Stan Stephens on tuning this off-road gem.
Carrying on with his series on tuning classic two-strokes, our Stan turns his attention to the Yamaha DT175 and 175 MX.
Yamaha was the first to see the market for a trail bike and originally started producing the DT175 in 1969 but it didn’t reach the UK until 1973, then all the manufacturers started to add trail bikes to their catalogue.
The DT175 was a twin-shocker with a five-speed gearbox, this was followed in 1978 by the DT175 MX which was a monoshock and had a six-speed gearbox. The DT175 had the same basic engine as the trials version TY175. Trials specialists Mick Andrews and John Shirt converted them into the 200 Majesty, using their initials to come up with the clever name. Back in the 1970s the DT didn’t seem to know if it was meant to be more at home on the dirt or on the road. I did a lot of tuning for riders for the road, it was THE wheelie machine of choice. Looking inside the engine you could see what I mean by not knowing if it was for trail riding or not!
The inlet port area and reed block were large but when it reached the inlet port window it was restricted with a cast ridge all the way around the port. To take advantage of the reed-valve design there was the rear transfer boost port but it was so restricted as to be fairly useless. It was as if the design was changed at the factory to soften the power delivery and make it more trailie-friendly. Looking at the port timing in the barrel you could see signs of why they did it. The transfer ports were very low and the exhaust port fairly high and wide. As a really basic explanation, the lower the transfers and the higher the exhaust port the peakier the engine will be. Also in a bid to soften the power delivery they had lowered the top of the exhaust port but only across the centre of the port, they left the width and sides of the port the same. This left the exhaust port very square and invited snagged piston rings. To finish the softening of the power, the head had a low compression and a wide squish clearance. Anything more than riding around town and the DT would struggle. To get more speed the trail gearing would be raised but then the engine did not have enough power to pull it. So it needed tuning! I have just had a DT175 head and barrel in for tuning. “I want you to tune it like you did for me back in the day when I was a teenager,” said the owner. Sadly, I think he also wanted to pay the same price! I looked up my old spec sheet but I could remember it anyway, so here goes for tuning the DT175. A lot of the tuning work on the DT is I think returning it to what the original designer had in mind. For a start the inlet port needs the restrictive ridge removing around the port window, it already has a large reed-block and inlet, with it unrestricted it looked right and as intended. The rear boost port needed making into a proper port and raising to 41mm, measured from the top of the barrel. The four main transfer ports are very badly cast and need a lot of work on them and to raise them 1mm to 41mm from the top. Even the bottoms of the transfer windows need flowing work doing. The transfer collection areas at the bottoms of the barrels also need tidying. You can see in the photo what I mean about the top of the exhaust port window being very square. When raising the top of the port I only raise the centre and shape it to the sides which apart from a little flowing I leave standard. I raise the top of the exhaust port by 3mm from 31mm up to 28mm measured from the top of the barrel. The roof of the port needs blending into the port outlet so that there is no necking in the port. And now on to the cylinder head. I used to have a special mounting for the DT head to mount it on the lathe but that has long gone or been used for something else. The problem with the DT is that the plughole is offset to one side and at an angle. I usually mount single cylinder heads with a mandrel screwed into the plughole but not on the DT. I came up with the simple cure of using six gudgeon pins on the six-head stud-holes and an 8mm bolt and domed spacer through the plug-hole to pull the head and gudgeon pin spacers onto the lathe face-plate. I machined 0.7mm from the head face to raise the compression and to bring the squish clearance down to 1mm. The DT 175 is actually 171cc, I usually try to come up with a big-bore idea for engines in these CMM articles. Wiseco and Pro X make 68mm pistons for the Yamaha 200 Blaster quad, these are the same dimensions as the DT175 and I am sure the 2mm over-size will do the job but check the liner thickness first. I am getting more and more of the less obvious engines in for tuning lately and although I know I try to only renovate and rebuild classic two-stroke engines like the 250/350/400 Yamaha twins and 500LC fours I suppose I need a little variation to let me photograph and write about them in CMM, so stay tuned!