It's so loud!
Slappin da chain mon, sometimes sounds like a chain saw, emitting noise over and above the braap or thumper exhaust on a dirt bike. Usually though, chain slap comes across as an annoying racket. Some chain slap is normal but constant slapping indicates something wrong with an aspect of the drive train.
Improper chain tension inhibits a dirt bike's overall performance affecting the power and torque. Eventually, the chain snaps but probably not before leaving some damage to other related parts like the swingarm, roller and chain guide. In fact, a good maintenance routine involves checking the swing arm, frame and sub frame that corresponds alongside the chain. If you find too much wear then the chain rubs alongside those parts which indicates a sloppy chain.
The culprit? Excess chain slack.
Excessive chain slack results from a number of scenarios the most common stemming from poor adjustment. Basically, you didn't cut enough chain off when installing. Size the chain with the rear wheel completely forward and measure off the rear sprocket. You want a two to three finger slack on the chain. For more information check out Dirt Bike Chain Tension - Changing and Adjusting.
After installing a new chain, inspect the chain slack as part of your pre-ride check thereafter because before long that chain starts slapping again. Over time, chains stretch and wear out. Think of it like a dried piece of spaghetti (brand new chain) then the wet soggy noodle after 10 minutes of boil (the clapped out chain). You can tighten a chain by moving the axle blocks but once you run out of room to adjust the blocks buy a new chain. This method stretches (no pun intended) your dollars but doesn't necessarily represent the best way. A chain wear tool offers a more precise method to measuring and you might find the chain needs replacing before you run out of axle block room.
Once removed, feel the difference between the old chain and the replacement chain. It's not even al dente! Remember to change the sprockets when replacing the chain and vice versa.
Excessive slack also occurs from a worn out chain guide, chain slide and/or roller though if you have more than one of these presenting as an issue riding probably proves a bit difficult, if at least not very enjoyable. Once past their prime, these parts allow excessive slack leading to chain slap.
Other less common but commonly overlooked reasons for excess chain slack include a loose rear axle or loose chain adjusters and improper rear suspension setup which can also cause chain binding.
Overall, out of spec or misaligned parts directly involved with the drive chain cause mismanagement of the chain resulting in chain slap.
Got other chain related problems? The fine articles below should get you back in gear: