Yamaha NXC125 20,000km

[ICE180]

great read

I have a 2t Runner 180sp with
172 head for higher comperssion
polini clutch
MHR racing varatior
PWk carb 24mm
Pm tunning expansion chamber
Racing stroke crank
Pm tuning bushes

The list goes on it is fun though when you turn up to a ride with bikes on a scooter and overtake them on twisty roads or chase them down

I just wanted a bit of extra speed for overtaking in the open,now I do DYO days at the drags and get 2nd overall to the disbelief of some harden drag peeps

keep up the madness love it

[spinnanz]

great read

I have a 2t Runner 180sp with
172 head for higher comperssion

my DNA180 could do with that motor!

[speedpro]

Took the scooter to work today. Sat on 100Kmh easy with traffic without sounding too busy. Accelerated just fine at pretty well any speed and pegged the speedo no problem. Cruised along from the bays up along Kepa Rd heading to Glen Innes. Sitting at the lights at the top of Apirana Ave thinking to myself how good the scooter was going when the light goes green. As I gave it a handfull to take off it hesitated just a fraction which got my attention and then got going fine. It had a few more really minor faulters going down the hill and then at about PaknSave the motor just cut. I've got it home, on the trailer, and have started looking at it. The starter motor has real problems turning it over but it spins fine with the plug out and makes a good spark. I wired the car battery in parallel with the same result. The starter motor really struggles. I put my finger over the plug hole and hit the starter and it blew my finger off the seat strongly. I've suspected, because of the noise returning, that the crank was floating around again and it has end float again so the loctite didn't last. My only real suspicion is that the cam timing has slipped somehow. It has been noisy but with a new chain and the tensioner only going out a couple of clicks I was thinking the noise was something else. Hopefully this lesson won't be too expensive. Full confession after the motor comes out tomorrow.

[speedpro]

Motor out, oil drained,?? what are those sparkly little chunks in the oil ??. Turns out the big end in the new crank has toasted itself. I think somewhere back in this thread I expressewd reservations about the rod as it had "GY6" stamped on it. I will fit the original crank over the weekend so it'll be back to 150ish.

I'll have a look at dismantling the crank. If a stock rod kit is cheap enough and long/short enough I might look at rebuilding it, or maybe a Pro-X kit.

[speedpro]

Post 17 refers to the rod, must have known something!

[speedpro]

After having a good look at the crap in the bottom of the crankcases I have decided to hold judgement on the rod. What I think has happened is that the Loctite was unable to hold the bearings. It has broken out into the oil and made it's way into the bigend where it has done a job on the bearing. The oil pump is scoured and I'm guessing the other bearings have had flecks and chunks through them as well. I've put it back together with the original crank as I still can't see anything wrong with it. I'm using the big cylinder so the motor is about 150-155cc, I can't be bothered working it out. The piston had been roughed up a bit as well and there was a bit of alloy stuck on the bore. I've sanded it smooth and taken the alloy off the bore to the point where I'm happy with it. I assembled it without a base gasket to make up for the shorter stroke crank so the compression is back up there. Even with the new cam chain the tensioner now only has about 4 clicks left to go. When running the engine makes some horrendous noises. The only thing it can possibly be is the cases or more specifically the housings for the crank main bearings. I'm now checking out new cases from the USA or possibly a 2nd hand bottom end from NZ or Oz, though I'm wary of buying an engine with the same issues.

The other simpler option might be to convert it to chain drive with a reguler 2-stroke motorcycle engine in it. I had a look today and it wouldn't be all that hard. You just need a swingarm that pivots on the same place the current engine/transmission pivots and the new engine mounted on the swingarm so it pivots with wheel movement just like now. The hardest thing would be getting a suitable rear wheel and sprockets.

This is getting annoying and wasting my time. Does anyone know of similar issues with crankcases in other Yamahas?

[speedpro]

I ordered the crankcases from the USA some time ago and got word yesterday that one is on order from Japan - yay!. The same one wasn't available in NZ either. The small crankcase cost more in NZ than I was quoted for both including freight from the USA.

If anyone knows where I could possibly get crancases a bit quicker I'd appreciate a call/msg. The commute is annoying and frustrating and expensive and a long time in the car.

Or another option would be a fix that doesn't cost more than crankcases and will last.

