Vintage Dirt Bike Q & A

Hello.. new guy here.. I just picked up a 75 mt250. I have wanted one since I was a kid growing up in the 70's and 80's. Well its here, in pretty nice original shape, I am happy with my trade (a 75 CB400f ss project..) bike starts and rides fine except when it warms up (5min..) it shuts off.. I know its electrical, short somewhere. It slowly dies, the I let it sit and it fires and runs fine. Any ideas? Until my manual comes in does anyone have a scan of the electrical diagram?

Also it has a Kei Hein? Carb on it.. I don't think its what they came with, or is it. Mikuni possibly. Thats all I can find wrong with it..

thanks
for your help
CHRIS

They did come with a kehin carb not a mikuni it was I think 28mm. It will have a one into two throttle cable on it if its the original carb and oil pump setup. I have a shop manual but I have to find it I can scan and send a diagram if it has one. I have a simaliar problem with a xl350 it runs fine down the street and then will just die middle of the road. Sits and starts then does it all over again. I was told it was probably the ignition coil (or the pulse generator the mt250 has points tho).Good luck figuring out the problem I will look for my book.

This has happened to me twice; once it was from loss of compression (blown ring), and the second time it was a stuck-but-leaking float valve in the carb. You could also check the fuel petcock filter, and it might also be a good idea to try a new spark plug if you haven't already.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Here's an MT250 wiring diagram:



Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Killer!... thanks. I am leaning towards coil. Will post my findings. C

Hey! I thought your name was GiftHOPDuane!! You have an evil twin trying to steal your identity

Haha.. yeah can't get them to change it.... its starting to grow on me.. got rid of the XL125 and the F7 picked up the Elsie, its really what I've always wanted..

Got it!!!! I had planned on removing all the lights etc.. so I stripped out the harness, wired the coil directly (I am waiting for a proper kill button and resistor..) bike has brilliant blue spark and starts on 1st kick, runs so well, loses power though, its got a fuel delivery problem, gave up for the night and went home. I am waiting on a 2nd dent free tank,and new seat cover, will address fuel issues then. I am very happy to have this bike

This is a familiar stator coil issue. The stator coil is down on the engine cases, at the end of the crankshaft. As the engine heats up, so does this sensitive bundle of wound up wires called the stator coil. As it heats up, the wires expand and cause a certain amount of stress, this can be enough to take it from just within functional to just outside of functional. When the bike is cold, take resistance readings between the combinations of wires at your stator coil plug in. Write down those color combinations and their readings. Hook it all back up and run her until she dies again. Repeat the same tests right away and compare to the cold numbers. My guess is that you will see a short developed during the heating cycle. For a less adventurous method, just heat the stator with a blow dryer until it's nice and warm to the touch to check for the resistance change. Ricky Stator can rewind yours if you can't find a good one.
Enjoy! Jon

Thanks Jon..

I removed the harness and it runs fine. took it out for a rip yesterday (despite the temperature, or lack there of..) ran great, I even chatted with my neighbor for 10 mins while it idled and it help a beautiful idle. I figure there was a short somewhere in the harness, kill switch, key switch.. somewhere! I might look at the harness, but I don't plan on using it so I might not.. In short it runs mint.. just bogs under throttle, I think its a fuel delivery issue, I have a dent free tank coming, will address it then. The best think about the bike is I was showing it off to a friend of mine and slowly kicking it over to prime it, it fired right up on the first kick (well not even a kick just a half lever push..) remarkable, I've never had a bike start that easy.

I know there are better bikes, or ones that are regarded as more collectable, valuable, lighter and with more power, but.... i've wanted one of these since the seventies and couldn't be more happy..

C

And being happy is what this hobby is all about!