air box removal

[Paulwhite]

good morning/evening! just a little help please cause im stumped

right iv managed to get the cbr 125 engine into the xvs bobber frame, but iv come across a problem that i need a little help with.

the air box is really bulky and has one of them flat square air filters inside, and it just doesnt go with the theme of the bike, plus it wont fit without some Major modification!

now iv experimented in the past with removing the air box on a xv 125 and putting a cone air filter on the carb instead, needles to say it didnt work and i found the only way to make it work was to attach the air filter to the air box as the bike wouldnt run without it.

now i was wondering is there any reason why the engine would need the air box? and is there anyway of making it work with a cone filter? im quite handy with fiberglass so i could make an air box to go over the filter but would like a better idea of how it works.

cheers.

.Paul.

[Paulwhite]

well im guessing that no one can help so iv ordered an airfilter and am going to experiment and see what the crack is.

[Airhead]

i guess you will be going down a tricky road of jetting, you may never get it spot on as there is more than just jets to consider. i'll give you an example...There were two versions of my bike (DT175MX), each version had different airboxes, the early one had a more restrictive airbox than the later one. They both had similar carbs but the later version had much richer jetting in order to compensate for its more free flowing airbox. Not only that but it has a richer throttle slide cutaway profile too so you see that jetting changes alone will not necessarily come to the right conclusion.

[Paulwhite]

well the way i think it works is similar to a sink. (bare with me on this)

you fill a sink with water (like the box is full of air) and its calm cause its contained,and therefore more "dense", so when you take the plug out (or when the intake valves open) the "dense" water flows at maximum capacity into the drain.

but if you make the water turbulent (like the air would be without the box) then its not as "dense" therefore it dont flow as well into the drain.

sort of like trying to catch your breath on a really windy day.

iv done a little research as there isnt allot of info on this matter, and all iv found is that the air box plays a part in the performance and that if set right could increase hp throughout the rev range. but if set wrong (or removed completely) then the engine will suffer for it.

so once i get the filter im gonna have a play and see if i can come up with a smaller alternative to the box, or try a ram air setup.

[Cynic]

I know when i had TTS tune my old 600 they only remove the airbox on full drag bikes.

I have read that having a pipe say 2-3 inches between the carb and the filter can help allowing some static air to help things along. How long exactly? dunno. Worth a try though, 125 motors havent a lot to loose.

[drewpy]

I have read that having a pipe say 2-3 inches between the carb and the filter can help allowing some static air to help things along. How long exactly? dunno. Worth a try though, 125 motors havent a lot to loose.

thats why race bikes have velocity stacks, its helps speed up the air into the carb, hence more power!

[Cynic]

thats why race bikes have velocity stacks, its helps speed up the air into the carb, hence more power!

Are'nt they unfilterd though with only a mesh screen to keep out rubber, (turns into grinding paste if it gets in the motor)

[drewpy]

same difference, air filters filter out the air before the tubes/stacks

[Paulwhite]

well i was thinking something along the lines of a side mounted velocity stack thingy going into a chamber with the air filter inside then that would force air into the chamber ready to be swallowed by the engine, but id have to have a play and see what i come up with.

[dan28]

can you put some pics up to show the air box an how it mounts to the frame etc

my xvs ran without filter (nt very good until warm)

can you not mount a cone filter directly to the carb

like this

Uploaded with ImageShack.us

( i expect this would be crap in winter due to carb iceing)

you could cut off the part of the cbr air box that mounts to the filter bolt it to the filter then use sections on f the cbr /xvs air boxes to make a mock up one that fits

then either recreat it in fibre glass

or fabricate one out of sheet metal

similar to this

Uploaded with ImageShack.us

[Paulwhite]

you cant just put a filter on the end of the carb, as the little 125 engine just wont work (this was my original point) the air needs to be sucked in by the engine so the vacume can suck the fuel from the carb, mix with the air then go into the chamber. the dragstar 125 uses a special pipe to control airflow into the engine where-as the cbr uses a box, both restrict air flow in some way. iv been doing some research into it and have come up with a new setup.

im going to use a silicone u bend with a cone air filter on the end, now to restrict the airflow im going to have to think of a way to put a sort of butterfly valve in there (similar to the throttle valve on the carb) then i can have a play with different setups to find the best for the engine. might even make it spring loaded then it would free up airflow when the engine needs it. in theory it should work and all id need to do is find the right tension spring and put a slightly bigger main jet in. but il order the parts, have a play and post the results

