Trouble-shooting.

Engine will not start.

• Attempt to drain float chamber
  • If there is little or no fuel in chamber:
    • Clogged fuel line, filter behind float valve, main filter, fuel tank breather hole.
    • No fuel in tank or fuel tap turned off.
    • Sticking float valve or faulty float.
    • Loose or disconnected fuel pump or relay wire.
    • Faulty fuel pump relay.
    • Faulty fuel pump.
    • Clogging within carburettor body.
• If there is fuel in float chamber then check spark plug.
  • Wet plug:
    • Mixture too rich
    • Carb flooded
    • Choke valve stuck open
    • Cylinder flooded
    • Dirty air cleaner
• Perform the spark test
  • Weak or no spark: See rough idle
• If there is a good spark then do this
  • Pour 1-2 cc of fuel into cylinders thru the sprk plug hole,
    reinstall spark plugs and try to start engine without choke applied,
    with full throttle
    • If engine starts:
      • Faulty choke system
      • Fuel comtaminated
      • Carb incorrectly adjusted
      • Intake pipe leaking
      • Clogged carb passage or jets
• If engine does not start perform compression test on each cylinder, see rough idle.
  • Low compression:
    • Valve stuck open
    • Damaged cylinder head gasket
    • Improper vlave timing
    • Improper valve and seat contact
    • Worn cylinder and piston rings
    • Damaged piston
• If compression is normal:
  • Improper ignition timing
    • Fautly pulse rotor
    • Generator with ferrous burrs

Engine is hard to start or soon stops.

• Check fuel supply to carb as above.
• If fuel supply is good then try to start with choke on
  • If the engine starts:
    • Clogged carb jets
• If engine does not start, check plug as above
• If no problem at plug perform spark test as above
• If spark is good then perform compression test
• If compression is good check valve train
  • See valve clearance
  • See cam chain replacement for valve timing
  • Worn camshaft
  • Loose valve adjuster and lock nut
• If valve train is good check ignition timing
  • If ignition timing is out:
    • Faulty pulse rotor
    • Generator with ferrous burrs
• Clogged carb passage or jets

Engine lacks power with normal loading.

Note:

• Check engine oil level and condition. Be sure the oil is not contaminated
  with coolant or metal burrs.
• Start the engine and warm it up to operating temperture, then check to see
  if the temperture gauge reading is abnormal.
• Check for engine mechanical noise: Cam chain and cam chain tensioner; improper
  valve clearance; worn camshaft and rocker arm sliding surfaces; broken piston rings,
  worn piston pin and connecting rod small end.
• Check that the wheels spin freely, check for brake drag, faulty wheel bearings,
  incorrect drive chain slack, bent fork, bent frame.

• Check exhaust smoke at half throttle
  • Blue smoke
    • Engine oil leaking into cyclinder
  • Gray smoke
    • Rich mixture
    • Worn piston rings
    • Incorrect ignition timing
• If exhaust smoke is normal check for surging during acceleration.
  • Causes of surging
    • Incorrect adjusted pilot screws
    • Incorrect synchronization
    • Intake pip leaking
    • Clogged crankcase breather
• If there is no surging check the spark plug
  • Wet plug
    • Loose connection between spark plug caps and wires
    • Dirty air cleaner element
    • Improper float level
    • Rich mixture
  • Cause of a light gray or white discoloured spark plug
    • Improper heat range
    • lean mixture
    • Improper float level
    • leaking intake pipe
• If plugs are good check the specified fuel flow capacity
  • Under the specified
    • Dirty fuel filter
    • Clogged fuel lines
    • Faulty fuel pump
• If fuel flows good check cylinder compression (above)
• If compression is good heck ignition timing (above)
• If ignition timing is good check valve train for surging (above)
• If there is no surging check for clogging in carbs
• If carbs are good
  • Seized cylinder and piston
  • Seized connecting rod bearing
  • Damaged main bearing

Poor engine performance at slow and idle speed.

