In early 2024 I started building up a bucket racer for the coming season's racing at the local kart track. The bike was in a bit of a sorry state, no brakes, forks were partially disassembled, engine had low compression on one of the cylinders and so on... I got the bike for free so I can't complain too much. The bike is based on a CB125T, the wheels and front end are from a TZR250 2MA.

I stripped the top end off the engine, the left piston looked pretty knackered. It'd had a seizure at some point. I had a second engine that came with a half decent set of pistons and cylinders. I gave the bores a quick hone, chucked a set of rings on the pistons - just to get an engine together so I could get it running and start going through what needed doing with the carbs, ignition system etc...

In and around working on the engine, I started making an exhaust and sorting out the brakes and suspension. I had a dummy engine in the bike that I could get in and out easily (nice and lightweight, no internal components) so I could mock up an exhaust. Lucky for me, I was working at a Yamaha dealer and there was an old set of header pipes from an R3 already chopped up in the scrap bin, I commandeered the pieces and cobbled together a 2 into 1 system mated to a $50 AliExpress muffler.

At this point the bike is pretty much ready to go, it's running and I've ridden it up and down the driveway. I took it down to the dyno at MCR so I could run it up properly, get some temperature into the engine and get an idea of what sort of power the bike was making. The first few runs went okay, the fuelling was way off (I'd bought some cheap carbs from Trade Me that were intended for a single cylinder 125). Before I could start swapping jets the bike just died... no spark. To cut a long storey short, the bike came running a TCI module from an old XJ750, it had decided to give up the ghost so that was game over for that tuning session.

In a bid to get the bike running and riding for the first race of the season, I cobbled together a set points, found some old coils that'd work and got them married up to the bike. They seemed to work okay although, they took a long time to set up and they seemed to float at high rpm giving an irritating misfire... they got me through the first round at the kart track which is all that matters! I ended up finding an electronic ignition designed for a Honda CB500 four that fits the 125 engine with a little fiddling. It basically goes in place of your points and you can use the same coils, pretty much plug in and play.

At the point the photo above had been taken I'd done a couple of races at the kart track on the 125 with the stock engine, it had run pretty well and I was getting comfortable riding the bike. Every year we put on an endurance event at the kart track, it's a one hour race with two riders, do as many laps as you can, most laps wins. So I teamed up with a friend's son who was keen to get into riding, I offered him a ride for the day which he was pretty stoked about. We did a couple practice laps, two points races and he had another couple practices when the bike started to make some rattley noises which just got louder and louder... I pulled into the pits, pulled the cam cover off and found the cam had eaten into the cam bearings which meant the valve clearances had gone from 0.002" to 0.020". Hence the noise. I only found out later that the oil pump pickup was completely clogged with years worth of crap, these engines have no filtration so what's in the oil stays in the system unless it makes it's way out when you change the oil. The set the stage for CB150T version 1.

Over the winter I had been messing around with some oversize piston options for the 125, the standard sleeves have plenty of meat in them, enough to accommodate a 47mm piston which takes the displacement out to about 145cc. I had a couple of piston options, but I ended up using a Honda C70 pistons with a low volume crown, I am aware you can buy off the shelf pistons for these engines from China but I wanted to make life difficult for myself and use Japanese quality parts. Using the C70 pistons also meant that I could increase the compression ration to around 11.5:1. It took a little figuring out but I managed to get everything machined up, assembled and ready to go for the next race meeting. I ran the bike on the dyno too which showed a decent increase in low end power! Peak horsepower is now 16.8bph at 11500rpm vs the 15.4bhp at 13000rpm of the standard engine. The most difference at around 10000rpm with around 20% increase.

There probably won't be too much more development on this bike in the future, I've since got a different chassis (keeping the engine from the Honda) to race next season. It's been a good way to start racing though, free bike, make it work, make it faster! I somehow managed to end up with third place in the local bucket racing series, probably due to a couple of the other more talented riders not making it to all the meetings. Good attendance counts for something I guess... Stay in school kids!