Airbox question

I'm a lawyer, not a dirt bike afficionado. But i'm doing my best to tear it down, clean it, add some goodies, and get it ready for the TAT this summer. This forum is just what I need--thanks! So here go the first of many questions I have:

I have a uni foam air filter. Just under that is a metal screen. No mention of it in the repair manual. Anyone else have this? Is this a necessary item? Should I grease both sides of the metal screen when I re-install? Treat the screen itself with anything?

The uni is shaped like the outer airbox (ie, flat and kinda kidney shaped). Are there filter options for this bike in other shapes--such as for the cone-shaped snorkle closer to the carb? Would the metal screen I mentioned still be used?

Thanks. Many more questions to come under other topics.

The metal screen is meant to keep the filter from catching fire if your pig backfires. Some people get less restrictive ones or remove them all together its your call. The UNI filter is probably one of the best you can use as the foam filters do a great job filtering, especially dust and sand. I think they make a pod filter that sits on the carb but not sure many people use them.

I've got the Uni as well. With it you do not need the metal screen as the Uni has one built in.

Also the metal screen is no guarantee that you will not get a fire. Mine was in and my stock air filter caught fire.

So far I like my UNI. I cant remember if I kept the metal one in or not. Ill have to look

Its a very good idea to keep the screen in place. It helps hold the filter in place so you dont get gaps that dirt can enter and helps prevent fire in case of a backfire. Ive never had it happen, but I guess anything is possible. You can get one that has larger holes for a little more airflow, but most of us dont need all of that performance. If you are running the TAT you want reliability. You dont need a f'ed up filter while out on the trail. Keep it simple. The UNI filter and No Toil oil are a great combo.

But the Uni, at least the one I got has the metal screen built into it. There is simply no room for the stock screen.

Great info. Thanks gents. My uni doesn't have it's own built-in screen. Curious. But the airbox is free of dust, and it sounds like keeping the screen is a good thing. Didn't know I could catch fire! My stock carb is nicely tuned, so i'm not going to mess with intake or outtake. Just wanted to make sure everything is as it should be. If I could have plated a new xr in CA, I would have done so. You take your chances on a used bike, but I think I've got a good one here. Looking forward to miles of smiles.

Please don't run after-market filters. There's a lot of bad advice out there that seems intuitively smart but turns out to be counter-productive. Put this gem in that file. Your bike is used so start using OEM air filters now to stop the damage.

If there is one area Honda has correct in an otherwise admittedly terrible intake design, it's the XR650R air filter. All these after-market air filters flow more air by allowing more air through the filter. There's only one way to do that and that is larger holes in the filter. Larger holes means more dirt gets through. Smaller holes means more surface area for oil, slower air speeds, longer path for dirt to be caught. It is really just that simple. It may not seem like much but remember those intake valves are traveling very, very fast. They are moving up and down several thousands of times per minute. So that piece of dirt than passes through your aftermarket air filter may only be traveling at 40mph, but the speed of the valve's travel may increase that impact speed to hundreds of mph or more. Plus add a few thousand degrees of heat in the chamber. That little sand particle WILL pit the valve with the energy of its speed and the hot environment. Pitting means knocking material off it and now you have a few items traveling out the exhaust valve/seat and so on. Perhaps taking out your seat and exhaust valve too.

I can send you pictures of intake valves from after-market filters and intake valves from OEM filters if you really insist. I bought my BRP used from a guy who hardly rode it and lived across the street from his professional mechanic. The bikes was maintained properly. It had about 2000 miles on it. He used aftermarket air filters. I had the head off to install a race head and the HRC race cam and sure enough there were the pitted intake valves.

Or, don't take my word for it, call Precision Concepts and ask them. They'll sell you whatever you want to buy but they'll also tell you what is the smartest decision. And they were the Team Honda shop for the XR650R. Siraton has moved over to Johnny's shop across town but he'll confirm it too.

Use the stock filters. Clean and re-oil and let set 24 hours before use. Get an excellent can of waterproof grease, Scuteriawest would be a good shop to find this at in SF. Grease all the lines where the filter meets the airbox frame and the outer cover. If you vent the cover then grease the seams of the vents where they meet the air filter.

