Vintage Dirt Bike Q & A

Can someone please tell me the differences between a 1976 MR 250 motor and a CR motor? Is the 76 MR motor like any of the 73-76 CR motors? Will a CR top end work on the MR low end? Thanks.

the port timing was a little milder and the head was made to lower the compression.a 75 or 76 cr 250 top end will bolt right on. the transmission on the mr 250 is a wide ratio for enduro races and the cr 250 has a close ratio set for motocross.the 73-74 cr 250 the cylinder and center cases are different and wont interchange but the internal transmission parts and clutch parts,main bearings and seals all interchange to the 75-76 cr and mr motors.
one other difference is the ignition systems on the cr and mr motors are different.the mr has a flywheel igniton similiar to the mt 250 and the cr has a tiny little rotor that allows way less flywheel affect and allows it to rev quicker.

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

Thanks for the info. Will a 74 CR topened work on the 76 MR motor? Also, will the 74 ignition fit too?

the 74 ignition could be bolted on but the top end wont work.the 75-76 cr and mr motors the cylinder bolts directly to the case with short studs and the 73-74 cr motor it has long studs that pass all the way through the top of the cylinder and bolts down the head.there really isnt anything wrong with the 75 and 76 cr motors and despite what a few may say its not hurting on power.in fact the power on my 75 model motor delivers power smoother while the 73-74 engine hits hard in the midrange then signs off early.the 75 and 76 engines seem to have more over rev.from what i have read alot of people modified the mr motor with the cr top ends back in the day and loved their bikes.

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

Thanks Dave... on last question... is the MR a 4 or a 5 speed trans?

5 speed.

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

Dave was right on about the MR250 and CR250 stuff. I have both an MR250 and 75-76 CR250s myself. Often guys want to make the MR250 into a CR. Even though the MR250 is based mostly on the 75 CR250 the motor has too many changes to ever run like a CR. Best to put a 75 or 76 CR motor in your MR to get it to go faster. I like the MR250 for what it is. A mellow, kinda heavy, trail bike that is fun to ride. I teach friends how to ride on the MR since it is fairly quite and has a forgiving powerband that is less likely to get away from them.