I own a YAMAHA DT250B (1975).
I want you to think about the problem from the symptoms.

・ When I started the engine for the first time in a month, oil spewed out from the oil breather hose and the ground was stained with oil.
・The engine started, but there was a lot of white smoke.
・Idle was low immediately after starting the engine, and the plug was wet with oil.
・ Oil consumption is very fast.

From the above, I thought that there was a problem with the oil pump.
Would changing the seals or gaskets fix it? And if there is another problem, where is it?

Which oil breather hose did the oil spew out from? The vent hose on the oil tank or the vent hose on the transmission?

Excessive Oil Consumption and smoke…..Oil pump check valve leaking allowing TWO STROKE oil to gravity drain from tank, flow through pump and fill up the crankshaft area. 

Oil coming from breather tube…. If it’s the transmission tube, it could be a leaking right side crank seal. 
If it is the oil tank breather, then it could be from over filling.

 

My .02¢ is a blown rh crankshaft seal allowing compression into transmission thus pressurizing cavity and forcing oil out breather tube, excessive smoke is also a symptom of blown rh crank seal.

Thanks for everyone's reply.
The breather hose is a black tube attached to the crankcase.
The breather hose is not attached to the oil tank.
Does that mean there is a problem with the crank seals?

2st oil blew out from this hose

Did you ever find your solution to this 2 stroke oil out the tranny breather hose?
I'm about to do the oil pump seal per recommendation but wanted to ask you
Thanks

SuperDad...  I don't recall you mentioning massive amounts of smoke and/or oil spitting out the exhaust on start up in your other thread? 

An easy test for the oil pump check valve is to simply disconnect the oil line to the carb while it's parked for several days... directing the hose into a container is best.  The check valve might drip a little but if you're losing a full oil tank in two weeks it would leak a couple ounces of oil in just 24 hours. 

If it is the check valve, then you need to have a way for the autolube oil to get from crankcase to transmission which would also cause excessive smoke, likely oil fouled plug, and not-so-great running... 

I purchased 2 different Yamaha Enduros that the right side crankseal was pushed out of the case, the bottom end had first filled with 2T oil, when first kicked over it pushed the crank seal out causing the symptom you described

Nailed it
my oil pump seal was damaged and the tank oil slowly drained into the crankcase as mentioned

thank you again forum