Yamaha NXC125 20,000km

[awa355]

Just finished reading the complete thread. .

It's a credit to you and your perseverence. Most of us would've taken the easy way out and traded the scooter long before now.

[speedpro]

I've finally stopped pissing round with other people's buckets and have yanked the motor from the scooter again. It only takes me about .5 hour now. Took the head off to have a look and it is all good. I fitted a piece of perspex with a hole in it to the head with the gasket still in place and measured the volume using a syringe and some oil. It is only 12.5cc. Taking into account that it is sitting on top of 203cc of swept volume and just taking the piston as flat that gives a compression ratio of 17.6:1. Possibly just a smidge high. I'm gonna take to it with the grinder and take out up to 8cc which will be about 11:1. The good thing is it will improve the breathing as well as lower the compression. The intake valves are badly masked around their outside edges so I'll open that right up. The exhausts are nearly as bad. It will affect the flow pattern big time in the cylinder with intake charge coming from both sides of the intake pair rather than more or less just from the cylinder inner side. I intend to leave flat sections where they are now to promote turbulence towards TDC so it shouldn't be a problem, though I may reduce their size if I run out of aluminium to remove from other places. Might see if I can take a picture showing what I mean plus for a before and after comparison.

[speedpro]

Been busy. Compression is now about 13:1 which is about as low as I can take it with just grinding out the head. The black circle outside edge is the size of the new bore. On the modified combustion chamber the black parts are untouched flat portions forming squish areas similar to the original chamber. I've advanced the ignition timing to stock and taken it for a thrash. It handles full throttle now but is still flattish. To be expected with the small ports etc. The valves look quite small in the modified chamber I think. In the original photos you might be able to see how badly shrouded the valves were originally.

[speedpro]

Things have happened and I've decided to sort it out properly. US$399.60 has been sent to Taida and a new cylinder head with big valves, big cam shaft and grunty starter motor is on the way. I'm going to order a 30mm PWK carb kit from Jiangwayne. The kit comes with everything including a new throttle cable and a selection of jets and a new intake manifold - US$149. I'll weld up some sort of exhaust for it. If anyone has a 250cc scooter muffler they want to get rid of PM me. I want to keep it quiet and discrete.

[speedpro]

3rd October, sent payment, got email back with photos of goods and documents.
8th October, parts on doorstep.

Carb is machined a bit funny but is still workable. Manifold has a sharp change in direction on the inner radius which I'm going to fix. The original Yamaha intake manifold was hand finished and was nicely curved. All the new bits are 3-4mm larger diameter so should flow heaps more on a suitably sized port.

[speedpro]

I've started looking at how to run the new throttle cable. So far it has eluded me how to remove enough front panels to run the cable in place of the original one. Even getting the throttle apart to attach the new cable is a mystery. If anyone can help with a diagram or instructions I'd really appreciate it.

Got a note from customs today. They need $132.57 before they release the new head and cam. $94.50 GST, $22+GST import transaction fee, and a biosecurity levy of $11.10+GST. The GST is on the goods plus freight so thats added 15+% to the cost for . . . . . . . . . not a lot. I suppose we have to protect that local spare parts industry.

[speedpro]

It felt like Christmas this mornng. Big box of goodies arrived. As I expected the hardest job has been running the new throttle cable through the frame and cables behind all the plastic. I pulled most of the panels off to get access and even then needed help from my son. The engine could be back together tomorrow and back in the scooter. I can't fit a thicker base gasket in and without measuring it I think the compression will be a bit higher with the new head.

The bits look good though. Nice big valves and decent ports.

[speedpro]

New motor all together and fired up nearly straight away. Ran sweet from the start though I am using the stock exhaust at the moment which has a much smaller pipe than the port. I've taken it for a quick squirt and it's sooo much better. The only annoying thing is the noise from the carb intake as the filter doesn't quieten it much. I'll give the original airbox and connector a try after I have the new pipe fitted. The throttle is sweet and I'd say it is marginally rich towards full throttle. It pulls to 80Kmh easy up a reasonably steep local hill and shoots to 100kmh on the flat. I thought it was running sweet accelerating around 80K and then the it came on the cam. Even the variator seems to be in the ballpark for thrashing round the block sort of riding. If the exhaust is as restrictive as I've been led to believe for larger motors it will be real interesting to fit a free flowing pipe.

