Vintage Dirt Bike Q & A

Guys,
I have a 79 CR250. In the process of restoring for vintage racing. Can't get it to run right and it's driving me nuts!

Here's what happens: It runs good all the way up to about 1/2 throttle. Anything more than that and it falls on its face, blubbers bad and smokes like crazy! I have replaced the crank seal on the clutch side, didn't help. I found that the intake boot had a crack in it and replaced it with a brand new one, didn't help. I also installed new rings and a new base gasket, didn't help. So, I thought maybe I screwed up the seal when I put it in, so I drained the oil out of the crankcase and took it for a quick spin and it still does the same thing!! I figured if it was sucking oil from the case it would run fine with no oil in there for a couple seconds, NOPE!!??

Can somebody give me some help? Please? I love this bike and I don't want to give up on it, but I'm at wits end here and don't know what else to do. Any help or advice will be greatly appreciated! THANKS in advance.

Benji

Clean the carb well and blow it out with compressed air, check jetting and try it. If still having the problem perform a leak down test to check for leaks in and around the motor

After all the work this may sound trivial to you, make sure the needle in the carburetor slide is locked in position. Sometimes the clip comes off or the retainer hangs.

If you unscrew the slide retainer you should be able to confirm the needle isn't moving in the slide body.

Rick

Too many variables and not enough data. If the bike runs well at part throttle I would say the possibility of a bad primary drive crank seal is low. Is the smoke white or blue? However you should remove both primary drive and rotor, and then check for any up/down play in the crank bearings. I have read that a bad bearing will allow the crank to move and deform a new seal. The bottom line is a leak down test.

I would recommend a re-build kit for the carb, particularly the needle and needle jet. Make sure there is a clip holding the plate in the slide that keeps the needle in place. What jetting is in use?

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

Wow, I finally figured it out guys!

Here is what happened: whoever had this bike before me must have scrapped the original air filter and cage. They took one from a modern day CR125 or 250 and stretched it around the mount for the original then just clamped it in place with the original clamp. Well those air filters have a rubber grommet in the center of them that the mounting bolt goes thru to attatch it in the airbox. So you all know where I'm going with this, about half throttle or a little more it was sucking hard enuf to suck the grommet into the tube in the airbox and therefore, cutting off more than half the airflow to the carb. Presto, it ran like crap! Took that air filter out and it runs like a dream, and man does this thing pull hard thru the gears!

So, I already have the right one ordered for it and as soon as I get it I'm gonna go tear up some dirt the old fashion way!!

Thanks for all the responses, and by the way, I love this site. Only 33 years young but I love the old iron! You guys know what I mean!

Benji