Wednesday 14 September 2011

Yamaha DT175MX


I was looking for a cheap 250 when I heard about a Yamaha DT175MX with only 4000 miles on the clock going for £250. The DT175MX was the first Japanese road bike to have monoshock suspension and sold very well, mainly because of this. The early version had a five speed box, CDI ignition and a high level crossover exhaust, making a very attractive package for its day. The gearbox was replaced by a six speed item shortly after its launch. Final models also having a box section swinging arm.

This bike was 1980 V reg model with a six speed gearbox and tubular swinging arm. The tank was a little chipped and the right-hand footrest was missing, but overall it looked okay. A test ride revealed it was either running a bit rich or needed a decoke and a few bulbs had blown, but I was happy enough so I bought it.

When I got home I set about giving the bike a decoke. The six nuts which held the cylinder head on could only be removed after removing the exhaust, which in turn needed the petrol tank and also the seat removed. All except the exhaust were easy to remove, the exhaust requiring a little wriggling and twisting to avoid the upper frame tubes. The barrel is held in place by four threaded rods with very soft alloy nuts. Two of the rods were stripped of thread, the only way to remove them was by locking one nut against the other on what was left of the thread and gently applying pressure.

When I polished the piston crown, I found that the bike was already on its second rebore at 4000 miles! Probably a credit to the previous owner, I thought. Carbon only seemed to accumulate around the exhaust port, the piston and head only having a thin coating. On reassembly I bunged in a new foam air filter as the old one had disappeared. Foam filters can be washed out in petrol and oiled, it beats paying out £5 a time for a paper element.

When I ran the bike it would not rev above 5000rpm, with thick blue smoke trailing behind. Back in the garage, I checked the oil pump setting which had been set with washers rather than shims - the air filter had been removed to help it burn this oily brew and it also explained the two rebores. After fixing that, the bike would hit 5500rpm and take off like it should.

The DT does take a few days to get used to as pulling away below 4000rpm would bog down the motor unless the clutch was carefully fed in. Pulling away above 5000rpm, however, produced some spectacular wheelies. Once, pulling away quickly into a space between two moving cars with the aid of my friend on pillion, the front wheel became airborne by about a yard, but the bike was perfectly balanced so I just carried on, reaching third gear before the front wheel kissed the road. In fact, a good DT engine would lift the front wheel slightly when changing into second from third under hard acceleration.

In town and on A roads the DT does make a good commuter. It accelerates faster than many cars and can be nimbly manoeuvred between queues of traffic. On my daily ride through Nottingham, a Honda CB250RS would be left behind at each traffic light GP. Once on the dual carriageway, however, the DT would lose its initial acceleration advantage and the RS would quietly run past.

Dual carriageways are not what the DT was built for. At the redline it would pull 65mph dead, any head wind could take this down to 50-55mph with the engine screaming in fifth gear. Of course, chinning the tank cuts down wind resistance but at over 6 feet tall, I would look like a real prat, especially remembering one occasion when a CD125 Benly quietly sailed past with its rider bolt upright. This was my most humiliating moment.

In the subsequent hunt for more power I read that if I added Boysen reeds to the engine, then combined with the Stinger exhaust already fitted, power would increase by 25%. I bought the reeds, cut the reed block back as instructed and bent the reed petal stop back. When everything was together I only noticed one difference, it became less of an on/off motor at 5500rpm, sort of taming the power band. Top end, economy and acceleration all remained the same.

Economy was poor, but I did seem to be riding around at maximum revs everywhere, so maybe 52mpg wasn't all that bad. At 7000 miles the exhaust pipe cracked clean in two, just where it exits the barrel. This also happened to a mate's bike, so it may be the weakest point on the system. A new one cost £65, but luckily Mutsui were offering pre 1980 parts at half price, so £32.50 wasn't too bad.

