Vintage Dirt Bike Q & A

After rebuilding my 78 CR125,I'm having trouble getting it to run correctly. It has the stock Kehin carb. I set the float at .79 from bottom of carb to bottom of float with it just slightly depressing the plunger. It usually starts okay, and if you keep the rpm's up runs okay, but as soon as you slow down and idle, it loads up badly. Then it is very difficult to get moving as it just bogs down, and sometimes dies. I took the seat and air cleaner off to see if there was a restriction, and there was none. As I looked down at the carb inlet when it was bogging, you could see it spitting fuel backwards through the carb. I could use some suggestions. Thanks, Mark

is your reedblock ok?

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The 76-78 CR125's were piston port and didn't have reed valves.

I'd start with a full carb strip and clean making sure all jets and drillings are clear.

I found the main jet to be a #125. Stock apparently is a #140. I am going to change that, and change one thing at a time. The bike also didn't seem to have full power, which I suspect is the leaned out main. The slow jet is a #45, and the air jet is a #200. Mark

I have the same problem with my 76. Jetting is 140M and 40P. I'm going with a Mikuni 30mm. The carb is just old and worn out.

Make sure your exhaust isn't clogged. Also is it burning tranny oil? It will smell like a car burning oil instead of a 2-stroke. It will also emit large amounts of goo out of the exhaust. If it is you probably lost a tranny side crank seal.

Newbie here, just brought back 75 MR175...restored lots of early 60's Greeves 2 strokes...this MR sweet wicked..froze up barn find....shock shafts bent badly along with the forks. Poor ole gal... now my favorite trail rider and hare scrambler:)

Point... Fought the kehien hard, rebuilt she still ran like she was posessed..bogged and rich, when I got her no air box or even a pod filter. Ran badly on the 130 main.. No difference all the way down to 118.
Finally switched to mikuni 26 mm with 180 main and needle 2nd from top...did the trick and mikuni bolted right on to original boots:) kept sweet low end great solid steady power throughout and nice powerband too. She is first kick starter every time idles herself up a few seconds then right down:)
It was like magic, no cutting out on the steep uphills or bogging down at all.

From what I understand same carb just jetted differently.

Good luck