This is a 1974 Yamaha DT360A with a little over 16k showing on the odometer.  The mileage is correct for the chassis, however the current engine was installed at 15k when I pulled the 360 out of it for a general freshening up.  The engine that is currently installed is a 496cc unit originally found in the Yamaha SC500.  This engine started as a set of empty cases and has been gone through and meticulously inspected during the build.  The crank was a NOS Yamaha part, the cylinder was re-sleeved and ported, the wide ratio four speed transmission was inspected and worn components were replaced.

The SC500 was not well liked by the magazines of the era and this motor required significant revisions to run as well as it does now.  The exhaust port was lowered during the re-sleeving process and widened.  The transfer port heights have been matched, the entry angles have been adjusted to take advantage of the knowledge gained in the 50 years since these came out.  These changes increase efficiency by improving scavenging and reducing charge loss.  Between the porting revisions and the custom made pipe, this motor makes strong bottom end and midrange with a hard hit on top.  Peak power is between 6500 and 7000 RPM, which will help keep this thing alive for a long time in contrast to the original SC which would happily rev itself to pieces.  Overall the porting is still conservative in the interest of piston and ring life.  I think it makes more than enough power as it is, though there is more in there.

Other improvements include a modern CDI from Rex’s Speed Shop, a Keihin PWK38 carburetor, a KX250 reed block and a custom made airbox.  This bike is making somewhere in the realm of 50 horsepower and some discretion is required to keep the rubber side down in first gear, despite 17/44 gearing for the street.  Top speed is in excess of 90 mph before it’s tapped out in top gear, it would go much faster with taller gearing but it’s already way faster than is appropriate for the brakes and suspension. 

This is a very unique bike and it is not for everybody.  The clutch pull is stiff, required by the amount of power and torque it has to hold, and it does vibrate.  The original SC had a chain driven counterbalancer but these were not reliable and the whole system was omitted from this build.  In my opinion it’s not bad at all for an air cooled big bore but I’m certain that some people would find it objectionable.  On the other hand, the original auto decompressor is intact and works great, making this bike surprisingly easy to start.  The oil injection of the DT was retained and works perfectly with no leaks.  No special maintenance is required, just top off the oil tank every few hundred miles.  Fuel consumption will depend on your level of self control, if you ride like me you will usually get 80 miles out of a tank before hitting reserve, which occurs when you’re down to about .6 gallons remaining.

To sum it all up, this is a very unusual, overpowered vintage machine.  It is easy to get into trouble, both in terms of the law and the limitations of the chassis.  I’m hesitant to offer it for sale, you should make sure your insurance is up to date and your estate is in order before throwing a leg over this thing.  Despite making “only” about 50 hp, this motorcycle accelerates explosively and will try to harm you.  Buy at your own risk, you will enjoy it tremendously but please be careful.
What is it worth?  I have no idea.  When it goes on ebay I'll probably start it at 5k and see where it goes.  I wouldn't mind keeping it but the wife wanted a bike and now my garage is jammed up.  Worst case I can rig up some pulleys through the rafters and hang it from the ceiling.


 

I remember the detailed progress updates on this interesting build. Pity it has to be sold after all that work but i'd say it'll create plenty of interest on Ebay. Good luck with the sale.
Talking vibration & remembering how my bosses one vibed without the ballancer--which i soon connected back up--I often wondered if a standard DT360A crank might vibrate less as it's designed for no ballancer. I don't know & i'm just reminiscing on old times, good time, & damn scary times haha.

interesting combo. any videos?

I will get video shortly. Haven't gotten any real good footage since before the new pipe.

For now, there's this:



 

I would like one of those pipes for my 76 DT 400 with stock motor. I was going to get an old KDX 400 or Rm400 pipe and try to adapt but yours fits so well in the frame. Are they making more? Great bike BTW.

thanks
Brett

Let the good times roll

Blasphemy!  Wasn't that a Kawasaki slogan? 

Haha.
www.google.com/search?q=let+te+good+time...vid:NbgkkvIvswU,st:0

I used to ride a KH250, maybe the phrase was nestled in my subconscious, still the OP has a gorgeous wheelie poppin' trail bike there

@BUGGIN

The pipe was a one off build for this thing.  If you want to check in with akara1 on here he might be able to make one for you.  Busy dude but it couldn't hurt to ask.  I believe the 75 400 uses the same basic pipe as the 74 360 and I know he has a design on tap for that model.  Only thing I would want to verify is head pipe diameter.