When I bought the bike the chain was brand new, so I looked after it with aerosol lubricant every few days and boiled it in Linklyfe every few months. With this tender care it lasted 7000 miles. Without it, I would imagine 4000 miles to be more likely.

The front forks are very strong, as I found out when I ran into our house! An out of control wheelie being the cause. They are easy to drain of oil and refill via screw-on caps, unlike some Yamaha forks with awkward circlips to remove. I used to fine tune the forks, finding SAE15 oil gave the best ride, whilst not producing too much dive under braking. A word of warning, though, make sure the forks are fully extended before removing the caps, as the springs are very long and powerful, launching the cap and spanner suddenly around the garage.

The monoshock, on the other hand, is quite a crude device by today's standards but it does work after a fashion. The upper pivot catches all the crud thrown up by the back wheel and needs regular greasing, otherwise it becomes stiff and affects the monoshock's operation. Another reason to keep it well lubricated is so the swinging arm can easily be removed every 12000 miles to replace the plastic roller, which protects the swinging arm from being massacred by the chain.

With winter fast approaching and the Pirelli tyres looking shoddy (7500 miles rear, 8500 front), I decided to buy a very trick pair of Michelin T61s - they look like motocross tyres and suited the bike really well. I once (and only once) grazed the footrest down on a sharp left-hander with absolute confidence, looking more like an ice racer. However, 4500 miles rear tyre life was a steep price to pay.

One dark cold morning, Derbyshire awoke in the grip of a snow blizzard and most people stayed in bed. I was overjoyed and couldn't wait to take the DT out. My ego was a little deflated, however, when to get up our drive I had to run alongside the bike. I carefully rode along the country lanes in second gear, with the just adequate 6V headlamp picking out a lonely trail. When I reached the main road, conditions were a little better so I went a bit faster, my confidence growing as I reached third and fourth gears.

I arrived at the bottom of a steep hill with cars abandoned all over the road. As I rode up the hill my back end would step out occasionally, so I stopped to lower the tyre pressures to 10psi. A chap rode slowly past on a Honda stepthru and shouted, 'Ha, get one of these, boy, a lot better than that fancy tackle.' I looked up to give a smart reply, when his front end slid away and he landed in a heap next to me. He got up unhurt but with severely dented pride as I burst into fits of laughter.

I rode on through Langley Mill and on to the dual carriageway, following cars at 30mph, single file on the inside lane. The DT felt very stable and eventually the outside lane tempted me - 35, 40, 45, 50mph on the virgin snow, rock steady until I hit a ridge of snow. The dance that followed must have entertained the car drivers no end, but somehow we stayed upright.

Winter had just ended when the DT developed an electrical problem, any light turned on would shine extremely bright and then blow. I bought a black box which does away with the battery and replaces the voltage regulator.....out on the road everything was okay until I stopped and noticed that white smoke was pouring from the box as it melted.

Eventually, I started to follow the vital wires until I came across a small piece of plastic between two spade connectors, located on the frame under the petrol tank. One of the connectors was corroded so it was cleaned and greased - everything worked okay after that. This insignificant piece of plastic was the rectifier, a vital component which had caused all the trouble.

After ten months the bike started to rumble from the bottom of the engine, the main bearings were shot at 10500rpm. When I split the cases, all the selector forks fell out. Five evenings work and I was back on the road, running the engine in for 500 miles, revving between 4-5000rpm. During that time it returned 77mpg but soon returned to the usual 52mpg when ridden normally.

I soon sold mine, out of town it was not much more use than a RXS100. On buying a DT the points to check are monoshock linkage, the exhaust where it exits the barrel and the wheels for dents. The oil pump is also known to fail through corrosion as rain water trickles in through the cable entry point and can't get out. And, of course, check the electrics. It is a fun bike but needs an hour's maintenance every week to keep it reliable, and a decoke every 2000 miles even if you use low smoke oil such as Belray S17. As a weekend toy, though, it's a lot cheaper than a TDR250.

Steve D.