Vintage Dirt Bike Q & A

Hi, I have a 1975 CR250M1. The book calls for the timing to be 2.3mm before TDC. I have retarded this some to about 1.8mm to try and reduce the kick starter kicking back so bad. I noticed the 1973-1974 book calls for timing of 1.5mm, which is way more retarded than the 1975. It is essentially the same engine, with identical ignitions, so I was wondering why the big difference and was wondering if I could reduce the advance even more down to 1.5mm. Anyone have experience with this or suggestions? The thing is very hard to kick over and when it kicks back it is brutal on the back of my leg!! Thanks a lot.

Don't know about setting '75 at 1.5 btdc, but the 75-76 M1s have 4mm longer rod affecting piston speed through the stroke, which may explain timing recommendations. Also, porting is somewhat different, but don't think that would make much difference in timing.

Honda Racing recommends 1.8 to 2.0 mm BTDC for '73 to '75 Elsinores, so at 1.8 mm you should already be there, but
I have read in other manuals that Honda recommends 1.5 mm BTDC for all '73 to '75 CR250Ms. So I guess you can try
further retarding and see if it helps.

If the "kick back" persist, you might want to invest in a compression release.

As far as identical ignitions, doesn't your M1 have a CDI?

dogger

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1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

You are not doing anything strange like running it without a base gasket, head gasket; or either one of thin material are you? Check some pictures of other heads and see if your's has been shaved for high compression or anything odd. I'd be interested to see a compression check reading to see if you are pumping 200 psi or something crazy. Your timing sure sounds fine, so I am thinking of what else could be roughing up your foot.
When you kick it, do you ease the piston up toward the top of the compression stroke then kick it though with commitment?
I can't imagine you have an 1/8" of carbon built up in there from neglect. Naw, that can't be it.
Does it actually start and run okay after the kicking dance is done?

This is in response to several of the questions above: The 1975 CR250M1 came with points ignition just like the 73-74, the CDI was optional. This bike is completely stock and unrestored. It has the original piston, stock gaskets, no carbon buildup, etc. I think I will do a compression check just for grins. Once the bike is started it runs great - good idle, good top end, no pinging, no overheating, etc. I have tried kicking it from just past top dead center and in just about every other position. I also tried kicking it with the spark plug in but disconnected and it doesn't kick back which I think sort of confirms that the kick back is ignition related. I think I will also try cutting the advance down to 1.5mm and see what happens. I have tried to find out why Honda advanced the timing so much on the 75-76 but can't find anything in the old magazines, etc. Thanks for all the suggestions, any more would definitely be helpful. It is a fun bike but the bruising is getting aggravating...

i too have a 75 model,the kick back is part of life with the beast.even with the timing set at the recomended settings.there are worst things to start.i have a 70 model Yamaha RT1M with a webco head.it has downright brutal kick backs.might wear a good boot when starting and build your leg muscles.these bikes were made when men where men.i had a 73 model CR250M and i still have a MT250.those two bikes kick back sometimes too.the timing is static on these bikes and it has no retard for spark,it will kick back no matter what you do,the RT1M has a spark advance on the flywheel,it still kicks back if you give it a half hearted kick.my 79 CR250R kicks back too,harder than the 75 model does,and it has a cdi ignition on it.my 75 runs the points setup.

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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!!