Hello all, 

Took the 250 out for a run as the weather seemed close enough to standard day conditions, and I've been wanting to test the bike out at highway speeds for about a year now. I thought I would post the results for anyone looking for similar data in the future. 

This was the first time since full restoration that I was able to really get the bike up to speed and gauge how things were running. Initially I had fuel starvation issues at the petcock, but once those were settled I was able to get a few good runs in. 

Though I had to tuck in a bit, I was easily able to hit 75mph without touching the redline, which was surprising to me! According to the CHT sensor installed, I was seeing temps between 280-330F depending how hard I was pushing. 

A plug inspection & scope of the chamber showing everything still looking good after a good cooldown. 

Bike modifications for reference: 
VAPE conversion (using capacitor in place of battery)
OEM Pipe & silencer 
VForce reed system (V4R01H is what I used)
Wossner piston (STD size)
Port work done by Eric Gorr Racing
OEM air filter

That's good to hear accurate results rather than a guessing game on paper. I'd say a standard motor would come nowhere near that figure even down hill with a tail wind.
Has it got any useful torque or is it all top end. 250's aren't that torquee anyway i guess, well not like a 360.
 

Sounds like it's running great!

What sprockets are you running?

Unfortunately, the bike was a paperweight with missing parts when I first picked it up with no possibility of it running, so I didn't have the chance to do a before run.  

It pulls very good down low, and I have no issue getting the rear wheel to break loose on command or lift the front in the air with the right application of throttle in 1st or 2nd gear. Again, I would have really liked to see where it started even if it was only using the butt-dyno to have a better comparison. 

Without the butt-dyno, I used the FSM & a Feb '77 article from Cycle World to "best" estimate where the bike started when new. In that article, CW claims they were able to get 16HP & 19TQ, and it certainly "feels" stronger than that now. 

I may be lucky enough to get this bike on a local dyno this summer though, and finally get some empirical results. 

As for sprocket sizes, I stuck with what was listed as OEM: 47/14.

I've been hesitating to reply because I don't want to take anything away from the great job you did on the engine.

A stock 77 DT250 is geared to hit redline right at 75 mph.  Gearing is locked to RPM so no matter how much power you have, you'll still hit redline at 75 mph.  

Japanese speedos tend to be optimistic...  (I've heard a rumor that in Japan the manufacturer was/is subject to huge fines if the speedometer reads low so they err on the side of reading high.  And also after nearly 50 years the tach or speedo might have lost accuracy.  

There are lots of phone apps that will give you GPS speed to check against your speedo...  if the speedo is pretty accurate compared to the GPS speed then your tach might be reading low...  not a big deal unless it's a couple of thousand RPM low near redline.  

You can also get a ballpark on the tach accuracy by noting the GPS speed at various rpm's in high gear and then calculating what the RPM at that speed should be and compare it to the tach reading.  

P.S.  With stock gearing and tire size, you should get about 10.8 MPH per 1000 RPM in high gear...  so 3,000 RPM should be about 32-33 MPH.