Nov 27, 2012
Lower and Quicker
The Yamaha DT250 is an iconic bike from the 70s. The first bike to move the Japanese manufacturers from the not-very dirt-useful ‘scramblers’ they’d been making to something you could actually take to work, to the woods, to the motocross track, and back to work. Powerful and light with high ground clearance – it was a winner and sold in the zillions.
The Yamaha DT250 (or DT3, such as mine) would also make a fine dirt tracker, but could be better with a few modifications. If the engine were lower in the frame, the handling would benefit from a lower center of gravity and the lost ground clearance would not be missed on the flat track, and additionally if the steering angle were steeper the turning (and direction-correction) would be quicker.
The picture below is of my engine’s cylinder and head. The head is not from my bike, but is an eBay purchase from the same year’s MX model which provides higher compression. The mechanic up the road is enjoying working on a piece of nostalgia – he used to race these hopped-up Yamahas back in the 70s and has retained the tuners tricks from those days. He has ‘ported’ the cylinder, which along with the high compression head will give a big boost in horsepower over the stock set up.
On the day I was at the shop cleaning the bottom part of the engine preparing for assemblyI was delighted to see what great shape the innards were. The clutch basket and plates looked to be near new and one of the issues the I assumed was going to be a big problem to solve required only a small 15 cent part. When I brought the bike home, it ran (for a while), but was hard to shift the gears. Later I discovered the the shifting rod could be pushed into the engine case quite easily and if it went too far – there would be no more shifting. I put a plastic sleeve on the rod to keep it from falling in the engine, but knew this was not going to be a permanent solution. I assumed I’d have to take the whole transmission apart and who knew what troubles one would find in there? But… turned out a simple circlip had moved from its position and once it was properly in place the shifter worked just like new.
Having a rare Saturday *and* Sunday weekend at the shop, the second day saw Saturday’s optimism slightly dashed – While the primary cover’s removal displayed a rather pristine clutch a deeper inspection brought forth red-flag damage to the crankshaft’s main bearings – the engine cases would need to be split open and the bearings replaced.
Here’s a picture of the engine split in two – the crankshaft is good enough to keep as it is and the main bearings are on-hand, but one of the ball bearings in the transmission will need to be replaced and that will require being ordered.
Here’s a list from last spring (when this page was originally written). All of the following have been done during the summer/fall 2012 –
Reassemble the engine.
Fasten and weld more bits to the frame.
Fabricate a bushing for the swing arm.
Paint the frame and swing arm.
Pull dent from and paint tank.
Build and modify the quick-change mechanism for the rear wheel (enable gearing changes at the track).
Modify as needed and install the triple clamps, install the front forks.
Mount tires to the wheels.
Fabricate axle spacers for front wheel and install in forks.
Install rear wheel w/ quick change.
Fabricate mounts for and install rear brake.
Mount handlebars, levers, throttle and grips.
Install the engine.
*Get the engine running.
*Mount tank and seat.
Finish number plates and mount.
Fabricate the hot-shoe from the sawblade.
* not quite done as of 11/27/2012 – but should be days away… Stay tuned.