Not too fast but very furious

[Lukas]

Now I see that after changing the cylinder I have to change the maximum RPM to 10500Rpm factory has 11000 moreover I am thinking about changing the gears I am looking for 15t front but I do not know if there is such, by the way this bike had to end up like this it was terribly catalyzed to the cut off

[Boatguy]

That sucks!

What motor is this?

[Lukas]

Originally Posted by Boatguy

That sucks!

What motor is this?

Zongshen RX3 EFI 82mm piston

[Working_ZS]

Originally Posted by Lukas

Now I see that after changing the cylinder I have to change the maximum RPM to 10500Rpm factory has 11000 moreover I am thinking about changing the gears I am looking for 15t front but I do not know if there is such, by the way this bike had to end up like this it was terribly catalyzed to the cut off

Ouch, damn, ouch!!

That looks like the piston skirt was hitting the crankshaft counterweights, identical to the problem that Choppercharles had with his kit. You can see scrape marks on the bottom edge of both counter weights in the video. There seems to be a severe lack of quality control on these big bore kits - hell, I don't think they are even using the right dimensional specs. They should not be experiencing this kind of catastrophic mechanical interference; this is basic blueprint reading and machining 101.

[Lukas]

I looked through his channel and the guy was turning the motorcycle up to 11000 RPM with a heavier piston so that's how it had to end up moreover on one video he has a carburetor and on the other he has injectors on, I looked and didn't see a lambda probe and the motorcycle ran normally so I for my kit will modify the limiter to 10000 RPM factory is 11000 RPM.In the comments it says it happened when he was driving at maximum speed for a long time on the road

[Working_ZS]

Originally Posted by Lukas

I looked through his channel and the guy was turning the motorcycle up to 11000 RPM with a heavier piston so that's how it had to end up moreover on one video he has a carburetor and on the other he has injectors on, I looked and didn't see a lambda probe and the motorcycle ran normally so I for my kit will modify the limiter to 10000 RPM factory is 11000 RPM.In the comments it says it happened when he was driving at maximum speed for a long time on the road

I'm curious as to what the weight is of the big bore pistons. Ideally, it would match the stock one, so as to match the crankshaft counterweights and not put undue stress on the connecting rod.

It's definitely a good idea to lower the rev limit on big bored NC250's, hopefully you'll be able to flash the ECU and prevent this from happening.

[pyoungbl]

ZS, when I weighed the BB piston it was 30% heavier than OEM.

[Working_ZS]

Originally Posted by pyoungbl

ZS, when I weighed the BB piston it was 30% heavier than OEM.

Wow, that's a lot. That would definitely require bringing the red line down quite a bit, and even then, you are still putting more stress on the connecting rod than the OEM piston assembly does; assuming that the new piston was machined correctly, which does not appear to be a given, in light of recent experience.

This is starting to become a question of whether the juice is worth the squeeze. Considering yours and ChopperCharles' experience with the BB kits, plus others that I suspect simply don't post up their results, I'm beginning to suspect that the answer is a firm no, at least in regards to the NC250 engine.

Now tuning the ECU with a stock setup is another matter, given that GPX manages to get 32 HP out of their FSE250 dirt bike with the NC250 in it; there's definitely more power to be had from the OEM 249cc set up. We just need to go about getting it differently, needing EFI tuning versus increased displacement.

[Lukas]

Originally Posted by Working_ZS

Wow, that's a lot. That would definitely require bringing the red line down quite a bit, and even then, you are still putting more stress on the connecting rod than the OEM piston assembly does; assuming that the new piston was machined correctly, which does not appear to be a given, in light of recent experience.

This is starting to become a question of whether the juice is worth the squeeze. Considering yours and ChopperCharles' experience with the BB kits, plus others that I suspect simply don't post up their results, I'm beginning to suspect that the answer is a firm no, at least in regards to the NC250 engine.

Now tuning the ECU with a stock setup is another matter, given that GPX manages to get 32 HP out of their FSE250 dirt bike with the NC250 in it; there's definitely more power to be had from the OEM 249cc set up. We just need to go about getting it differently, needing EFI tuning versus increased displacement.

The nc250 engine on Aliexpress are sold in 300cc version with the same head and shaft, and you can't get much out of mapping the injection, also heavier piston means different crank, and thus more Newton meters and better start.The fact that someone does not fit the cylinder is rather a matter of what Chinese factory it was produced in and you also need to be a little lucky and save the bike, because in this frame this engine is strained because of the weight.

[pyoungbl]

Going for a big big bore, with its heavier piston, should not be a big problem if the crankshaft is also balanced for the new weight. In fact, the new setup might even be smoother than when it came out of the factory. That's exactly what I found when I did a BBK on a Ducati Multistrada. Once the crankshaft was correctly balanced the engine was much smoother running even though it had higher compression and more displacement.

The inexpensive big bore kits being installed only include piston and cylinder. The pressed crankshaft remains balanced for the stock piston. Lowering the redline sounds like a must to me. If one were to balance the crankshaft it would require splitting the cases, then sending the crank, rod, and piston out to a specialist. I'm guessing $500 worth of labor in total. That takes a $200 mod up to $700 in order to gain a few hp...probably less than 3-4 hp. Once done the bottleneck is intake and exhaust. This quickly becomes a point of rapidly diminishing return in terms of power for money spent. BTDT.