Is a K&N Dangerous to my bike ?

[Matt-]

Well i have read somewhere that a K&N on some 2 stroke bike's can very dangerous, is this correct ?

The bike is a 1991 Honda NSR125FM De-restricted with Full Arrow exhaust system, the k&N is a cone type filter, when i have ridden it it seems to run fine ?

Many thanks
____________________
RIP jack (wheelie king), you will never be forgotten.
Current Bike's - Road Legal TZR 125 Pitbike, Aprilia SR50R 2010

[Bezzer]

A K&N in most cases is more free flowing, more air into the system=weaker mixture=meltdown in some cases dependant on how weak the mixture is and more so with a 2T than a 4. If you bought it like that with everything fitted, pound to a pinch it's been sorted......hopefully.
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I used to be indecisive but now I'm not quite so sure.

[Matt-]

[moonzoomer]

Check your sparkplug for colour, this will give you a basic idea if the bike is running rich or weak due to the mods, also check your carb jets sizes and compare to original spec, normally jet size will have been increased after an exhaust and filter mod

[Matt-]

[moonzoomer]

https://www.verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm

[Bomberman]

Spark Chart
I always read (on here mostly) that they would run far worse on my TZR than the original so I avoided them. If the carb has been set up though, fair D.
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'Allo! My name is Inigo Montoya, you killed my father, prepare to die!
'89 TZR 125 - '94 GPZ500s - ZK3 GSXR600 Alstare

[Matt-]

Think i am going to re fit a standard air box
____________________
RIP jack (wheelie king), you will never be forgotten.
Current Bike's - Road Legal TZR 125 Pitbike, Aprilia SR50R 2010

[U_W v2.0]

if its had a K&N filter fitted with a bigger jet and you reinstall a stock airbox you will need to reduce the jet again
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BCF's biggest cunt list: Cansa, Pits, Rob

[Matt-]

We will see what happens, i have bought a standard JC20 Air box just in case, so if i get the jetting right will it be ok and safe and not harm the engine ?

What jet size do i need ?

jet's and tuning really confuses me

Matt
____________________
RIP jack (wheelie king), you will never be forgotten.
Current Bike's - Road Legal TZR 125 Pitbike, Aprilia SR50R 2010

[U_W v2.0]

best thing to do would be get the haynes manual or manufacturers manual for your bike and check the carb jet sizes in there.

if its a truely stock air box for your bike then the sizes in the manual will be fine.

however, if you already have the K&N + correct size jet for it, why not leave it as is?

what was the result on checking your spark plug?
____________________
BCF's biggest cunt list: Cansa, Pits, Rob

[Matt-]

[_Iain_]

[Matt-]

Very helpful sir

Karma Given, i did know some of what you said about ratio's etc but never ever had to re Jet anything, 2 Stroke's really interest me so it's all now being sunk into the brain.

So standard Air box back on?

As i have a ARROW exhaust system will i still have to re jet, i would of thought NOT as my NSR is naturally DE - restricted as it's an import and full power it should have correct jetting, being honest an arrow exhaust system wouldn't really upset the ratio anyway, it's only lighter, slightly bigger expansion chamber ?

to be honest as well i only like K&N for noise
____________________
RIP jack (wheelie king), you will never be forgotten.
Current Bike's - Road Legal TZR 125 Pitbike, Aprilia SR50R 2010

[Fizzer Thou]

[Bomberman]

[Robby]

[_Iain_]

D'oh.

I knew what i meant
____________________
Please be aware that the above post may be full of complete nonsense.
Riding: '07 KTM Duke II, Baotian BT49QT-20 Driving: '88 Volvo 340

[Matt-]

Thanks for all the reply's, I have bought a old standard air box off eBay which will hopefully fit my bike, bought a new air filter element for it as well, il put that back on, if it runs better I am going to keep the standard on their, the after market exhaust system should not make much difference to how my air / fuel ratio as its not a high end pipe,
The derearitcted bikes were jetted high anyway for the winter, so my arrow shouldn't make all that much difference, it's just lighter.

