Vintage Dirt Bike Q & A

as per the subject line - what can I expect? I would think the CR motor is stronger than the stock MR motor.

Should have a little quicker acceleration. You won't have the low-end grunt of the MR, unless you maybe use the heavier flywheel, requiring the use of the MR stator and points. You probably will have a little more high end oomph, but actual wide open speed might be a little lower since the gears are spaced closer. You won't be the first one to do this. Give it a shot.

I think I may try to use the MR250 stator and flywheel if they work - (bolt on?) to gain back some low end and have the ability to run the headlight (not like I iwll ever use it)

the mr flywheel should bolt on just fine.the cr 250 of that year is capable of 8000-8500 rpm so you might look at the flywheel rpm rating,its stamped on the outside face of the flywheel somewhere.you will have a lot more midrange and top end power with the cr motor.the transmission is close ratio and first gear is a much taller ratio.

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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!!

My winter/someday project is to re-build an MR engine and put it in a CR frame! I have all the parts, it's the time that isn't there.

Another project: To do the top end of my original, running MR250, it's never been apart!

As a past owner of a CR250m and a current owner of a MR250 here are my thoughts. With my CR I used to do alot of ice riding and now I have owned my MR for 2 yrs and this is the first time the river froze to be able to try it. In this respect I'm a little disappointed with the MR, the power is naturally down from the CR,about 10hp and the gears are too wide.The MR revs nice in 3rd then you go to 4th and you drop out of the powerband.You could fix that by changing sprockets but you would lose top speed also.CR was not affected as much to the amount of snow on the ice either.In the best conditions both bikes had the same top speed.In the end tho the MR is a better woods bike than my CR was and that is where I ride the most now days.But if I had a 75-76 CR motor I would give it a try,I think with mixing and matching parts,you could have a nice bike.

No way is the CR 10 hp more than a properly running MR. Internally, the only difference in the powerplant is a 1mm lower exhaust port on the MR. The taller gears (top=0.718 MR; top=0.862 CR) and flywheel weight won't affect horsepower, but will affect the time it takes to get there.
Besides the porting, the main difference is in the carburetion. Stock, both have 34mm Keihin's but the CR uses a larger main and no powerjet (140-158 main recommended). The MR, on the other hand, uses a 105-115 main, plus the powerjet giving a totally flow of 130-150.
The CR was listed from the factory as being 33 hp-- magazine tests I've seen say anywhere from 29-34. The MR's horsepower is unlisted. But the MT250 which has a completely different carb, cylinder, head and exhaust than either the CR or MR puts out about 21-23 HP or about 10 less than the CR. The MR is somewhere in between and--when jetted and running properly, a lot closer to the CR than to the MT.
I just put a fresh top end on my MR and took it out to Stillwater yesterday. I had to switch from a 14 front gear to a 13 to keep my speeds down. I forgot just how much power these things have. Too bad the suspension sucks and they are a little porky.

I'm sitting here with the December 1975 Motorcyclist magazine, and it states "on the dyno low 20's".It also says "they made a damn fine playbike,but if they expect to go out and play with the good ol boys, Husky and Penton, Honda will have to make some changes.It is simply impossible to find fault with the rest of the bike.Handling,suspension,riding comfort are as nearly perfect as anyone is likely to get.Only the power,or the lack of it,bothered us.Dyno tests on a CR250M1 show as much as a 6hp gain by removing the bolt on muffler and rejetting.The MR is a different bag where removing the muffler-spark/arrester would not appreciably help.Our test MR250 ran clean and smooth throught the throttle range so we didn't suspect improper jetting as the cause of the bike being down on hp". Bottom line is it's not a CR,but I think you could make a nice bike by combining the two,Honda tho decided it must have not been worth it and cancelled it instead. By the way, here are some pictures of mine so you can see I'm not riding some claped out piece of sh*t
http://i94.photobucket.com/albums/l119/ ... p_0699.jpg

http://i94.photobucket.com/albums/l119/ ... p_0698.jpg

http://i94.photobucket.com/albums/l119/ ... p_0702.jpg

Good lookin' bike!! Maybe my High Zoot 0.040 over piston gives me an extra 5 HP !! I would guess mine is putting out mid 20s, but obviously it is only a guess and a biased one at that. It was scary strong Wednesday, but maybe part of that was an unfamiliar track coupled with the MR's suspension. I can actually tell when it is time for a fresh top end by two things: 1) a noticeable (but not dramatic) drop in power and 2) three or more kicks to start it instead of one or two. Another thing I finally did was just to give up on jetting it for 32:1 or 40:1 and went kicking and screaming back to what Honda recommended 31 years ago. It doesn't smoke more than the less-oil mixes and actually runs crisper. But it does cost twice as much in oil and my friends all roll their eyes at me for not accepting their "science of modern oils" lectures. How is yours jetted? I went down to 42 on pilot and really need a 40 in warm weather. I experimented with the main and went back to stock 110 with clip on notch from top. Never tried changing the power jet. Any chance you'd sell a copy of that article?? I've bid on that magazine a couple of times on F-Bay and always lose.