[PhilDawson8270]

you cant just put a filter on the end of the carb, as the little 125 engine just wont work (this was my original point) the air needs to be sucked in by the engine so the vacume can suck the fuel from the carb, mix with the air then go into the chamber. the dragstar 125 uses a special pipe to control airflow into the engine where-as the cbr uses a box, both restrict air flow in some way. iv been doing some research into it and have come up with a new setup.

im going to use a silicone u bend with a cone air filter on the end, now to restrict the airflow im going to have to think of a way to put a sort of butterfly valve in there (similar to the throttle valve on the carb) then i can have a play with different setups to find the best for the engine. might even make it spring loaded then it would free up airflow when the engine needs it. in theory it should work and all id need to do is find the right tension spring and put a slightly bigger main jet in. but il order the parts, have a play and post the results

For optimum throttle response, you should have 130% of the engine's capacity of filtered air between the filter and the butterfly.

[Paulwhite]

For optimum throttle response, you should have 130% of the engine's capacity of filtered air between the filter and the butterfly.

so thats 16250cc of filtered air????? (my engine is 125cc) i dont get how you have come up with that figure. and why its important.

[PhilDawson8270]

0.125litres x 1.3 = 0.1625litres so, 162.5cc of filtered air.

As above, it's to do with providing smooth air flow, and having readily accesible air for the engine to take on quick throttle openings, this is the job of the airbox (plus protecting it from heat), adjusting the dimensions and volume can affect how the engine runs.

130% isn't a hard and fast rule, but is a popular starting point for air intake systems in motorsports, generally rallying where a lot of people still run airboxes, there's a lot of research around the K&N site about it, if you read around.

[Paulwhite]

0.125litres x 1.3 = 0.1625litres so, 162.5cc of filtered air.

As above, it's to do with providing smooth air flow, and having readily accesible air for the engine to take on quick throttle openings, this is the job of the airbox (plus protecting it from heat), adjusting the dimensions and volume can affect how the engine runs.

130% isn't a hard and fast rule, but is a popular starting point for air intake systems in motorsports, generally rallying where a lot of people still run airboxes, there's a lot of research around the K&N site about it, if you read around.

ahhhh ....cheers m8 dont suppose you could post some links to some good sources? (so i can do some research and get a better idea)

[dan28]

from what you said above what about fitting a hypercharger with the xvs air filter element inside

i ve fitted one to mine but the vacum isnt strong enough to open the flaps so i need to think of something it makes an evil crackle at high revs though

[PhilDawson8270]

Best thing to do is get down to a library and take a good old fashioned, no bullshit book out about 4 stroke engine tuning.

[Paulwhite]

little problem ....iv got 3 kids under 4 years old the youngest being only 2 weeks old so library is a no go, but internet on the other hand, i can use that anytime iv had a read on 4 stroke tuning and come to the conclusion that unless you have a small fortune to waste then there is no point trying to tune the 125cc 4 stroke. but im not trying to tune so to say, im just replacing the original airbox cause it wont fit.

and a hypercharger? .....no point id have to put a new engine in every week

[PhilDawson8270]

little problem ....iv got 3 kids under 4 years old the youngest being only 2 weeks old so library is a no go, but internet on the other hand, i can use that anytime iv had a read on 4 stroke tuning and come to the conclusion that unless you have a small fortune to waste then there is no point trying to tune the 125cc 4 stroke. but im not trying to tune so to say, im just replacing the original airbox cause it wont fit.

and a hypercharger? .....no point id have to put a new engine in every week

If you're not attempting to get it running as well as possible with a smaller airbox, then just make something fit and play about with it till it runs how you're happy with. It all depends what you're aiming for to be honest. I found a lot of internet articles often came with a lot of garbage in them too.

[Paulwhite]

If you're not attempting to get it running as well as possible with a smaller airbox, then just make something fit and play about with it till it runs how you're happy with. It all depends what you're aiming for to be honest. I found a lot of internet articles often came with a lot of garbage in them too.

well im trying to get the same performance that i had before with the original box (if not a little more) but iv searched the internet and havent come up with any decent info on the matter, well not for a 125cc engine anyway.