hi ineed your help i need a fast pre75 engine to race decided to go for a 74 yz250 but they are so rare but the mx250 engines are ready avalible now hope all you experts will help me out is there much between the two engines in hp numbers what i have found is yz 34hp mx 31hp cr19hp
if its that close mx it is as all the ignition and carb will be specials

YZ was special & lightweight in motor & frame but is the first generation DT1 shape so will only fit that frame directly or any Dt Rt up to "73. MX is a heavier first of the later b\generation shape. Good solid motor & take more abuse. Talking comparing new with new not 45 years old of course. & the MX won't fit in the early frame as are a longer motor. YZ in original form has a chrome lined cylinder, that's YZA & B. B being the first monoshock worksy type bike. Then in "75 the mono arrived with the MX250B shape motor same physical shape that your talking with MX. All a bit confusing around that time with lots of changes. What your plan?, sounds like a story brewing.