From 1 to 4000rpm the bike is ok, but from 4 to 8000rpm the engine seems to loose power and after 8000rpm the power is back. The sparkplug on piston number 4 was black, so i replaced the plugs, start it again and the results was the same. Can the problem be the coil or the pick-ups?

Check your compression on each cylinder. It sounds like you have holed a piston or lost a piston ring.

Hi,have a compression test to make shore that the machanics are ok first, (could be worn cam,worn rings) if ok then you have to make shore alternater (stator) is giveing wright volts.
then if you still cant find the falt,you will have to take it to a tunning station where the can put it on a rolling road so they can see how it performs in reel time,just food for thought and all the best nice bike hope you get it soted.

change faulty sparkplug or ht lead,check lead by holding sparkplug end near to engine nearly touching,have someone turn engine over whils checking for spark jumping,if spark present then not lead,change spark plug,if sparkplug ok then check pressure in cylinder, poss piston damage

Hi and welcome to fixya,

you can narrow down the problem by synchronizing the carburetors. Check for timing as well (use a timing light) .. It is important to time and synchronize 4 piston engines. This diagnostic check will make sure you get the most power out of your current engine configuration.

hope this helps

it might be excessive piston slap,=pistons too loose in bore so loosing compresion.ignition timing too advanced so pinking.carberation too weak,

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Try a new air cleaner, or strip down carbs and clean all jets .

Hello,

Remove both sparkplugs from engine. Plug them back in the sparkplug cables and let them rest on the engine.
Make sure your battery is well charged.
Check for sparks while kick-starting...
Interchange the sparkplugs in the cables:
-Same plug give sparks: replace the spark plugs.
-Same side of engine sparks: interchange the low-voltage connector at the ignition coils.
Give it a try:
Same engine side sparks: Replace the defective ignition coil.
Other side of the engine sparks: Check the circuit harness for loose connectors, and/or Clean-replace the breaker points ( left side of the engine carter ).

Hi and welcome to FixYa,

Initially if electrical, disconnect the black/white wire from the engine stop switch, try and observe if problem persists:

• if yes, then a loose / dirty / faulty engine stop switch;
  
• if not check:
  

1. 2 connectors to CDI;
   
2. connector from pulse generator (black/yellow - blue/white & green/white wires);
   
3. black/red wire from exciter coil;
   
4. blue/yellow & green (to ground) wires on the ignition coil;
   
5. sparkplug wire & cap.
   

Good luck and thank you for asking FixYa.

i think the engine oil is the problem.
because i think at above 80 degrees it is loosing its viscosity due to which the piston is slipping inside the cylinder.
change its engine oil .