Vintage Dirt Bike Q & A

Recently purchased a 73cr250m on ebay. the bike started on the first kick but would never get up to rpms(seems like it was loading up and never clearing. I put in a new plug , cleaned the carb and put in fresh gas. Non of this made a difference! I then decided to look a the timing and upon pulling off the timing left cover I had a small amount of gas and oil come out and everything seems wet( seemed to be mostly gas- maybe 1/2 ounce or more. compression on the bike is 140. Sure could use some input----on where this gas is coming from? and what i should do?.

P.s. I was able to ride the bike through my neighborhood for some time but it never would reach high rpm's

Thanks, Jon


73 z50
77 z50
86 z50 chrome special
05 crf 450
73 cr250m

Sounds like time for new crank seals. Since seeing what bikes you own, I'm thinking that this is your 1st 2 stroke. No matter what your compression is, you still have to have the crankcase sealed on a 2 stroke. If you were smoking real bad it would be a sign that the seal on the gear box side was bad. Either way the motor has to come apart and you would need to replace both seals, and of course check every thing else out also.

thanks for information and comments. It is true I no longer have any two strokes but I fell in love with the elsinores after riding my friends dad's in seventh grade. His dad was a dentist and was never around-what a machine. Back to the problem... Do you think seals are the sole reason for the bike not running well or could it be electrical or carb. related. Also, do you or any one else know of some vintage company's that would do a nice job fixing this type of problem. Thanks in advance. Jon

In addition, if any one else might shed some light on this bike I sure would appreciate it --Thanks again.

The seals are prime suspect at this point. The pre-mix you found behind the ignition is being pushed out past the crankcase seal when the crankcase is pressurized. If you have taken your 4-stroke motors apart this will be easier, but you should have a factory manual to-hand. In some cases - and I do not know this particular engine - you can get the seals out without splitting the crankcases. You do need to remove the ignition and primary drive gear to get at them though. However, if you can wiggle either the crankshaft, and/or the connecting rod up-down I would have the motor split and the bearings checked.

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

i had a 74 CR250M that did the same thing-the left crank seal was blown so bad i cant believe it would still run.Remove the stator and point plate.You will need a special puller to remove the rotor-do not use a jaw puller.The crank seal has a large o.d. which makes it easy to remove with a hooked seal remover tool-dont damage the case or crank!! its easy to drive a new seal in with the appropriate driver.take note of the depth the origianl seal is so when you install new one the same depth.its really an eay fix

FWIW: A bike with 140PSI of compression should still run hard with a leaky seal. The important thing is you wouldn't want to run it. Leaky seals may produce a lean condition causing the engine to overheat and melt down.

There's other odd behavior that happens with a leaky bottom end: Throttle response is funky. Revs may build slowly, power non existant low in the RPM range, roll off throttle and the engine takes 20-30 seconds to return to idle.

You really want to fix the bottom end before you finalize everything else. There's nothing wrong with rebuilding the carb, resetting/replacing the points, setting the timing, just know that when you do fix the leaks you will have another round of fine tuning.

I wanted to thank all of you for the great information. I still can't figure out how I got gas into the case. It had no left gasket and maybe the leaking carb gas just ran down and found a path inot the case. Is that possible ? If I do the seals where could I get the tool to pull the rotor off, get the seal and do I need to do the right side as well? thank again and look forward to kicking up some dirt. Jon

It could have leaked into the side cover if the overflow hoses from the carb weren't routed right or were missing.

I think http://www.denniskirk.com has the pullers although they are ubiquitous. A cycle shop close to me, Zoom, had a spinning display with tools ranging from pullers to tire irons on it. They are only ~$10-$16 and well worth it.

I do a lot of shopping on eBay but I don't reccomend it for someone in a hurry and summer running out. Takes a week to 10 days for an auction to end and another couple of weeks for the item to get to you.

I make a lot of my own gaskets. Ace Hardware has huge rolls of gasket material they sell for a couple of bucks.

If gas is leaking anywhere, make sure you fix it. I've had 2 fires in the last 3 years and could go the rest of my life w/o having another.

Another personal op thing, don't tear anything down until you have the replacement parts in hand. Sometimes it may take 6 months to get the part you need and a poorly running bike is better then a basket case.

The gas got into the left case through the leaking seal. On a 2 stroke, the gas flows through the intake and into the bottom of the crankcase before it passes through the transfer ports and into the area above the piston to be burned. It is likely that at some point the petcock was left in the on position and the carb filled the crankcase with fuel which then seeped through the seal into the left cover. Since that is a points based ignition, I would make sure that all of the fuel is out of the ignition area before you try to start it again.

You should be able to get both the seal and the flywheel puller from your Honda dealer. Pick up a Clymer or factory Honda shop manual from them as well. As far as the right seal, it wouldn't hurt to replace it as well. If the bike is making a lot of white or blue smoke, that would indicate that the right side seal is leaking transmission oil into the combustion chamber.

Great information!!. Last question, can I definitly get the left seal from the outside? How about the right case-outside behind the clutch? thanks, Jon

You can replace the seal behind the magneto easy enough but you have to split the cases for the other seal. It only comes out from the crankshaft side. I would do the other seals while you have it apart and maybe the main bearing if they appear worn in the least.

Your really taxing my memory,as I sold my 74 cr about 28 yrs ago but I have rebuilt a 74 motor for a friend 10 yrs ago and do now own a 76 mr250 which is totally different as it has a flywheel.If I remember right all that is needed to remove a cr rotor is a bolt that threads into the rotor and then pushes on the crank as opposed to the bolt that threads into the end of the crank and holds on the rotor,hope this makes sense. The original manual that came with the bike was very good on the service aspect. Nathan has kindly provided copies under the Manuals header on the main page. Good luck, Gary

it will come out like i stated above