[speedpro]

First short run on the motorway today. Quiet run at 80K to Royal Road on the NW motorway using hardly any throttle. Coming down the on-ramp back to Lincoln Road I gave it a squirt and it was up over 100K halfway down the ramp. Took it back to 90K and cruised back with the slow cars. It'll handle taller gearing easy and I already have "up gears" fitted which are about 10% higher gearing. The carburation is faultless as far as throttle response goes. If it's moving it does whatever is asked with no hiccups or staggers. Even from a stop at lights I can just nail it and it leaps away which could be expected I suppose considering it's 200cc with good ports, carb, cam, and a variator transmission geared for only about 120K tops.
Hopefully I'll get some time in my mates workshop this weekend and come up with a better exhaust then I'll check the jetting. I wonder just how bad the standard exhaust is considering how well it runs now.

I've been thinking this setup would make an awesome bucket sidecar power plant though the engine would have to be smaller and some support would be required to handle the loads when cornering.

[speedpro]

Put it on the dyno. As it was it only made 11.?hp but over most of 70-120Km/h.
It did seem rich just going by how the graphs looked so dropped a jet size. Immediately picked up 2hp.
Dropped another jet size for another hp and the same again.
A further jet made no difference so back to the previous. We suspected the pod filter may be restrictive so did runs with it off and it definitely picked up .25-.5hp. Then we tried a couple of velocity stacks which showed promise but none were long enough.

It's making 15.?hp now pretty well between 70-130km/h which was the only range we were checking on the dyno.

There's clearly something holding it back looking at the graphs which is most likely the stock exhaust complete with the catalytic converter so that puts the pressure back on to finish the new one.

The stock transmission seems pretty good. It picks up nice and holds the motor off maximum revs until about 100-110km/h when the transmission maxes out after which the motor revs further until it pops on the limiter at 130Km/h - on the dyno at full throttle. I'll have to check which rollers are in the variator as it seems it will pull heavier rollers without too much fuss.

Riding back home it is definitely perkier and much happier at full throttle. The intake noise is getting really annoying so I'll have to look at an airbox or something and include a velocity stack.

[speedpro]

I've finished the pipe and have the new muffler fitted. Now it really does bark. It sounds really similar to a bucket racer FXR150. We actually compared it side by side to a 20+hp one today in the workshop. It's running lean at larger throttle openings now so should make more power when a bit more fuel is tipped in. Photos to come.

[speedpro]

Nice looking pipe, shame it doesn't work.

I've gone up in the jets to the point where it really didn't want to know and it simply doesn't work. Possibly if I could clutch it and get it over the stumbles it might be OK but with a CVT transmission it isn't an option. I will try a few extensions to see if it simply needs to be longer but I think I may need to make another one.

I put the original pipe on and went back to the smaller jet we figured was best on the dyno. It runs sweet. I gave it a squirt on the NW motorway and it pulled 110Km/H easy peasy.

I'm thinking the exhaust needs to be freed up but not tuned at all as if there's a dip in torque the variator won't let the engine get over it.

[speedpro]

The new starter has always seemed to kick off well but if the engine didn't catch pretty well straight away it then struggled. I added some new hefty wiring in parallel to the existing starter circuit from battery to solenoid to starter motor and paralleled the ground wire from the battery to the engine. It "seems" to spin a little more energetically and this arvo when it didn't catch straight away a 2nd push of the button caused it to turn faster than previously and it fired up fine. No way of measuring it but it seems to start better.

I took my son to town for a look at Motomail on Saturday. We cruised in at a steady 90Km/H until we came across a few cars really crawling along. With the traffic in the middle lane I needed to speed up to merge without causing a disruption. A bit of throttle saw 110Km/h easy. On the dyno it was hitting the rev limiter at 130K and I'm reasonably confident it will pull it even with the stock exhaust.

Off topic - Motomail have an Aprilia 850cc scooter which makes 75hp. Supposedly it will do 200Km/H. Cool looking bit of kit.

[speedpro]

Dead flat bit of private test track, no wind at all, and it pulled round to the "oil change indicator" no problem. Just changed the oil, Mobil1 4T, and loosened one intake tappet a whisker. I've taken it to Manukau to work and back to Henderson recently. Just cruised along at about 90K without any problems, just taking it easy.

One little issue I've found is that in the new road works going to Maioro Rd where it's only one lane the traffic really crawls. It isn't always possible to sneak past and I just have to stop/start creep along with the cars. It gets a bit ragged as the clutch takes up and I figure it's getting a bit overheated with the continual slipping. The stock lower gearing would probably be better in this situation as the clutch could fully engage at a lower speed.

I've had a good chat to someone that knows "stuff" about this style of engine and I've got some good pointers for the exhaust. That'll be a little project once I'm back from holiday.

[frogfeaturesFZR]

Good work !