RD 125 LC YPVS issues at 7500RPM

[madtalavera]

my RD had an ignition failure. Really don´t know exactly what was wrong, but engine don't start, and when it do, only worked 2 or 3 secord, with too much smoke. My mechanic says "it is ignition failure", so changed ALL the ignition system, CDI included. The engine now start and runs well with no gear. But when i go in 2nd march or more and acelerato to 7500rpm+ engine dont work. It's like they have limitation and sound like engine stops 1 sec, slow down to 7000rpm, starts again with poweful pull and stops again. Constantly, between 7000 - 8000 rpm.



When turn the key to "ignition position" sound YPVS valve moving, so i assume it works correctly, but at this RPM is when YPVS must work and just my engine have failure. Maybe YPVS don´t work?



The ignition system my motorbike have now is from DT 125, so... maybe I have CDI limitation? How to get it off?



On other hand I changed the spark plug to see how it is, and I see it completeleyt black, with some oil liquid type. It indicates rich mixture of gasoline, right? this mixture can cause the problem?

[simon998]

Sounds like the ypvs isn't opening, take the cover off the side and check that it's opening when you're rev'ing the bike

[Dave B]

engine don't start, and when it do, only worked 2 or 3 secord, with too much smoke.

That is a sign of leaking crank seals. That explains the black oily spark plug too. The crank seals let the engine suck in the oil from the gear box.
I think you might need an engine rebuild to change the main crankshaft bearings and seals.
When you have done that, it is a good idea to clean out the inside of the exhaust silencer too. If the engine runs with leaking seals for a time, it gets full of oily carbon.

[museumman]

Only change the Cdi with a non-Yamaha one like Ignitech. Yamaha ones can give a spark but still break down.

I doubt it will be the ypvs. If they stay part-closed, the bike just feels less powerful, but will still slowly pickup speed with momentum, like a restricted moped still can. If the ypvs supply fails, they usually stay open after the test. 

Be careful of NGK spark plugs. I've avoided them for the last 2 years. Had a bike start to die always just before 8K, it was the new looking NGK plug.