Craig's Scrapbook - Home

 
health
  technical
  motorcycles
  cars
 
 
 

 

 

Virago foam air filter modification

Problem
The standard XV1000 & 1100 Virago air filter is not exactly free-flowing. Yamaha have crammed a small paper element into a compact side pod that owes more to aesthetic compromise than optimal design. Being so small for two big cylinders, it is guaranteed to stifle the engine and clog up pretty quickly. Yamaha replacement elements sell for a premium price.

Foaming at the mouth
If you want to escape these penalties in horsepower and portable property, you could try fitting an aftermarket filter. I had planned to select one of the excellent Uni sock filters for my bike, hoping that I might somehow modify and retain the outer pod chrome to keep the visual balance of the bike as well as some measure of rain and bird [poop] protection. This would have netted the ultimate performance bonus, but I was put off by the time and effort it would have taken to contrive this arrangement. The Virago has made me really lazy. I expect to ride it, not fiddle with it.
So, with my fuel economy steadily worsening as the old paper element became more and more matted and clogged, I eventually threw off my procrastination and came up with a compromise:
Enlarge - finished elementThe standard paper element, I've discovered, can be modified to hold a piece of foam that offers less restriction and the advantage of multiple reuses. Maybe it won't breathe as well as a Uni sock at 7000 rpm, but it does mean that no other modifications to the air pod are necessary, and the bike retains its standard appearance. Total time to complete the conversion is about an hour or two.
Unless, that is, you don't have a suitable piece of foam rubber hanging around (I was lucky). If not, you'll first have to find a piece from a rubber supplier (Clark Rubber in Australia), a hardware store, or perhaps a packing company. I used a small sheet of ordinary density stuff that was about 11mm thick (0.43in), but it could probably stand to be a little thicker. In fact, next time I service it I'm thinking of making mine into a dual-element by adding a sheet of thin stuff (~4mm) over the top which will catch the bulk of the dirt first.
Once you've got a piece of foam, the next step is to attack your old paper element with a pair of pliers. You want to pull off the filter paper and most of the glue without mangling the plastic body or the wire gauze underneath. The glue is super tough, so be firm but careful. If you wreck the metal mesh, you should be able to source something to replace it at the hardware store. If you've got it intact, separate it from the paper and straighten it out. Don't worry if you have to sacrifice a few strands on the edge to get it free from the glue. Don't worry about residual, hairy glue that might be left on the plastic either. As long as it's a vaguely straight surface that the foam can seal against you'll be okay.
Enlarge - inside viewNow, because the foam is much floppier than thick corrugated paper, you need to brace it to stop it from being sucked into the engine. I achieved this with five small ribs of fencing wire running lengthwise through the filter body. Coat-hanger wire would do nicely too. You simply drill matching pairs of holes in the plastic end plates, thread your wire through, and bend the wire's ends so that it can't come out. The wire should be positioned just below where the foam will sit. In my own plastic element body this meant drilling holes along a contour at the very edge of the indented segments of the end plates (see photo). The metal mesh sits directly on top of these ribs and together they make it impossible for the foam to be sucked through into the inlet tract.
Foam dimensionsNext you can cut your piece of foam. I've provided the rough measurements of my piece here if you want to use them as a starting point. I've deliberately made the foam generously oversized so that when it's in place it is actively pushing against the edges to create a seal. You have to sort of tuck it into place. The dimensions aren't critical, but better to err on the side of too much here.
Once it's in place, you need a system to stop it springing outwards too. The simplest thing I could come up with in a hurry was to simply lace it down. In the last photo you will see that I've drilled eight holes along the very outside of the straight plastic sections. I chose some good quality braided synthetic chord and just laced it up like a shoe. Don't tighten it down and compress the foam too much. Just pull it snug.
Enlarge - ribs, lace holes, and cut foamBefore you assemble it together for the last time, make sure that the mesh is between the ribs and the foam and that you've oiled the foam. You can buy special filter oil from motorcycle shops or just apply gear oil and squeeze out the excess. Also, run a bead of grease or petroleum jelly along the edge of the foam element and/or its mating surfaces on the plastic to assist the seal and prevent grit from sneaking past the foam. While you're at it, do the same for the main sealing surface along the bottom of the plastic body—the one that bolts up to the airbox itself. This will help the seal there too (it's a pretty slack arrangement and can use a bit of help, especially now that you've reduced it's stiffness, drilled holes in it, and looped string through parts of it).
Service intervals will vary with the amount of dust you encounter, but you can usually get a fair idea of how dirty the foam is just by looking at it. A lot of backyard mechanics think that it's a good idea to give foam elements a final wash with detergent and water, but the people at Uniflow say that this shortens the foam's life, and do not recommend it. Whether this is true for all foams I don't know, but I never use water on foam elements anyway. I use a few small consecutive washes of mineral solvent (turps, kerosene/paraffin, petrol/gasoline all work—but the latter variant, being of the volatile, low flash-point kind is unsafe to work with, of course…).

