K & N Filter on a Honda Cub

[Newtsalad]

I've been reading quite a lot of posts put up by folk putting K&N's on their Cub, and having difficulty setting them up.

ag did this on the 125cc lump he put in his 90. it took a lot of effort to get it right, and much experiments with jetting etc.

The wiser folk on here rightly say, leave it well alone. They really do run best with the airbox.

However, for the young & foolish who insist on the K&N's. I've had flashback 32 years to an RD400 I had. I went for the cafe racer look, and accordingly I insisted on running bellmouths.

I recall that to get it running right, I simply restricted the airflow by placing two metal strips over the carb inlets.

I actually used some of the metal bands off packaging. I made them the width of the inlet then added 3/4" and turned 3/8" over at each end.
This sat across the inlet and was held in place by the bellmouth. I also used a piece of stocking to keep out stones, the piece of banding supported this.

The bike ran perfectly, just as it should. I was always amazed, at how much of the inlet area I had to cover to get it to equal the original airbox and filter.

Just a thought, for those who insist!

[aliengravy]

Or just buy loads of jets and get fiddling

Trouble is, as Fuzz has said on here many times, a hot summers day it will be bogging down, and on a cold damp winters night it will go like stink!! The weather will effect the jetting!

You will also burn more fuel, and it will make more noise! Its pro's and con's all the way!

Personally my bike is just a "toy" so i dont have to rely on it! If i used my bike every day for commuting i would leave it well alone!

But hey! Bags more fun

[frenchy3]

I had an rd 400 and 350 lc with k@n,s and never got the bikes to run as well as the factory airbox setup. Japanese factories do fit cheap parts but have huge budgets for development and they generally get it right. The k&N does benefit from some airflow restriction or acceleration device like a bellmouth to work at its best,as air velocity needs to be correct. A rolling road tune with a bike specialist with access to all the range of jets,slides and needles etc is often the only way as they can monitor the mixture at all rpm and loads. My best result with a K&N was a panel filter inside the standard airbox(less restrictive and you can wash and re-oil so they last as long as the bike) The amount of oil you put on the K&N is also critical as too little results in incomplete filtration and too much restricts the airflow and either just runs out of the bottom or is ingested by the engine. Weather has a large effect as well most bikes and cars run better in cool damp conditions but this is more noticeable with the K&N set up. They can suffer from carb icing in winter as the air vapour freezes in small air drillings and normally the pilot jet as well and the bike will cut out at low rpm. This then needs a supply of hot air via a ducting from around the engine and you lose the benefit of cool dense air and lose power.
Steve.

[richg]

on my old 70 i used to have i put the chrome air box cover on it and i could not get it to run right. I had to cut up an old plastic bottle and put that under the chrome lid and only then would it run right.

[Bobn]

As as has been said, they are a nightmare.
I've found too that it makes the set-up more sensitive to temperature, so you end up having to change jets in summer / winter. They don't work in the wet and you get carb icing problems. I've only ever used them as a last resort (on a chop for instance).
My best advice would be not to forget about the jet needle when tuning carbs. Everyone fiddles around with the main jets but the needle has a huge effect on the running.
I usually found that main jet up 2 sizes, needle down 1 or 2 clips did the trick.