Also the exhaust has a lot of carbon in it?

Would this be a symptom of it running lean?

Matt
____________________
RIP jack (wheelie king), you will never be forgotten.
Current Bike's - Road Legal TZR 125 Pitbike, Aprilia SR50R 2010

[Kickstart]

Hi

It is a bit more complicated.

There are several ways the carb controls the mixture, depending on how much throttle you are using. At idle and just off idle it is controlled by the idle air screw or idle mixture screw depending on the bike (similar functions, one controls the fuel through the idle circuit, the other the air through the idle circuit). The needle controls the mixture for most of the throttle range (the shape of the bottom of the carb slide controls it on the transition from the idle screw to the needle) and at large throttle openings it is controlled by the main jet. Some carbs also how power jets to give further fuel on very large throttle openings.

With these combinations it is entirely possible for the engine to be running rich at one point in the throttle / rev range and lean at another.

Also on a basic slide carb there is nothing to make up for user error. Whack the throttle wide open at low revs and it is going to give a really dodgy mixture. Gas speed through the carb will be low, hence the air pressure drop through the carb will be minimal and little fuel will be drawn from the float bowl (hence most carbed larger bikes using CV carbs which are far more idiot proof).

Added to that there are the resonant frequencies that Fizzer Thou touched on. The lengths of intakes sets up pressure waves at certain rpm which bounce around forcing more mixture in. However this can also land up with air going through the carb, then bouncing back and then through the carb again at certain revs (you might be able to see this with no air filter in place, with a mist of mixture appearing to sit in the air before the carb).

A K&N cone filter can be freer flowing and hence lean the mixture off in general (although I wouldn't guarantee it, and it might well have a smaller area than a conventional filter). However they also have a metal end cap which can screw up the pressure waves through the carb.

It is possible to get them to work but takes a bit of time playing around with jets.

You shouldn't have to worry about the 2 stroke oil on a bike running an oil pump. However is a bigger issue on a bike running premix.

[skatefreak]

For the carbon deposits in the exhaust I suggest for standard pipes, this:

https://t2.gstatic.com/images?q=tbn:ANd9GcQdsm7-ejPR78CyyU_t6llTwnzPBbQCwPYp7DZwt0z-cRYWeyPu&t=1

Pull off the engine, bung a carrot in the end and throw some of this in with water.

It will bubble for half an hour or so, drain, repeat then rinse and run to dry.

Be warned, if you have a non standard pipe you may have aluminium in there and this may corrode it badly

-Jvr

[Matt-]

So what would you suggest doing Keith ?

I am really confused on what to do for the best, i want the bike to have some reliability so the K&N Filter is a DEFINITE no.

I want to keep the arrow exhaust as it's light weight, and a better sound plus i don't want to have to buy a standard exhaust.

I have some sense in mechanical knowledge of 2 stroke's what i have learn t on here.

Any idea's what i can do ?

Matt
____________________
RIP jack (wheelie king), you will never be forgotten.
Current Bike's - Road Legal TZR 125 Pitbike, Aprilia SR50R 2010

[Bezzer]

For all you know it might have already been sorted properly for it's current set up and running fine mixture wise.
For peace of mind have a dyno run to show how the fuelling is then you can take it from there knowing it's either OK or you have a benchmark to sort it out .
____________________
I used to be indecisive but now I'm not quite so sure.

[Matt-]

Yeah i know but for the hassle it's worth i am going back to standard, and if the k&n get's wet it will be UN - reliable

Will this jetting kit be ok? it's an upgrade kit for a Honda NSR 125 ?

https://www.dellorto.co.uk/merchandise/products_details.asp?PartNo=NSR%20JET%20KIT&CategoryID=2&PartsectionID=24

Matt
____________________
RIP jack (wheelie king), you will never be forgotten.
Current Bike's - Road Legal TZR 125 Pitbike, Aprilia SR50R 2010

[Bomberman]