Firing incorrect
- Air filter restricted. Clean or replace filter (Chapter 1).
- Spark plugs fouled, defective or worn out. See Chapter 1 for spark plug maintenance.
- Spark plug caps or HT wiring defective. See Chapters 1 and 5 for details of the ignition system.
- Spark plug caps not in good contact (Chapter 5).
- Incorrect spark plugs. Wrong type, heat range or cap configuration. Check and install correct plugs listed in Chapter 1.
- Ignition control unit defective (Chapter 5).
- Ignition HT coils defective (Chapter 5).
Fuel/air mixture incorrect
- Main jet clogged. Dirt, water or other contaminants can clog the main jets. Clean the fuel tap filter, the in-line filter (XRV models), the float chamber area, and the jets and carburettor orifices (Chapter 4).
- Main |et wrong size. Check the jet sizes according to the Specifications in Chapter 4. The standard jetting is for sea level atmospheric pressure and oxygen content - if you are constantly running at high altitude (where the oxygen content of the air is reduced), the mixture will be affected.
- Throttle shaft-to-carburettor body clearance excessive. Refer to Chapter 4 for inspection and part replacement procedures.
- Air bleed holes clogged. Remove and overhaul carburettors (Chapter 4).
- Air filter clogged, poorly sealed, or missing (Chapter 1).
- Air filter housing poorly sealed. Look for cracks, holes or loose clamps, and replace or repair defective parts.
- Fuel level too high or too low. Check the float height (Chapter 4).
- Fuel tank breather hose obstructed. If the breather hose is blocked a vacuum can form in the tank which will restrict flow.
- Carburettor intake manifolds loose. Check for cracks, breaks, tears or loose clamps. Replace the rubber intake manifolds if they are split or perished (Chapter 4).
Compression low
- Spark plugs loose. Remove the plugs and inspect their threads. Reinstall and tighten to the specified torque (Chapter 1).
- Cylinder head(s) not sufficiently tightened down. If a cylinder head is suspected of being loose, then there's a chance that the gasket and head are damaged if the problem has persisted for any length of time. The head nuts/bolts should be tightened to the proper torque in the correct sequence (Chapter 2).
- Improper valve clearance. This means that the valve is not closing completely and compression pressure is leaking past the valve. Check and adjust the valve clearances (Chapter 1).
- Cylinder and/or piston worn. Excessive wear will cause compression pressure to leak past the rings. This is usually accompanied by worn rings as well. A top end overhaul is necessary (Chapter 2).
- Piston rings worn, weak, broken, or sticking. Broken or sticking piston rings usually indicate a lubrication or carburation problem that causes excess carbon deposits or seizures to form on the pistons and rings. Top-end overhaul is necessary (Chapter 2).
- Piston ring-to-groove clearance excessive. This is caused by excessive wear of the piston ring lands. Piston replacement is necessary (Chapter 2).
- Cylinder head gasket(s) damaged. If a head is allowed to become loose, or if excessive carbon build-up on the piston crown and combustion chamber causes extremely high compression, the head gasket may leak. Retorquing the head is not always sufficient to restore the seal, so gasket replacement is necessary (Chapter 2).
- Cylinder head(s) warped. This is caused by overheating or improperly tightened head nuts/bolts. Machine shop resurfacing or head replacement is necessary (Chapter 2).
- Valve spring broken or weak. Caused by component failure or wear; the springs must be renewed (Chapter 2).
- Valve not seating properly. This is caused by a bent valve (from over-revving or improper valve adjustment), burned valve or seat (improper carburation) or an accumulation of carbon deposits on the seat (from carburation, lubrication problems). The valves must be cleaned and/or replaced and the seats serviced if possible (Chapter 2).
Knocking or pinking
- Carbon build-up in combustion chamber. Use of a fuel additive that will dissolve the adhesive bonding the carbon particles to the crown and chamber is the easiest way to remove the build-up. Otherwise, the cylinder heads will have to be removed and decarbonised (Chapter 2).
- Incorrect or poor quality fuel. Old or improper grades of fuel can cause detonation. This causes the knocking or pinking sound. Drain old fuel and always use the recommended fuel grade.
- Spark plug heat range incorrect. Uncontrolled detonation indicates the plug heat range is too hot. The plug in effect becomes a glow plug, raising cylinder temperatures. Install the proper heat range plug (Chapter 1).
- Improper air/fuel mixture. This will cause the cylinders to run hot, which leads to detonation. Clogged jets or an air leak can cause this imbalance. See Chapter 4.
Miscellaneous causes
- Throttle valve doesn't open fully. Adjust the throttle grip freeplay (Chapter 1).
- Clutch slipping. May be caused by an incorrectly adjusted cable (see Chapter 1), or loose or worn clutch components. Refer to Chapter 2 for clutch overhaul procedures.
- Timing not advancing. Check as described in Chapter 5.
- Engine oil viscosity too high. Using a heavier oil than the one recommended in Chapter 1 can damage the oil pump or lubrication system and cause drag on the engine.
- Brakes dragging. Usually caused by debris which has entered the brake piston seals, or from a warped disc or bent axle. Repair as necessary.
- Fuel flow restricted. Check the tap and its filter, and all the hoses from the tank; on XRV models check the in-line fuel filter. If the breather hose is blocked a vacuum can form in the tank which will restrict flow.
- Fuel pump flow rate insufficient (XRV models). Check the pump (Chapter 4).
- Modification to exhaust system. Most aftermarket exhaust systems cause the engine to run leaner, which make them run hotter. When installing an accessory exhaust system, always check whether different carburettor jet sizes are needed and rejet the carburettors accordingly, if necessary (see Chapter 4). Often the best way to determine this is by running the bike on a Dyno.