compromise.

Basically it is true that the more fuel / air you ram into the cylinder the more power you will get. But a carb doesn't really do that, it just allows in the fuel / air demanded by the engine up to a limit. You can use a larger carb to raise that limit, but if the engine cannot draw in more mixture it is a waste, and you will likely loose out at the bottom end.

All the best

Keith
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[Leviathan]

ah, right now i see where iv been getting confused, i was under the impression that the carb would allow for extra fuel intake as the jets got wider (which i assume it still does) but what i didnt realise was that the engine is the limiter(which is what you were saying all along), the limit is when the piston stops moving down drawing the fuel in and returns to compress it, the idea of turbo chargers and super chargers are quite impressive, yet obviously you need an engine that can afford to compromise... its a shame that they dont make small ones for a laugh.

anyway cheers for that, at last finally i have got there.


cheers,
G

[Kickstart]

Hi

Trouble with both superchargers and turbochargers (well, technically a turbocharger is a type of supercharger) is that they take power to drive them. With a turbo this means that you likely loose a load of power at low revs and gain some at high revs, but also give it a trickier to control throttle response (ie, you open the throttle, the engine responds to that, the turbo gets speeded up by the engine waking up and then that shoves more fuel / air into the engine).

Carb jets flow fuel, and the size of the jets affects how much fuel they flow, but you need the air to go with them. Fuel air ratio should be around 14 parts air to 1 part fuel (by weight, not volume), but you have advantages running it a bit either side of that. Bit more fuel and you get more power, bit less fuel and you can reduce fuel consumption (at the expense of emissions of oxides of nitrogen going through the roof).

However it isn't only the size of the jets that affects the fuel they flow. As mentioned earlier the carb works by the pressure difference between the air going through an atmospheric pressure (basically the atmospheric pressure presses on the fuel in the float bowl). If you do something strange then this can have odd effects. For example full throttle on a slide carb at low revs and the air pressure through the carb will be barely lower than atmospheric pressure, hence barely any fuel gets pushed into the fuel / air mix going to the engine.

Worse than that run a wrongly designed turbo setup and you will pressurise the air going into the engine, and land up with it at a higher pressure than the air pushing on the fuel in the float bowl. Hence no fuel goes to the engine.

All the best

Keith
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[Leviathan]

actually i didnt know that lol,

so if you dont mind me asking, why dont turbo chargers and superchargers come in like powervalves, engage when the engine is up to speed?

and so with the air filters does that mean say a huge air filter or no filter at all (excluding enviromental conditions) would perform badly because the suction would be drawn from the air instead of the float chamber? or is there some kind of force stoping this that im not taking into account?

cheers for the information,
G

[Kickstart]

[Leviathan]

lol, so the engine doesnt actually use suction pressure to move the fuel in alone? it s the air that sucks the fuel in using the atmospheric pressure? i mean how do airplane engines work at high altitude where pressure changes?

that makes alot of sense, at a guess id say thats the best way to make sure the fuel is regulated and mixed with the air as it enters the cylinder, but what just occured to me is that that a i thought a small or blocked air filter would cause the engine to run rich,
how does that work when the fuel is drawed in by the air passing?

cheers kickstart im slowly getting there,
G

[Leviathan]

lol, sorry i have another question,

so am i right in thinking the carb restriction is the diameter of the carb itself? seeing as the end of the carb where the air filter goes on is big, and the other end going into the manifold of the engine tents to be smaller?

hope im getting somewhere lol,

cheers,
G

[Kickstart]

[Leviathan]

right exellent, i think i get it now, its amazing how much technology went into carbs and for them to change the system completely with injectors etc.

but this sorta stuff isnt covered in a haynes manual lol,

where did you get all this knowledge anyway kickstart??


cheers,
G

[Kickstart]

Hi

Too much reading over too many years.

In many ways electronic fuel injection is simpler than carbs.

Mechanical fuel injection seem to land up being very complicated.

All the best

Keith
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