I use to own a Yz250b,that I bought off Vince Strang,they were pretty rare.It was a big leap ahead of most other MX bikes at the time,and so was the price.I think I paid $1500 for it second hand,more than double the $650 I had paid for a new Dt250 in 1974.it looked trick with the strap on tank,and red and white paint.It had a lot of light weight parts,but was still heavy overall,and as I found out when the chrome stripped out of the barrel,and I smashed an engine case,the parts were dear,luckily DT3 bits fitted,and it still went as fast.I soon learnt how to replace big end bearings,as the bike shop charged me $300 labour,and it was locked solid when I got it back,so I had to split the cases and adjust the crank anyway.I agree they weren't that easy to ride fast,you really had to keep on the gas,to get it to work,we had a 100m stutter section,and the only way was flat out,and hang on,that's where I learn't to hang on to the bike,no matter what happened,and most times you will stay on.
Along with the YZ250b,I had a YZ125D,and a RM125X,3 of the real Motocross game changers back in the day,I wish I had kept them!