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Compression low (continued)

J Valve not seating properly. This is caused by a bent valve (from over-rewing or improper valve adjustment), burned valve or seat (improper carburation) or an accumulation of carbon deposits on the seat (from carburation or lubrication problems). The valves must be cleaned and/or replaced and the seats serviced if possible (Chapter 2).

Stalls after starting

U Improper choke action (carburettor models). Make sure the choke linkage shaft is getting a full stroke and staying in the out position (Chapter 4).

□ Ignition malfunction. See Chapter 5.

□ Carburettor malfunction. See Chapter 4.

□ Fuel contaminated. The fuel can be contaminated with either dirt or water, or can change chemically if the machine is allowed to sit for several months or more. Drain the tank and float chambers (Chapter 4).


D Intake air leak. Check for loose carburettor-to-intake manifold connections, loose or missing vacuum gauge adapter screws, caps or hoses, or loose carburettor tops (Chapter 4).

U Engine idle speed incorrect. Turn idle adjusting screw until the engine idles at the specified rpm (Chapter 1).

Rough idle

[7] Ignition malfunction. See Chapter 5.

□ Idle speed incorrect. See Chapter 1.

□ Carburettors not synchronised. Adjust them with vacuum gauge or manometer set as described in Chapter 1.

□ Carburettor malfunction. See Chapter 4.

H Fuel contaminated. The fuel can be contaminated with either dirt or water, or can change chemically if the machine is allowed to sit for several months or more. Drain the tank and float chambers or fuel rail (Chapter 4).

□ Intake air leak. Check for loose carburettor-to-intake manifold connections, loose or missing vacuum gauge adapter screws, caps or hoses, or loose carburettor tops (Chapter 4).

□ Air filter clogged. Replace the air filter element (Chapter 1).


Poor running at low speeds


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.

U 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.

Incorrect spark plugs. Wrong type, heat range or cap

configuration. Check and install correct plugs listed in Chapter 1. U Ignition control unit defective. See Chapter 5.

□ Pulse generator defective. See Chapter 5.

□ Ignition HT coils defective. See 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 float height (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.

Compression low

□ Spark plugs loose. Remove the plugs and inspect their threads. Reinstall and tighten to the specified torque (Chapter 1).

□ Cylinder head not sufficiently tightened down. If the 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 bolts should be tightened to the proper torque in the correct sequence (Chapter 2).

L 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). G 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). C 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 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).

Q Cylinder head warped. This is caused by overheating or

improperly tightened head bolts. Machine shop resurfacing or head replacement is necessary (Chapter 2).

E Valve spring broken or weak. Caused by component failure or wear; the springs must be replaced (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 them (Chapter 4).

J Timing not advancing. The pulse generator or the ignition control module 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).

L~l 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).


ref-38 Fault Finding

3 Poor running or no power at high speed


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.

r 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. See Chapter 5. \Z Incorrect spark plugs. Wrong type, heat range or cap

configuration. Check and install correct plugs listed in Chapter 1. IZ Ignition control unit defective. See Chapter 5.

Ignition coils defective. See Chapter 5.

Fuel/air mixture incorrect

D Main jet or fuel injector clogged or wrong size. Dirt, water or other contaminants can clog them. Clean the fuel filter, and the carburettors (Chapter 4). The standard jetting is for sea level atmospheric pressure and oxygen content.

□ Air bleed holes clogged. Remove and overhaul carburettors
(Chapter 4).

D 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).
D Fuel tank breather hose obstructed.

Li 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

D Spark plugs loose. Remove the plugs and inspect their threads. Reinstall and tighten to the specified torque (Chapter 1).

D Cylinder head not sufficiently tightened down. If the 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 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 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). D Cylinder head warped. This is caused by overheating or

improperly tightened head 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 replaced (Chapter 2).

J 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 or lubrication problems). The valves must be cleaned and/or replaced and the seats serviced if possible (Chapter 2).


Date: 2016-01-14; view: 714


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