Find and fix an exhaust leak

Home mechanic (basic tools) · Time: 30 to 60 min (estimate) (estimate)

An exhaust leak lets hot gas escape before the silencer. You hear a ticking or puffing that follows engine speed, often loudest when cold. The most common place is the joint where the pipe meets the cylinder head (the exhaust port gasket), then the joint between the header pipe and the silencer, then rust holes and cracked welds. A leak near the head also lets air in on deceleration, which causes popping, and on fuel-injected bikes it can upset the oxygen sensor reading.

Safety

• The exhaust and catalytic converter stay very hot for a long time after the engine stops. Work only on a cold exhaust.
• Never search for a leak by holding your hand on or near a running exhaust.
• Run the engine only outside: carbon monoxide.

You need

• A new exhaust port gasket of the right type (copper, graphite or steel ring). Old gaskets are crushed and do not reseal.
• New nuts or studs if threads are rusted.

Steps

1. With the engine cold, look along the pipe with a torch. Black soot streaks or a grey-white crust round a joint show where gas is escaping.
2. Start the engine cold (outside). Hold a strip of paper or a light rag near each joint, not touching. A leak flutters it in time with the engine. Stop the engine.
3. Let everything cool fully.
4. Check the port nuts or flange bolts at the head. Loose nuts are a common cause. Check the clamp and bolts between the header and silencer, and the silencer mounting bolt.
5. If only loose, tighten evenly to the manual value, alternating between the two nuts a little at a time. K-Light 125: muffler connector (header flange) bolts 5 to 9 N·m; muffler mounting bolt 22 to 29 N·m.
6. If the leak is at the head joint and the nuts were tight, replace the gasket: undo the port nuts and the silencer mount, slide the pipe back, remove the old gasket from the port (a small screwdriver helps), clean the port face with a wire brush.
7. Fit the new gasket. Some engines need a dab of grease to hold it in place; do not use sealant on a port gasket.
8. Refit the pipe, start all nuts and bolts by hand, then tighten the port nuts first, evenly, then the silencer mount.
9. Run the engine and check again. Recheck the nuts after the first ride once cold.

Check your work

• No ticking, no soot streaks, no flutter at the joints.
• Pipe sits square with no strain on the mounts.

If it did not fix the problem

• Rust holes or a cracked weld: a temporary exhaust bandage can get you home, but the pipe needs welding or replacement. Do not patch a catalytic converter section.
• Popping on deceleration continues: Popping or banging from the exhaust when slowing down.
• Rattle rather than a puff: Check exhaust mounts and heat shields.
• Broken stud in the head: workshop job; drilling out a stud can damage the head threads.

Reference values (K-Light 125)