1974 Yamaha DT500

[turbodan]

This is a very unusual machine, but we have some unusual people on here so maybe we can make a deal.

It is a 1974 Yamaha DT360 with a 1973 Yamaha SC500 engine.  It is a street legal, 496cc single cylinder, air cooled two stroke:





I built the engine from a set of bare cases as a sort of Plan B in case I was unable to get satisfactory performance out of the original 351cc engine.  I ended up getting very good power and torque out of the 360, but the 500 motor was already done, so I put it in just to see what it would do.  Since it was a clean sheet, bottom up build, I took every opportunity to address any deficiencies or shortcomings in the original 1973 design.  They were numerous.  The stock porting was awful, the transmission wasn't great.





I believe this was the last NOS SC500 crank in the world, what better purpose for it than a project like this. Bore is 95mm BTW, that piston is quite substantial.



If you're familiar with big bore two strokes, you have some idea what to expect from a machine like this. It produces torque and thrust from very low on the tach. It pulls from idle with ease. The first bump in the torque curve hits at about 2800 RPM, and power builds from there. By 4000 RPM it is on full boil and the front wheel is in the air. It revs to about 7000 RPM before you need to shift. It's making somewhere in the realm of 45 horsepower, and it compares favorably to a modern 250cc two stroke, just with more torque. It'll wheelie at 60 MPH with only a tug on the bars and a twist of the throttle. To say it's overpowered would be an understatement, but it's in a good way.







It's running a modern CDI system from Rex's speed shop with a 12v lighting output. The stock oil injection from the 360 was retained and works beautifully. Just put gas in the tank and top off the oil every 500 miles or so. It's currently geared 17/44 to enable cruising at up to 60 MPH, though it is best as an around town bike. Top speed is in excess of 90 MPH but I wouldn't recommend doing a whole lot of that with these drum brakes, which were insufficient even for the stock 22 horsepower this thing made originally.

It'll still do all the same stuff the original DT360 would do, it'll putt around on trails and offers well mannered, tractable power at low revs. If you grab a handful of throttle, you will get it, but it is only on command. The ill-tempered nature of the original SC500 has been completely stomped out and replaced by a controllable, metered power delivery.

It is located in Albuquerque, NM. Test rides are welcome. I'm not sure what to ask for the price. This is a unique and tremendously enjoyable bike, but I realize it is a in a weird sort of niche category.

I guess I'd go for $4500 or best offer. I don't really want to sell it but I have nine bikes right now and I need to start peeling some off.

Here's a bit of a virtual test ride.  Note that I live at 5700' elevation, so it's down about 20% on power with the air density up here.  You would get all that back at sea level.  I can only imagine how much fun that would be.

https://youtu.be/Uvo546xPaEE?si=GsO_8YDXiscvBwMM

[tseohs]

Super cool bike!

My parents live just north in Corrales and on a dirt road! Would love to have something like this to scoot around town on.

Maybe I'll look you up for a 'test ride' when I'm in town next.

[turbodan]

That would be a good plan. If this one doesn't fit the bill you can see how you like some of the other bikes. 

[stuartinmn]

Cool bike.  I once owned a Yamaha RT360, I think it was a 1973 model.  The compression release was inoperative on mine, I recall it was a bear to kick over.  The SC500 engine was a real powerhouse in its day.

[turbodan]

They are a real bitch without the decompressor. The kick lever reduction ratio was designed with that feature in mind.

The original SC made good peak power but the delivery overall was pretty awful. They made little bottom end, hit very hard in the midrange and revved to the moon on top. Dirt Bike magazine tested them in '94 along with a few other infamous infamously overpowered vintage bikes:



I am of the opinion that revving an engine like this beyond 8000 RPM is ridiculous and shouldn't be necessary to make gobs of power. Didn't help reliability either. This was addressed when I had the sleeve made, which offered a rare opportunity to make substantial changes to the porting.

 
It still makes peak power around 6500-7000, and a bit more than it did originally, but the improvements in scavenging pumped up the low end and midrange.

The original porting layout worked best within a narrow range and poorly outside of that. Modern porting is designed to aim the transfer streams toward the center of the bore and use opposing transfer flows to cancel out most of their velocity once they converge in the cylinder. This fills the bore efficiently by minimizing charge loss and intermingling of fresh air/fuel with exhaust gases. 

I picked up more power overall with the porting redesign than I got from the pipe. That's part of the fun with these vintage bikes. There's a lot more power in them if you can find it.