[speedpro]

Decided to have a look at the guts of the long stroke crank and see if there was a fixable reason why it stopped and got all hard to turn. The "Trimac" guys had an idea that the two flywheels may have squeezed together and be jamming the rod, which seemed reasonable whilst talking to them. We pressed the pin out about 1mm which completely freed the rod but it was still problematic so pressed it completely apart. It took about 4 tons to press apart which was a clue that it hadn't just squeezed up a bit. What a mess. The pin has major damage with serious brinelling and the rollers all fell out in pieces and the cage was mostly gone. Seeing the damage I've had another very very careful look at the genuine Yamaha crankshaft and there could be freeplay in the big end on that as well. I'll degrease it tomorrow and jig it up to measure with a DTI. General concensus is that with such a lousy oiling system ther ehas been some crap fed to the bearing which has wrecked it. Not many options there unfortunately. I will check the TKRJ and ProX catalogues anyway. The cranks only cost US$122 so it's hard to justify a rebuild and could be a really good excuse to get another (longer) stroke crank. Will need another (bigger) cylinder if I do that, and a cam, and a carb, and a pipe, and somewhere else to live when the wife finds out.

[speedpro]

Rob at ESE pressed the Yamaha crank apart tonight. The pin has serious wear but the bearing seems good. It looks similar to the other crankpin but not as bad so it would seem that is where the noise has been coming from. It could be just the extended thrashing on the NW motorway but I really didn't expect a roller crank to wear so badly on the pins. I've raced old Z1s which have roller cranks and Honda MB100 2 stroke bucket racers and never seen the wear like in the Yamaha scooter.

So now comes the decision on what bore and stroke to make the motor. They go all the way to 288cc but that would raise a pile of other issues. Hopefully I can cancel the order for the crankcases.

[speedpro]

Hopefully I can cancel the order for the crankcases.

They arrived today.

I also got an email from Brad at MrCycles replying to my email from last night. He offered to take them back if I didn't need them and noted that they had been shipped. Pretty good I reckon.

[speedpro]

I have to commute to Manukau every day now, 30Km each way. I was getting a ride for a week but that isn't happening any more so I have to get the scooter going again. I've ordered another +3.5mm crank and a bigger 65mm cylinder and forged piston. It'll be about 200cc so will require a bit of tuning to get right, maybe even a new carb. I'll set it up so it revs lower to hopefully make it last.

[speedpro]

Bits arrived from Taiwan on Thursday 26/4. I only ordered them on Tue 17/4. It looked like they put everything in a box and put about 50 stamps on it. It was going to be 2 weeks until they were sent but as I'd been emailing them about some other stuff my bits got bumped up the order. Great service, kept me informed and with the tracking number it was easy to follow.

The crankshaft is different to the last one. The new one is fully machined and "looks" a lot flasher. The machining of the spline looks a lot cleaner as well. Like the last piston this one is a very nice forging but strangely the cylinder has a steel sleeve whereas the last cylinder was chromed aluminium. Wish I could afford a good cylinder head but realistically I don't need it, I hope.

[speedpro]

It lives. I dummied it up to check that nothing was going to go "bump". With the low compression base gasket (thick) I couldn't get the cam sprocket onto the cam as the chain was too tight. With the high compression gasket (thin) I can just get it on with a real good shove.

It's actually hard to turn over by hand. Once together, and with some fuel tipped in to fill the carb, I hit the start button. I have discovered the next thing I need - a high torque starter motor. If I give it a couple of goes it will turn the motor over but it isn't a sure thing. I have to give the kick start a pretty good prod to turn it over and you definitely couldn't turn it by hand like before.

It all sounds good which is a relief after all the mucking around. A few good lessons have been learnt. I'll need to sort the jetting tomorrow and after a few miles I'll see if I can put it on a friend's dyno and check the timing as well.

[speedpro]

Done about 125Km on the new motor and it's running sweet. Still only using 1/4 - 1/5 throttle except for short bursts. I've just topped it up with Mobil 98 but it still pinks a bit I think though less than before. I'll try jetting it up probably this weekend and I'll slot the pickup mounting holes so I can adjust the timing. I've got the technique for starting it now. You leave the throttle completely alone and just let the starter motor struggle with it. If you touch the throttle it stops the motor turning and seems to flood it after which it needs to be turned for a while on the starter until it clears out and then it slowly catches and starts running. If I've messed it up and had to turn the motor for a bit, it quickly gets to the point where it won't start turning the motor when you press the button. In that case I stand on the kick start and then hit the starter. The combination is enough to turn it over and generally it'll keep turning until it fires.

The motor was genuinely tight to start with but it is noticeably freer spinning now.

[speedpro]

250-300K on it now. I've given it a good handful here and there only to discover that it is severely limited. General concensus says it is the exhaust primarily but it needs a bigger carb as well and ideally a new big port head and cam. I'm leaning toward a complete variator, pulleys and clutch upgrade as well. There are plenty of writeups extolling the virtues of aftermarket bits. I've dropped the timing back a couple of degrees which has stopped the pinking with the added bonus that it seems a little easier to start.
Stock pipe will be off this weekend and I'll spend a bit of time machining a new exhaust spigot and hopefully find a bit of pipe to use. I haven't decided to buy or make a muffler yet. I might possibly go for 2 pit bike mufflers stacked, very styley.