2020 RIEJU RS3 NKD EURO4

[Jaxx]

Now where to begin this story... Some time ago I've acquired a garage nearby my home, which has plenty of space. Much more than my RX bike actually needs and I've always wanted second bike, which would be more sport/street orientated. Of course it is an option to put 17" motard wheels to RX, but that appeared much more complicated (speedometer can't be readjusted for different tire size, MOT papers-homologation are not valid for 17", and so on...) As I'm quite familiar with Derbi D50b motor now, I started looking for Aprilia RS4 (from 2011), unfortunately Derbi GPR 50s are more rare than snakes with legs here, only scooters and few GPR 125 four-strokes were sold...

It thought that would be an easy task, but I couldn't find a single bike that was worth buying. I was little surprised that I couldn't find any RS4 on sale, but there was plenty of 2006-2010 RS50 (first generation with derbi motor). On the pics they all looked ok, but in reality... For serious project, for someone who has the time for strip bike down completely and clean/fix/repaint all parts, maybe... Then I've found this bike:


RIEJU RS3 Naked EURO4 Black series

The bike is 2020 year with only 680km on clock. All restrictions still on place. Ok, I know, it's AM6 motor, but price was very good and bike is practically without scratch, only some very light scratches on outer plastics of mirrors and end of clutch lever!

[Jaxx]

I've already ordered some upgrade material:

- Unrestricted Polini ECU/CDI for EURO4 bikes with variable igniton curves
- Tecnigas G box pipe (should be the quietest 70cc sport pipe on the market)
- ATHENA 70cc AM6 Sport Aluminum cylinder kit, 47.6mm bore, single ring piston, 1:13.9 compression
- PHBG 21 carb
- Malossi 5 disc clutch upgrade, reinforced

[Jaxx]

Still waiting for upgrade parts... I don't rememeber situation like this - many cylinder kits are currently not available and delivery time extends to one month or even more. I hope I will get at least CDI soon...

[Jaxx]

I have finally found some spare time to continue work on the project. In august I've got most of the remaining parts, couldn't get Polini CDI though and got Naraku CDI instead.

I've found out this is one of the last small two-stroke sport/road bikes made... As you might know, in 2021 EURO4 emission regulations were superceded EURO5 (EURO4 lasted just 3 years!) and most manufacturers just couldn't adapt that, mostly because it would raise the bike cost well above price segment where this bikes should be... Aprilia (Derbi?) didn't bother and stopped production of all 50cc two-stroke bikes and scooters, Rieju and a few others (Sherco, Beta...) somehow managed to adapt AM6 motor to EURO5 with implanting third (!!!) catalytic converter in the exhaust (making exhaust so 'full' that it certanly doesn't need any aditional restrictions ) but that's just on enduro/motard bikes. I presume production costs of sport bikes are higher, as Rieju stopped production of RS3/RS3 NKD models in 2020. From 2021 on this kind of bikes are "history".

[Jaxx]

Before I remove everything, let me show you (if someone is interested ) what's about that EURO4 stuff anyway.


The hose with plastic box in the middle in front of cylinder: SAS(secondary Air System) also known as AIS(Air Induction System) is not strictly EURO4 thing as it appeared on some, but not all 50cc two stroke EURO3 bikes before. It supplies fresh air (oxygen) from airbox to exhaust to help catalytic process and to extend lifetime of catalytic converter. On bigger four-strokes it's active, driven by a servo motor, this one is passive - just a simple valve.


There is aditional rubber hose that's connected to carb intake. that's only exception to otherwise standard Dell'orto PHBN 16 with vacuum for petcock and carb-heater connections.


Under the fuel tank, that hose goes to solenoid valve, which is controlled by ECU unit.


ECU goes to the automotive-style connector on upper left side of the previous photo. This thing is not just simple CDI as with pre-EURO4 bikes. It has much more connections (inputs) and monitors engine temp, engine rpm to open solenoid valve that lean out A/F ratio under certain circumstances. How is this rpm/temp/solenoid function exactly "mapped" I do not know and any info on that is impossible to find...

[Jaxx]

Here is stock piston from Rieju:

Looks like somebody didn't observe the rules for proper run-in Now at least I know, why it was so hard to start bike from cold and it smoked a lot...

[Jaxx]

One thing I like with AM6 is that there is no swingarm bolt through engine case... That makes removing engine from frame very easy.

Fitted Voca kevlar clutch disks and stiffer springs (at first wanted Malossi, but it was out of stock...)

Didn't have to adjust center nut as I stayed with same number of discs (4, some kits have upgrade to 5) and they were about same thickness.
Stock clutch on Derbi D50b looks much beefier than this btw.

[Jaxx]

Fitted Athena Sport cylinder:




checked the squish:


At the end of solder wire (thats at cylinder wall) it is 1.0mm, exactly as it should be. But notice that at inner edge of squish band (toward sparkplug) it's just 0.55mm! That means that squish gap gets smaller toward expansion chamber?

Exactly as on this sketch that I've found on mopedarmy.com:


At first I thought that they send me wrong piston (one with higher dome) but I have checked part numbers they are ok. What do you think?

[Jaxx]

After extended search I've found out that everything appears to be ok. I was baffled at first, because that guy at mopedarmy.com forum that draw this little sketch blamed squish profile to cause pinging (detonation) and spent much time for re-shaping head from converging squish gap to diverging, just to find out in the end that pinging stayed, and the problem was probably in too much ignition timing...! Then there are buch of 2-stroke dirtbike forum so-called 'experts' talking shit that diverging squish gap in 100% cause detonation and (beacuse 'pushing' charge in wrong direction) blowing out head O-ring, etc... Well, it looks like it ain't that simple! For max. rpm race engine it certainly may not be the solution, but for sport cylinder that doesn't rev past 11.000 rpm (quite low 182°/125° exhaust/transfer timing) it should make optimal turbulence...

[Jaxx]

The bike is more or less finished, right now i'm doing a couple of heat cycles.











I'm having a problem with choke cable... PHBG 21 needs 1 cm longer inner cable. Anyone knows the easiest way to shorten outer flex cable without damaging inner cable?