I have a 1989 Yamaha FZX 750, and I can't find a replacement air filter for it. What's the best method to clean the air filter?
I agree with NoCarrier, you really have to tackle the source of the problem, which is the ability to get a new filter.
I'm iffy about recommending eBay, but this guy has a stellar rating, and seems to specialize in motorcycles. He has a listing for this filter with a quantity of 10+. Communicate with him to see if this will work out for you, and consider buying multiples to help offset shipping costs (he's in the UK).
Another option is an after-market filter. It may be possible to modify the airbox to allow a custom filter to be used. This isn't plan A, but it's better than trying to keep a paper filter going past its expiration date.
Since it's paper based, your ability to clean it is somewhat limited. Mechanically agitating it (i.e. with a brush) is iffy since you run the risk of damaging the filter element. You can try shooting compressed air from the opposite direction of the filter but that really only removes the larger chunks of dirt, and if you're not careful, can also damage the element. If you look closely between the pleats, you find that the microscopic pores in the paper element are all clogged with particles that aren't readily removable.
Long story short, any cleaning you can perform without damaging the filter will have negligible gains in airflow.
You definitely should try and find a new filter and because the filter is paper based your options are limited cleaning wise. If you own an air compressor, try limiting the amount of air coming out to the closest lowest setting and blow air gently into the filter which might help clean it slightly, but eventually you won't be able to clean it properly any more and you might be opening up a can of problems the longer you leave it.
I have a Yamaha SZR bike, 4-year-old, and I used to change the air filter every 8,000 km. It's an orange paper air filter. I use a method to clean the air filter every 1,500 km and it really works if you do it carefully. It takes time.
Soak the air filter for 3-4 hour in detergent solution in a bucket, you can see a huge amount of dirt and dust settle in the bottom of bucket.
Now open the tap and just allow fresh water to pass into it and make sure that no detergent remains.
Never try to wash it by brush or anything. It is wet paper and if you do that you could damage it.
Allow the filter to dry completely, just like clothes.
Use this technique to clean your air filter after every service. No service center, no matter how good it is, has the time to do it the proper way.
How about a vacuum cleaner? Try using the upholstery brush, which will limit the amount of vacuum applied (due to air leaking through the bristles). Don't actually brush the filter.
That said, I agree that you really should find a new filter. Even if you manage to clean it, the paper will eventually disintegrate due to age, and then you'll have to replace it anyway.
I cleaned mine with petrol, a lot of it. It actually cut out all the grease because I could see the petrol turn black in colour when it leaked out from the other side.
But unfortunately now my bike sounds like it doesn't have an air filter. The filter is damaged or there is some air leak. So I suggest changing the air filter because it must be changed every 10,000 km (at least for mine, a KTM Duke 200). And remember, if the sound of your bike changes after you tampered with the filter, it's consuming more petrol.
I use dishwashing liquid because of its grease cutting qualities you still have to leave it to soak for a couple of hours but when you take it out of the bucket it is much cleaner and to stop the breakdown of the paper I add a cap of fabric softener then when you leave it in the sun to dry it has a fresh lemony smell when it's dry