Vintage Dirt Bike Q & A

I want to check to see if my crank seals are leaking on my 75 cr-250. How do I do this?

If your bike runs like a dog under throttle and has thick white smoke out the exhaust you probably got a bad seal behind the primary drive which is letting transmission oil into the the crankcase. If your bike idles overly-fast then the seal behind the rotor is letting air into the crankcase directly and leaning out the mixture. The definitive check is to buy a leak-down tester which measure the ability of the crankcase and cylinder to hold pressure.

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

Bike breaks up under full throttle. There doesnt seem to be any white smoke,but it does smoke some(blue).Spark plug is wet. I am running a 36mm Mikuni with a 300 main. I tried diff jets . I installed new points and condenser. I cant tell if its ignition or carb problems.

I ran a 36 Mikuni on mine with a DG pipe (stock head and cylinder). I ran a 250 Main and it ran great. I have heard several people advise a 300 main but for my bike it was way too rich.

You can easily test for air leaks using items purchased at your local hardware store. You take off your exhaust and plug the exhaust manifold with a rubber plumbing plug and hose clamp. You take off your carb. and install PVC fittings (using your stock carb. clamp) attached to a standard water pressure gauge. Take a pump and put 6 lbs. of air in this setup and check to see if you lose pressure (10 minutes should be sufficient). Use a spray bottle with water and a few drops of dishwashing liquid to spray around the head gasket, base gasket, exhaust manifold and intake. If you are losing pressure and do not see any bubbles in those locations, then you have a leak at your main seals. Pull the rotor off and do the same test there.

I hope this helps.

Thanks for the info. I am gonna re-jet and see if it clears up. I may even rebuild the stock carb and run it , that way I will know if its carb related.