Exhaust and silencer

The exhaust carries burnt gas from the cylinder head to the back of the bike. It has a header pipe bolted to the head, a silencer (muffler) at the end and, on newer bikes, a catalytic converter and an oxygen sensor in the header. Two-strokes use a fat expansion chamber that is tuned to the engine.

What it does

The exhaust lets the engine breathe out, cuts noise, and on newer bikes cleans the gas: the catalytic converter burns leftover fuel and carbon monoxide. The pipe and silencer rust from the inside, mounts crack from vibration, and carbon builds up, especially on two-strokes and engines that burn oil. A melted or blocked catalyst chokes the engine.

Signs it is worn or failing

• Many modern two-stroke engines have a power valve in the exhaust port. Carbon makes it stick and the engine goes flat at one end of the rev range: Two-stroke is weak at one end of the rev range.
• Exhaust much louder or rattling.
• Ticking or puffing from a leak and popping on overrun.
• Pipe glows red or catalyst overheats: stop the engine.
• Low power from a blocked silencer or catalyst, or a blocked two-stroke exhaust.

When to replace it

When it is rusted through, cracked, or the catalyst is broken or blocked. Two-stroke pipes can often be cleaned: Decarbonise a two-stroke exhaust.

What to match when buying

• Match make, model and year. A wrong pipe changes power and fuel use, and on injected bikes can trip the warning light.
• On bikes sold with a catalyst, buy a part with one. The K-Light silencer carries approval marks; many countries require a marked, legal silencer.
• Loud "free-flow" silencers and removed baffles are often illegal and can make the engine run lean and hot.

Fitting notes

• The exhaust stays hot long after the engine stops; let it cool before touching.
• Always fit a new exhaust gasket. Fit all bolts loosely, tighten the header first, then the silencer mount.
• Home job: Find and fix an exhaust leak, Check exhaust mounts and heat shields.
• Steam from burnt fuel condenses inside the pipe and rusts it from within. Chrome can hide the holes.

Reference (K-Light 125)