You'll see MUCH more airflow from a vented cover than from after-market air filters too. But depending on what else you do that mod might mean rejetting.

If you need a mechanic I know a great guy in Petaluma and Berkeley Honda also gets a lot of great word of mouth, though I've never used them.

I hear ya, Bump. Smart advice. I guess I'm lazy to change because the uni comes so highly recommended by so many people, and i don't want to monkey with the carb. poor excuses for actual facts, i recognize.


Hypothetically, and physics-wise, I would think that if two filters had equal filtering effectiveness, a filter that has a greater surface area would allow more air flow without necessarily allowing dirtier air. Maybe I should invent an airbox mod so that "SFXR Filters!" could be added. I could quit this job and make a living in motorcycles! My american dream.

You'll find a lot of things on this bike that come recommended by a lot of people, but not from Ogilvie and Team Honda. They get to use anything they want. Ogilvie started designing that bike in the mid 90s. The showroom bikes were essentially de-tuned race bikes. When he gave Johnny and Henge the full race XR650R with teh race piston on race gas you know what they told him? They said it was too much and the bike was coming out form underneath them. So obviously the stock air filter ain't the problem.

A friend of mine who was a Supercross mechanic schooled me. I was an after-market filter guy too. It just made sense. On two-strokes it can make sense. There are no valves, top-end rebuilds are cheap, and there's lots of oil in the fuel. Not sure about physics. But the oil coats all the surfaces and the idea is for the air to have a convoluted path and run into as many oily surfaces as necessary before heading into the engine. So you'd need some speed for it to happen efficiently but not so much the air isn't scrubbed. At this point it's important to remember that an internal combustion engine is really just an air pump. By removing restrictions on the intake you can increase the efficiency of the pump. In cars aftermarket filters are competing with disposables, so that's more of a clear win. In any case, I doubt you'd notice the difference between filters in respect to performance.

Run it without a filter down the street and I doubt you'll feel a usable difference.

You might also check the valves to see how much adjustment is left in them.

Have you gotten the GPS thing figured out yet?

SFXR wrote:I hear ya, Bump. Smart advice. I guess I'm lazy to change because the uni comes so highly recommended by so many people, and i don't want to monkey with the carb. poor excuses for actual facts, i recognize.


Hypothetically, and physics-wise, I would think that if two filters had equal filtering effectiveness, a filter that has a greater surface area would allow more air flow without necessarily allowing dirtier air. Maybe I should invent an airbox mod so that "SFXR Filters!" could be added. I could quit this job and make a living in motorcycles! My american dream.

What foam filter oil or spray are your guys using?

I use the Uni filter with PJ-1 Super Tacky Spray, but was wondering if others like something else better

I have been using No Toil for several years with no issues. You can throw it in the washing machine to clean them. Its bio degradeable oil. You may not want the SO to see you doing it though.

+1 No toil.

Man, Bump is coming through like Mr. Honda, himself (or better)! Thanks!

I recently bought a UNI filter to replace the worn out stock air filter. It came with a metal plate, not to be confused with the "backfire" screen in the airbox.

The plate did not fit very well, at all, so I omitted it from the install. I used generous amounts of grease to seal the perimeter of the filter to the airbox. It's all clean inside (to the eye and finger test), but I'm going back to the stocker after reading Bump's comments!

Hell, the pig has lasted ~15K (90% dirt) with a stock filter, and runs like new. Why screw with things (why did I do that? - don't answer that )?

Dave

Bruce Ogilvie was pretty well known for preaching about the Honda air filters being best. I just don't want to reinvent the wheel.

Or waste money. That's what boats are for!

Bump - wish I'd read your post before I got the Uni (never put it in though) - Going to keep runing with the stock filter - thanks for the advice

If you read about the guys doing really long rides, like some of the guy on The HUBB-that's a great web site, and I think there's a few over there who've ridden a BRP around the world, many try to use Honda filters. They roll them up and stash them in places. The filters are not fragile. One guy I remember carried either a cover or a 2nd air filter cover to use in really dusty conditions. He said the decrease in power wasn't noticable at touring speeds. Made me think of creating some sort of fliter cover for the vented side panel on my bike. I just remembered it after reading your post. My idea was to use a stock filter and cut it to cover the vent. Then take a 2nd vent cover and cut out a frame for the filter that would match the vented side panel. I think I never could come up with a proper way to attach it.