Performance and jetting
The foam element made an immediate improvement to my bike, especially at higher revs where I could feel it pulling harder and breathing much more freely.
However, with my aftermarket shotgun pipes, not to mention the Hitachi carbs' lack of throttle pumps, the jetting seemed to be a fraction on the lean side following the above treatment. It started pinging a little when the engine was loaded heavily (such as when cracking open the throttle in top gear when riding slowly). This occurred with ordinary Australian unleaded which is only about 91 RON octane (or 87 octane in the American AKI system). I decided to raise the jet needles and thus richen up the midrange. The idle circuit seemed rich enough already (I almost never need choke to start it) and the main jets can stay standard for now (they don't see that much use anyway).
The Hitachi needles aren't adjustable, so I achieved the desired effect by placing small washers underneath them. Once the carb assembly is out of the bike (follow this procedure) it's an easy modification. The trickiest part is finding washers to suit. I found some of appropriate thickness and outer diameter, but had to drill out the centres to fit the needles. At first I made them too tight. The needles bind on the washers if the centre hole is not large enough because the washer sits on an uneven surface. If this happens you're likely to think that the needle is seated when it's actually way higher than it should be (use a felt marker on the needle to double-check your work). I ended up with a 4.5mm centre hole which lets the needle move around nicely.
My washer was about 9.5mm in total diameter, and about 0.8mm thick. This lifted the needle less than the equivalent of one notch of a typical adjustable needle's increments (0.9mm).
If you've got a later model bike with Mikunis, you'll have to investigate their needles yourself to see if they are adjustable. I suspect that they are not, in which case you should be able to use washers also. Try the 0.8-1.0mm thickness range as a safe starting point.
This jetting adjustment worked beautifully for me. I am very happy. The pinging problem disappeared and the engine's grunt and responsiveness increased quite markedly. So much so that I'd say it is a necessary step to realising the real potential of the foam filter element. Later model Viragos with Mikuni carbs might respond differently.

Airbox modification
In the top picture of the finished filter assembly I've pointed out the two extra inlet holes that I've drilled in my unit to increase airflow into the box. This barely increases intake roar at all (darn it) but does augment the narrow standard inlet somewhat.
There's precious little free space to drill out the plastic backing plate. I didn't want to cut the chrome outer cover, which is about the only other alternative.* If you're really careful, you should be able to figure out where you can place these two holes on the left side so that they don't interfere with structural plastic. You can start with smaller holes and gradually ream them out until you can't make them any bigger. It all helps.
*Apparently there's another way of increasing the airbox inlet area. Thanks to Jeremy for pointing this out. He has placed washers underneath his chrome cover mounting points so that the chrome cover no longer seals against the main body. In fact, he has a gap of about 1/4 inch (7mm) running the whole way around the airbox now. This allows a bit of rain to hit the filter, but he is running a foam element also (a K&N one) which can tolerate a bit of water better than a paper element can. Very crafty.

Another approach
John "Flagman" Finn has produced a comprehensively illustrated tutorial on adapting a Screaming Eagle / K&N paper filter assembly to the Virago.

High-mileage Hitachi carburettor owners might like to refer to the notes on the main page regarding carburetor synchronising.

Back to my main XV / Virago page.


Copyright © Craig Forsythe, 2013. All rights reserved. Contact.