Vintage Dirt Bike Q & A

I have a MT250 that I have reserected from the scrap pile - looks ruff but I finally got it running decent. I pulled in the garage the other day and the engine just suddenly went wide open. I got the engine stopped - now I can't get it going again. Since I had an aftermarket carn on it - I took the carb off another bike that is running great - and still nothing. The spark is not blue - but it wasn't when I started. What now - I want this beast to terrorize the neoghborhood again

By wide open it sounds like you mean the throttle stuck open. Was teh throttel slide open or was it at idle and it just revved like mad? If throttle was closed and it was revving like mad it could be a vacuum leak. Check your carb boot/intake for cracks and proper seal to cylinder and carb.

Also ensure your head and base gaksets are not leaking. A compression check would be prudent. Should have at least 100 PSI to run, should be more like 120-130 optimally.

Also check your points and ignition timing.

Yep leak down test would be my 1st course of action

If the throttle slide is closing OK, most probably you have an air leak. Check the joints between the carb and the cylinder as well as eyeball the base gasket to eliminate the easy stuff. A leak down test is then the definitive way to find out if you have a leak from the inside out. If yes, suspect both the crank seal behind the ignition, but also the crank bearings themselves. I have heard of situations where the crank is oscillating in a worn bearing and flexing a good seal to let air in. You should not be able to move the crank up or down at all....

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

When I had that problem, it was a crack in the rubber intake manifold between the carb and cylinder. I tried every fix I could think of, but it ultimately required a new manifold.

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Steve Amling

1976 CR 250M
1980 Yamaha DT100G
1980 XR 200
1981 CSR650

Check the throttle cable ends. Did it snag on something on the way into the garage that pulled the cable housing out of it's seated position in the carb top or the throttle tube area?

Also, you need a two-stroke pressure tester kit, not a leak down tester. The pressure tester kit allows you to pump air into the cylinder to 6 psi. Then if it doesn't hold that pressure you have a leak that needs fixing. While the engine is pressurized with the pressure tester, spray soapy water around the seams of the crankcase/cylinder/head/intake and crank seals to see where the leak is. It's a great idea to do this everytime you service the top end.

Get 'er going! Jon