Rough idle.

Cylinder compression

Check cylinder compression:

• Obtain a compression tester for petrol engines with a 12mm thread.
• Check that the engine has the correct amount of good oil in it.
• Check that the choke cable is not sticking.
• Drive the bike for 10 minutes or so, until the engine is fully warmed up.
• Turn off engine, put bike on center stand and into neutral.
• Remove one spark plug from the cylinder to be tested,
  fit the compression tester to the spark plug socket,
  turn engine kill switch off (so that the engine doesn't start),
  open throttle fully and turn on starter motor, as the engine turns over the
  reading on the tester will climb, when it stops climbing (after 5 seconds
  or so) take your reading.
  
• Service limits (Bros 400cc): 14 ± 2 Kg/cm² at 400rpm.
  or 199 ± 29 psi.
• Service limits (Bros 650cc): 13.5 ± 2 Kg/cm² at 400rpm.
  or 192 ± 29 psi.
  

Poor compression can be the result of:

• Worn piston/piston rings/cylinder wall.
• Valve clearance too small.
• Damaged or bent valve or valve spring.
• Faulty head gasket.
• Clogged air filter.
• Cracked or warped cylinder, cylinder head etc.
• No oil in engine.

Further test:

• If there is poor compression add 5cc or fresh motor oil to combustion
  chamber thru spark plug hole and repeat compression test.
• If correct compression returns then the problem usually is with
  the piston rings / cylinder wall.
• If correct compression does not return then the problem is usually with
  the head gasket / valves / cracks.
• If the head gasket is leaky then you may be able to detect carbon-dioxide
  in the cooling system or motor oil contaminated with froth.

Cam train.

• Improper valve timing.
• louse cam chain.
• Incorrect valve clearance.

Ignition system

• Clean and gap spark plugs (0.9mm).
• Do the "spark test" by removing a spark plug from the engine, touch the
  plug outer against the bare metal frame and start the engine,
  check that the spark is strong and consistent. Repeat for all plugs.
  (Note: turning on the starter motor for more that a few seconds may damage it.)
• Check the secondary circuit resistance of the ignition coils/HT leads/caps
  by taking the two caps off the spark plugs and measuring the resistance
  between the caps, Service limits: 30-36 KΩ
  If outside service limit replace caps/leads/coils accordingly.
• Check the primary circuit resistance of the ignition coils by unplugging
  the two input wires from the ignition coil and then measuring the resistance
  of the coil Service limits: 2.2-2.6 Ω (20°). If outside service
  limits replace coil. For an accurate reading take into consideration the
  resistance of the multi meter probes and leads.
• If no spark at any plugs check spark unit.
• Ignition timing is not adjustable on the
  Bros J/K.

Carburettor and fuel supply.

• Check for fuel leaks in the fuel tank, fuel tap, fuel filter, fuel hoses,
  fuel pump and on the gaskets in the carburettor (especially the float chamber cover).
• Check the fuel filters in the tank and the other one for a clog.
• Clean and inspect carb internals (See carb settings).
  • Check carb balance/synchronization.
  • Check vacuum piston rubber diaphragms (Be careful, extremely expensive).
  • Check air cut off valves (expensive).
  • Check the choke cable for sticking.
  • Check fuel/air mix, check spark plugs, see the spark plug page.
  • Check float chamber gasket.
  • Check float valves rubber tip and seat for damage.
  • Check filter behind float valve seat for clogging.
  • Check/clean jets.
  • Check float level (see carb settings).

General

• Causes of motor oil or coolant contamination.
  
  • faulty head gasket.
• Causes of blue smoke from exhaust.
  • Worn piston rings/cylinder wall.
  • Leaking valve oil seals.
  • Too much oil in engine.
  • Excessive oil in air box, clogged/full crankcase
    breather tube.
• Causes of gray smoke
  • Rich mixture
  • Faulty choke
  • Clogged carb