Years ago when I used to ride the dunes I'd take a large tupperware container with extra filters preoiled and change the filter when I refueled.

If you watch the sales you can pick them up pretty cheap.

Tennessee sure is pretty country.

ded6080 wrote:Bump - wish I'd read your post before I got the Uni (never put it in though) - Going to keep running with the stock filter - thanks for the advice

That metal screen (the loose insert, I don't know any pig-filter with metal IN them) is there to support the filter's rather large surface!
If the filter clogs up (we're riding dirt remember?) then there will be a pressure-differential hence a force on the filter, and that force will bend it inwards, towards the carb. In extreme cases it may tear even, and this a screen will prevent. But also, our big thumper will cause the filter to oscillate, so KEEP that screen IN !!

As far as filter oils are concerned - there's only 1 in my book. Bell Ray's is like the condom (note: THE, not A), the Best There Is. It also is the only product I use from them because this filter oil is what they're known for, not motor oil or why.
Wipe your entire inside of the airbox with this stuff also (NOT the cover, you're going to battle to keep it clean!), and use clean toilet paper (white of course) to wipe-check if there's any debris inside when you change (change yes, not clean, you must have at least 2 of them!) the filter - if there's ANY in there you've got a problem which you'd better sort ASAP.

Bump made 1 important remark: let your filter-oil "set", and allow it to do so in a horizontal position. Any filter oil has a part in that must evaporate first, he calls it curing. Once done it leaves a very tacky layer which will catch any debris.
Honda's filter is fine but a bit restrictive. Uni's filter is a dual layer, a coarse outer for the larger particles, and a finer inner for the secondary filtering of the finer particles, and this 'sandwich' filters very well! The BEST thing though is the black profiled rim which fits perfectly in the airbox's groove!!! Grease this (plain lithium or MS grease will do, no need for waterproof grease unless you go swimming) and end up with a perfect SEAL...... and this is where this filter shines, it is superior than Honda's in this respect!
Now, the inventor of the foam filter (no prizes: it is Twin Air, Dutch) made a point of exceptionally well sealing the edges - for a reason of course. THIS is where most filter lack, and it is not for nothing that we all (we do don't we?) grease this well. But also grease is not perfect, in fact it is a patch - hence UNI's profiled rim.
If Honda's filter had this then I'd agree with Bump, however not now: Uni's filter filters fine, very good even, and when partly clogged-up I bet it flows & filters better than Honda's, for Honda's lacks the coarse outer, it will block up quite soon. But use whatever of these two (or 3, Twin Air) brands you prefer, they filter well - but keep them clean. Let them cure, grease/seal the edges (especially with the Honda filter!!), and regularly clean, oil & check the internals of the airbox.

A note for those seeking the maximum flow out of their UNI:
The black rim kinda 'masks' a large part of the filter's inner surface as it has a 'lip' a bit too wide on the inside/filterside. Granted, due to airflow a part of this lip will bend 'open' hence will allow airflow from underneath it, but still this is a kind of restriction.
Cut the lip with a very sharp scissor narrower, ideally almost to where it is glued to the inner yellow foam. Such will flow more air, and surface-wise this is a rather large improvement. Mind, this is NOT detrimental for the filtering at all, in fact, when partly clogged-up, it is even better.

IF cotton-gauze filter exist for the Pig (like K&N, dunno if they do) then a word of caution: DON'T frigging use them on anything dirt-related!!
Cotton-gauze filter (also oiled, same like foam) are notorious for 2 things, which are 1) flowing more air indeed, and 2) letting an astonishing amount of rubbish through when they're new/clean! They only start to filter better when they're partly clogged, this UNlike a foam filter (or paper for that matter). Also, that gauze is fragile, a small stone will ruin it at speed, punch right through.
So you've got one & keep it clean, you 'recharge' it regularly..... poor bastid, you're killing your engine in the dirt.
Not with a reputable-brand foam filter, and UNI's one of them. One wonders who makes Honda's filter