i have a 1975 issue of Dirt Bike.the 75 cr250M1 put out 26.9 at the rear wheel.it makes less peak power than a 73-74 cr250 but it has a wider spread of power and they gave it alot more over rev.it compares power wise pretty well to other bikes of the period.i have ridden a 73 and i currently have a 75 cr 250 and it doesnt have the strong midrange explosion of the 73 model but it comes on at a lower rpm and revs out much further.its possible the 1 mm lower exhaust port cost 2 or 3 hp.that would put the mr at 23 or 24 hp.the mt might make 18 or 19 or so at the rear wheel.if i had a mr i would put on a cylinder from a 75-76 cr 250 or do some porting and copy the cr exhaust port timing if possible.

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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 Mitchie- I can make a photo copy if you want,do you want the April 1976 artical in Cycle? Did Dirtbike magazine have any articals? I have never had the carb or engine open,it still even has the yellow paint on the masterlink.When I get a chance I'll check the jets. Yours might run crisper because if I remember right say 20:1, more oil means less gas which would be leaner.32:1 which is what I'm running, is less oil more gas=richer.Despite my comments, I'm happy with it,I like the lighting and large tank,and where I normally ride it's plenty fast.

If you are going to woods ride I would not really suggest putting the CR engine in the MR. The MR has a wide ratio gear box for trail type riding, with a lower first gear for slow stuff and steep hills.

The 73/74 engine made about 25 HP at the rear wheel in stock form. The 75 might have shown more power on the dyno, but the 73/74 was faster on the track, and Honda offered a race cylinder to pep up the 75/76, they were pretty sick actually!

I am making in the mid 40's to low 50's HP with the 73/74 cylinder at the rear wheel, depending on which version of race engine. I just did a 75/76 engine and it left tonight, testing this weekend hopefully. The new 75/76 engine clearly hit harder down low than my 73/74 engines, but it seem to be a little shy on top end, could be a jet change, it seemed slightly rich. Could need some tweaking on the port timing, will know more after the weekend, and the true test at the nationals next weekend. I did like the snap it had down low, it was brutal spoke breaking torque!

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Sam Alexander

1974 Honda CR250M
1976 Honda CR250
1974 Maico MC440
1974 Maico MC250
1977 Maico AW400

sam, the 73 made 28.8 at the rear wheel according to dirt bike way back then.its faster on the track if the rider keeps it in the meat of the powerband which is from 4000-7000.honda mainly wanted to make the 75-76 model easier to ride for the most part.the 73 jumps from 12 hp at 4500 to 27 at 6500 then goes on to make the 28 at 7500 then signed off.

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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!!

btw what was the gp cylinder like on the 73 model.i have been thinking about getting the gp cylinder for the 75 model.sounds like it has lots of beans.

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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, the test report I saw on the 73/74 was just shy of 25 HP at the rear wheel for the Elsi, and the same report showed the YZ at a little over 28 HP at the rear wheel. Could have been the dyno or it could have been a slower engine, some ran better than others.

I have an NOS 75/76 GP race cylinder, all that has been done is the port windows cleaned up and they changed the exhaust port to the same shape as the 73/74 exhaust port. I have not checked port timing compared to stock, I run drastically different port timing on all of those engines, different type combustion chambers and custom pipes, along with our intake manifold and a worked over VM Mikuni. That GP cylinder for either the 73/74 or the 75/76 will probably only get you a couple of horsepower at best, and you may loose low end in the process. My engines hit brutally hard down low and rev out too, in fact myself and my customers have been accused of cheating with big bores and such because they have a lot of grunt like an open bike, I can assure you all of the engines that leave here are legal and all of the horsepower came from several years of hard work and testing. I continue to test all the time and I have a new version for the 73/74 that will be tested soon, and I will continue to try and improve on the 75/76 engine. The 75/76 cylinder should have more power available do to the transfer design, it's just a matter of figuring out what port timing it likes.

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Sam Alexander

1974 Honda CR250M
1976 Honda CR250
1974 Maico MC440
1974 Maico MC250
1977 Maico AW400