Vintage Dirt Bike Q & A

My 1975 cr250m1 runs very strong but when you go full revs with throttle full open it does not runsmooth it burbles, if i back the throttle off or change quickly through the gears in short burst it is fine. If i put it in third and let it pull up a hill it pulls well from low rpm not breaking down under load but once the revs go high it bubbles again with no extra power. the bike idles well. It will even notice the bubble if you just sit there with the throttle on( not full out). If i do quick burts of the throttle is goes smooth.
i had a look at my points (i noticed a bit of oil leak in there ) ??
I also checked the carby and have shifted the needle from top to bottom, although i did notice a very very slight improvment when i leand it down.
i just recently bought this bike and was told the it had a recent engine overhaul, it has good compression and starts very easy. has any one got any ideas , has it happened to someone else. hope to hear your thoughts, regards steve

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I love doing up old vmx bikes then riding them, bring back great memorys

This is exactly the same problem I am having with my '74 MT250. I have not been able to figure it out yet. Let me know if you do figure it out?
Thanks
Newfie

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Newfie
1974 MT250
1974 MR50
1975 CB750F
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You're rich guys. On the high end of revs, the main jet size determines rich or lean. You are rich in my opinion. When you are at high revs, the slide is almost all the ways up and the needle doesn't really do that much. Open up and see what you have for a main jet size. It is probaby a 145. Higher #s are bigger hole and therefore make it richer. Lower #s make it leaner. I'd go two sizes down and see how it feels. Slide needle down as much as it will go. Needle up makes it richer in the mid range. Lower makes it leaner in mid. Then ride and see what you think. Adjust accordingly.

Robert

P.S. If you really want to know more about carb tuning, check this out.

http://ericgorr.com/techarticles/carbtuning.html

Ok, just to advise step by step, i checked the original factory specs, my main jet is 155 standard , I was only able to get from my honda dealer in stock was a 150 and 155. i bought both, i have put in the 150 and noticed it was a bit more worst at high revs, throttle responce is excellent but still burbles at high revs. My next step is to go richer ,as i have 2 155 jets, i am going to drill one out say to 160. My manual says that i can go from the optiond of 150 to 160. I will advise on this post how that progresses. If that does not fix i will have to look at my electrics. I have a 6 volt ac coil that is on my honda TL250 which will be first option to try. This step by step action may be helpful to others with this problem. but still ,i would like to hear from others how may make my problem a short cut.
regards steve

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I love doing up old vmx bikes then riding them, bring back great memorys