I just acquired a 76 DT250B. Form what I can see it is pretty original, but a few things have been changed. 
The rear sprocket is new, but there isn't a chain. Looking at the service book I got with it the chain is a 520? The tank isn't original, it looks to be from an earlier model. Should be the orange one with the stripes from what I can tell.
The old girl has been sitting for a very long time and mud dabbers have made nests all over. The exhaust pipe is plugged with mud. The odometer shows a bit over 4000 miles.  Once I clean it up what would be the best way to put it back in service?  I just acquired a 76 DT250B. Form what I can see it is pretty original, but a few things have been changed. 
The rear sprocket is new, but there isn't a chain. Looking at the service book I got with it the chain is a 520? The tank isn't original, it looks to be from an earlier model. Should be the orange one with the stripes from what I can tell.
The old girl has been sitting for a very long time and mud dabbers have made nests all over. The exhaust pipe is plugged with mud. The odometer shows a bit over 4000 miles.  Once I clean it up what would be the best way to put it back in service? Here are a some pictures. 

 from the UK.

Looks a nice tidy bike.

Clean carb, check air filter, change oil, clean faces of points and, before you ride anywhere, replace both sets of brake shoes (still available from Yamaha).

I already have a carb kit and fork oil has been changed. I feel that flushing out the oil tank would be a good idea along with cleaning out the mud from the muffler.  I will report back on the progress that is being made.

 

Greetings from Tennessee. 
   Great looking bike is the carb kit aftermarket?

Welcome from NZ. You have a very good model there. I had one once as a dirt play bike. Never broke it.

The carb kit came with the bike, not sure who makes it. The guy who gave me this project bought a bunch of parts from Rocky Mountain ATV before deciding to move it on to me. I will check and see who made the kit.
Cheers
John
 

The carb kit came with the bike, not sure who makes it. The guy who gave me this project bought a bunch of parts from Rocky Mountain ATV before deciding to move it on to me. I will check and see who made the kit.
Cheers
John
Well I looked at the carb kit package and there is a sticker over the label. Did take a picture of it. 
Rebuilding the carb is my goal for today. 

beautiful bike, congratulations !
I would use the carb kit for gaskets, o-rings, other non-metering parts but not jets.
If your bike has correct Mikuni jets then I would clean them and reuse.  To tell if they are Mikuni you got to look for a square with a square within it, its tiny and I have to use a binoculars turned around backwards!
Also might want to start her up with premix after doing all the stuff already recommended + flushing, refilling bleeding and adjusting the lube pump so you can check its output without risk of running without oil and having a meltdown

Yes I think that is a good kit came from Niche cycle you can double check the jet, needle for there logo. Mikuni 

I learn something new here from you guys almost every day!  I didn't know Niche (or anyone!) had carb kits with genuine Mikuni jets!