Dirt bike getting a little louder? Performance doesn't quite seem the way you remembered it? Chances are you need to repack your muffler (or silencer).
The way a dirt bike's muffler works is very simple, and so is repacking it. In a nutshell, your muffler is a large tube, with a narrower, perforated tube inside it. Between the two tubes is “soft stuff”, generally fiberglass. When fuel explodes in your piston, the sound and pressure wave travels down the exhaust to the muffler. The soft material absorbs a lot of the sound waves – it's the same science that allows movie gunmen to fire through a convenient pillow to muffle their shots.
In your dirt bike muffler, the fiberglass is assaulted by hot gases, moisture from condensation (and washing if you don't remember to plug the open end), and vibration. The spaces in the fiberglass get clogged up, and the material gets hard. Repacking the muffler simply means replacing the fiberglass.
First, check to see if you can get a repack kit for your specific dirt bike (if the muffler is OEM) or pipe (if it's after-market). A kit will have all the material you need and make the process a little easier. If a kit is unavailable, pick up some fiberglass packing from your local bike shop and at least as many rivets as your muffler already has holding the ends on.
And just because there's always one guy wondering... DON'T use fiberglass insulation meant for anything other than mufflers. It simply won't hold up.
Put on some long sleeves and have a pair of gloves handy. You'll be working with fiberglass and, even though the exposure is minor, the less you come in contact with it, the better. It actually wouldn't hurt to put on a dust mask either – again, better safe than sorry – because fiberglass dust and strands are easily airborne, and you don't want spun glass in your lungs if you can avoid it.
Remove the muffler from your dirt bike. On some bikes you'll be able to remove just the muffler, others will be easier if you take the whole assembly off.
NOTE: Some mufflers will use screws to hold everything together instead of rivets – FMF pipes for example. In this case, remove the screws and skip the part about rivets.
Using a small drill bit – the one provided in the kit, or one around 5-millimeters or 3/16-inch, give or take – carefully drill out the existing rivets holding the front or inlet end cap on. I can't cover every make and model of dirt bike or pipe, obviously, so use some common sense on the drill bit size. You want to drill out the shaft of the rivet, which is smaller than the head you can see. It's also a good idea to use a piece of masking tape to mark the end of your drill bit, so that you don't drill in too far – about 1 centimeter or 3/8-inch should do it.
Put on your glove and dust mask.
When the rivets are removed, you should be able to take off the inlet cap. It may pop right off, or it may be a tight friction fit. If it's tight, use some gentle persuasion with a plastic or wooden mallet. You can try a rubber mallet, but the mallets rounded edges will make it hard to connect with the edges of the cab. If you don't have a mallet, a chunk of lumber works. Use gentle taps and work your way around the cap.
When the cap is off, you should be able to carefully slide off the outer sleeve and packing material, leaving the inner sleeve attached to the inlet cap.
I haven't seen it, but I've heard that on some mufflers, the inner-sleeve is loose. If that's the case, you'll pack the fiberglass around the tube and insert the whole assembly into the outer sleeve.
Remove the old packing material, place it in a plastic garbage bag, and discard.
Gently clean the inner sleeve with a wire brush, and make sure all the openings or perforations are clear.
Now take your piece of new fiberglass packing and wrap it evenly around the inner sleeve. The packing should be just slightly wider than the diameter of the outer sleeve. It's okay if you have extra fiberglass left over. If you pack in too much, it'll actually decrease performance and increase the sound.
Now, rather than wrestle with loose ends and having wrap unwrapping when you don't want it to, put a strip of masking tape around the fiberglass at each end, and one in the middle. The masking tape will keep everything together for assembly, but burn off once you start riding.
Slip the outer sleeve back over the fiberglass and inner sleeve, and into the inlet cap. If you think the cap has too loose a fit, you can apply a thin bead of hight temperature silicon just before sliding everything together. Check to see if the silicon has a cure time, and don't run the bike before it's cured.
You can take off the gloves and mask now, if you want.
Make sure all the holes line up. Put the new rivets in.
Reinstall the muffler on your bike.
As always though, I might have missed something, or you might have an awesome tip to add. Feel free to add any comments that'll help out all of us who ride.