Spark weak
- Battery voltage low. Check and recharge battery (Chapter 9).
- Spark plugs fouled, defective or worn out. Refer to Chapter 1 for spark plug maintenance.
- Spark plug cap or HT wiring defective. Refer to Chapters 1 and 5 for details on the ignition system.
- Spark plug caps not making contact. Make sure they are securely pushed on to the plugs.
- 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).
- Pulse generator coil defective (Chapter 5).
- Ignition HT coils defective (Chapter 5).
Fuel/air mixture incorrect
- Pilot screws out of adjustment (Chapter 4).
- Pilot jet or air passage clogged. Remove and overhaul the carburettors (Chapter 4).
- Air bleed holes clogged. Remove carburettor and blow out all passages (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 level (Chapter 4).
- Fuel tank breather hose obstructed.
- Carburettor intake manifolds loose. Check for cracks, breaks, tears or loose clamps. Replace the rubber intake manifold joints if split or perished.
- Incorrect carburettor jet sizes. Check according to the Specifications in 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. I his 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).
Poor acceleration
- Carburettors leaking or dirty. Overhaul the carburettors (Chapter 4), Timing not advancing. The pulse generator coil or the ignition control unit may be defective. If so, they must be replaced with new ones, as they can't be repaired.
- Carburettors not synchronised. Adjust them with a vacuum gauge set or manometer (Chapter 1).
- Engine oil viscosity too high. Using a heavier oil than that 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 (Chapter 7).
- Fuel flow restricted. Check the tap and its filter, and all the hoses from the tank; on XRV models also 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).
Miscellaneous causes
- 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.