RX3 / NC250 gearbox issues.

[NzBrakelathes]

Had this issue for more then a gear where it would jump out of 3rd to nothing or 2nd or 4th or 5th and bash around like crazy.
Stripped engine out and down and I cannot see any bad wear on the gears yet the fork for 3rd looks to have wear marks on it as if the grear can touch it.
Also the tabs on forks are very low so the raised part of finger is hardly or one fork not there at all.

I get the feeling this engine is a very early made (cases made 2009-07) yet bike is a 2014 from factory and sold in China.
There have been some minor changes in the shaft design and 1 being the big nut to hold sprocket rather then the big bolt.

I suspect it wasn't made perfect as China stuff often is and then it fails within a modest amount of time (15000 Km or so - if in fact real milage) and changing out to newer made more maturely manufactured stuff or just less wear stuff will solve the issue.

Anyone else in the world had this issue with 3rd jumping around to no drive or being rammed to another great?

There must be a hell of a lot of forced pushing the gears apart to force the fork to push into the drum and drum being forced to spin to another gear.

[NzBrakelathes]

I pulled the 2 gears off that are related to 3rd gear - slight wear and my supplier says that it doesn't need much to cause trouble.
I honestly would have thought the wear isn't much BUT........

Here are the forks - you can see they are kinda ground weird IMO



Here are the 2 gears that engage when you go to 3rd - not much wear but you tell me? I am no expert....

[JerryHawk250]

Looks like it wore just enough to make it slip.

[franque]

I'd replace the complete transmission including the shift forks and drum, as I'm sure it shouldn't cost you too much. I've seen shift forks like that that caused trouble, I'd just play it safe and replace everything.

[NzBrakelathes]

The other thing I found was the rear shock spring is backed off so much the spring was actually loose!

Anyone have any clue as to stock setting or where it might be good for a big dude 120Kg?!

No wonder the back would slam me up the ass!

[pyoungbl]

A general rule of thumb is that the suspension should sag under rider weight about 25% of total travel. Thus, you should adjust the pre load to the point where you have the suspension compressing that amount when you are sitting on the bike...with the zero setting being when the rear wheel is off the ground. I use tape on the bike chassis and tape on the swingarm with horizontal marks on each...with each tape being directly above the other. Measure the distance in mm with the rear wheel off the ground, the measure with the wheel on the ground and rider seated. The difference should be about 25% of total travel. If the spring is too strong you won't be able to get that amount of sag and you need a softer spring.

[NzBrakelathes]

I now have a couple of engine manuals - first I had didn’t show exactly my engine and was an older version on NC, now I've got one that’s got closer to my engine and shows certain areas as per my engine.

I also scored some USA RX3 & 4 parts diagrams but be aware there are slight changes thru out the years etc.
RX3
RX4
TT250
NC250
NC450
Manuals are not perfect as they change designs slightly at times but helps in assembly n what washers n bits go where or possible details on bolt lengths - but careful as older to newer there are minor changes n info could be wrong.
I layed out parts together to check what pond or washers went with what parts but found 1 manual to not show the same as my engine n to the other manual.

Still useful etc

[franque]

Is the NC450 manual for the dirtbike style motor, or the RX4 style? I might be interested in a copy of the NC250 and 450 manuals.

[NzBrakelathes]

Originally Posted by franque

Is the NC450 manual for the dirtbike style motor, or the RX4 style? I might be interested in a copy of the NC250 and 450 manuals.

RX4 as best I know - I doubt the difference for reference is too much different etc.

[NzBrakelathes]

Had another look yesterday and I had it slightly wrong.
4th gear is what’s got a bit of wear on gears but not 3rd, the fork for 3rd moving to engage had wear marks n no more seat/step for gear and I’ve replaced all 3 forks.
The gear was touching the fork and an NZ customer had a similar issue with a bent fork.
I’m sure the drum was slightly in wrong rotation cause it never displayed in 3 correctly n with fork wear or bent it would surely roll past or before the correct position of the drum n not connect to earth that wire to show 3rd

Let see in a few days if fixed or if I’ve not learnt enough!
Best bet id say and was suggested to just replace both shafts drum forks pin star wheel and gear actuator - but I’m on a budget n rather find and pin point the issue then just parts cannon at it.
We will see......

[NzBrakelathes]

Total FAIL!
After making a few BIG Farkpups and finally assembling the Bursht it still pops outa 3rd gear.
Soooooooo......
There are at least or have been 3-4 changes to gear box shafts with or without bearing under some gears and 3 different sprocket mounting hardware, old and new fork design (but don't think new forks work on old drum).
Best just replace both shafts complete and drum with newer style forks etc etc etc..................................
I suspect the 3rd gear "tips" on the shaft under load - so is the bearing or shaft or the inner of the gear worn? Hence the rubbing on the fork I suspect.....
DAMN it all! It has been a massive headache n learning curve etc.
AJP PR5 manual and an original Zongshen manual for torque specs has fair confliction specs - who know what is right or wrong.......

[franque]

It could be that the cases have bad tolerances, too. Sometimes the cases flexing can cause shifting issues, too. If it were me, I'd look at newer cases, measuring and comparing them.

[NzBrakelathes]

Originally Posted by franque

It could be that the cases have bad tolerances, too. Sometimes the cases flexing can cause shifting issues, too. If it were me, I'd look at newer cases, measuring and comparing them.

Already replaced a case and no point as even locals say replace all “made in china” parts due to quality issues

Same done to an NZ customer
20 or more changes made to engine over 10 years tells you things not made right

[NzBrakelathes]

3rd gear fork gets “squished” between the dog gear if dog gear fully engaged
Maybe that’s why I had wear marks? New it’s like this too

[franque]

Maybe it needs a shim?