• Topics: 38
  • Replies: 447
Nov 30, 2017
North UK

Hey dude
The CDI cheat never works on a 125 – whoever said so is a faker
“A few”  250 cdis allow 1 wire to be cut – which helps the 250 engines rev slightly better on its STOCK coils.

If you had upgraded the electrics on the bike, it should be very spirited by now.

Sadly, as i’ve experienced fixing customer RC bikes the 4-7K rpm issues is usually caused by either of these:

  1. Ignition coil isn’t getting 100% electric juice  (suspect = cdi / magneto )
  2. Spark Plug could be sooty  (trust me! – RC bikes do not tolerate sooty plugs more than the old gen bikes & electrical changes = RC bikes have some woeful EFI electrics but i digress!)
    So –> Sooty plug = Intake issues / weak sparking ? / fouled plug (annoyed plug)
  3. Air filter needs attention = more air is sucked in as you go faster but the carb likes a clean filter & not restricted to less air flow than before
  4. Clutch needs looking at (any RC over 4K must have a new clutch set as the cheap organics wear down fast –> hence the gearbox drama and false N’s)
    Search “Clutch” at the top of the site which explains what kind of power issues you get on a clutch that has worn down a bit (Symptons)
  5. However , we can’t rule out a weak spark or a misfire – Make sure the “HT” cap is under 7k ohms  – > RC have an EFI HT CAP for the spark plug that is not that much better than 2007 however too much ohms –> weak sparking -> delayed RPMS
  6. Intake Manifolds need investigation – > you need to take out the fairings  and possibly the carbs off – then inspect your intake manifolds – there should be zero holes or zero leaks  (protects piston rings from dirty particles being sucked in too)
  7. Battery – If it isn’t 12.8v fully charged when off = inspect magneto , all electric juice starts from there before it feeds the rest of the bike too.
    If you also switch full beam –> engine off -> indicators / hazards on –> if the dash  “DIMMS”   -> check battery is holding up juice / not dropping volts too quickly  (battery should not discharge quickly)
  8. intake lines (vacuum) – only lose clamps can cause leaks or  a very old “stock” line that is rotting inside  Vacuum performance is needed for the fuel pump & fuel tap to be effecient in high revs  (4-11k rpm) (more demand = more fuel). Big lines help the vacuum & fuel system to run a bit better (colder air/colder fuel) on a hot engine.
  9. CARBS!
    i saved this for last as it’s usually a time consuming process but you may need to inspect your “black CV Sliders” inside the carb and clean them
    CV sliders control most of the carb work, so  as you rev the bike , the sliders go up  , they release more fuel!

    If the bike is hanging at certain rpms or stuck at certain points –> CV Sliders in the carb need cleaning (common) ,  Clogged jets in the carb , float bowls need inspection to make sure they are all at the same height  (not bent floats)

    When you look at your own spark plugs , thats when you get a true story of whats happening to the bike.  A healthy motor = all coffee brown color , no black anywhere.

    Spark plug color hints  = (Intake system / Fuel System / Sparking Electric system conditions) 


So it’s a case of verifying what i said above & most times some TLC on the intake system & carbs wakes it up.   Electrics are common on RC but not impossible to fix,  if you have upgraded electrics , that’s a good thing Less drama down the road!

Let us know how you get on, or if i missed something too.👍

Also have a look at this , as some of my replies can apply to your situation too.

// Meditation doesn't mean you have to sit still....

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