Note:

• Check engine oil level and condition. Be sure the oil is not contaminated
  with coolant or metal burrs.
• Start the engine and warm it up to operating temperture, then check to see
  if the temperture gauge reading is abnormal.
• Check idle speed: specified 1,200 +/- 100rpm

• Check engine condition with choke applied
  • Better
    • Intake pipe leaking
    • Improper carb adjustment
    • Clogged carb slow jet
    • Improper float level
• If it does not improve remove air cleaner and check again
  • Better
    • Dirty air cleaner
    • Improper carb adjustment
    • Clogged carb air passages
    • Improper float level
• If it does not improve do spark test on all plugs (above)
• If spark is good check carb synchronization (carb overhaul)
• If synchonization is good check cylinder compression (above)
• If compression is normal check ignition timing using timing light with "F" mark
  • Abnormal
    • Improper ignition timing
    • Faulty spark unit (CDI)
    • Pulse rotor
    • Generator
• If ignition timing is normal check valve train
  • Problems
    • Improper valve and seat contact
    • Valve stuck open
    • Improper valve timing
    • Damaged cylinder head gasket
    • Worn cylinder and piston rings
• If valve train is good clean carb air passages and jets
  • Better
    • Clogged air passages or jets
• Does not improve
  • Seized cylinder and piston
  • Seized connecting rod bearing
  • Damaged main bearing

Poor engine performance at high speeds

Note:

• Check engine oil level and condition. Be sure the oil is not contaminated
  with coolant or metal burrs.
• Start the engine and warm it up to operating temperture, then check to see
  if the temperture gauge reading is abnormal.
• Check idle speed: specified 1,200 +/- 100rpm
• Check that the wheels spin freely

• Check for spark plug (above)
• If plug is good check exhaust smoke at half throttle (above)
• If exhaust smoke is good check engine performance with choke applied
  • Better
    • Lean mixture
    • Intake pipe leaking
    • Improper carb adjustment
    • Clogged jet
    • Improper float level
• If performance does not improve check performance without air cleaner
  • Better
    • Dirty air cleaner
    • Rich mixture
    • Clogged carb air passages
    • Improper float level
• If performance doesn't improve test specified fuel flow
  • Under the specified
    • Dirty fuel filter
    • Clogged fuel lines
    • Faulty fuel pump or relay
• If fuel flow is good check cylinder compression (above)
• If compression is normal check ignition timing using timing light with full advance
  • Abnormal
    • Faulty pulse rotor
    • generator with ferrous burrs
• If ignition timing is good test ride bike and check for valve surging
  • Valve surging
    • Improper valve clearance (too small)
    • Weak valve spring
    • Worn camshaft
• Check for clutch slippage
  • Slipping
    • Improper clutch lever free play
    • Faulty clutch lifter system
    • Worn or warped clutch discs/plates
    • Weak clutch springs
• If clutch is good clean carb air passages and jets
  • Better
    • Clogging
• Does not improve
  • Seized cylinder and piston
  • Seized connecting rod bearing
  • Damaged main bearing

Common faults on the Bros

• Stater motor does not work
  • Starter relay switch
  • Worn carbon brush terminals in starter motor
  • Open circuit in stater button wire on right hand switch gear
• Hard starting
  • clogged carb
  • Incorrect carb adjustment
  • Vacuum in fuel tank caused by clogged breather hole
  • Faulty fuel pump
• Engine stalls as if it were out of petrol
  • Clogged breather hole in fuel tank
  • Faulty fuel pump
• See rough idle

Ignition timing


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Ignition advance for Bros 400 J/K
10° BTDC, "F" mark, at idle (1,200 rpm)
32° BTDC at 9,000 +/- 200 rpm, full advance

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Ignition advance for Bros 650 J/K
10° BTDC, "F" mark, at idle (1,200 )
31° BTDC at 7000 +/- 200 rpm, full advance

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