Vintage Dirt Bike Q & A

thanks ray, i tried what you said, and everything is working ok. the gas comes in, i raise the float and it shuts off. the one thing im not surehow to check is the same thing, but with the float bowl on. gas does not run out of the overflow when i turn the fuel on, but for a second or two a few drops will come out after i turn the gas on. i was also wondering if there should be any compressive force pushing out of the intake port on the cylinder. is this the force pushing gas out of the carb? or is it just an overflow of gas..... thanks for your help, i cant answer bike questions, but i am master certified ASE mechanic. i just dont know anything about 2 stroke bikes

I would take another look at the float valve combination. If the plunger is worn (very likely),
it may cock in the seat and not close reliably. This would cause exactly what you have going
on. Also check the float level to make sure the tang isn't bent at some bizarre angle. This
could also allow way more gas to enter the bowl than needed before shutting it off.

There is no positive pressure anytime at the intake of a two stroke. This sounds like a simple
case of excess gas in the carb. Since this is a 35 year old carb, my advice is get a rebuild kit,
rebuild it and set it up properly. That should eliminate this problem and preclude many future
ones.

dogger

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1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

This sounds like a textbook case of a float level that's a lot higher (lower in mm - see diagram below) than the correct 20mm, but not quite high enough to overflow.

Re the pressure out the intake... my CR125 did this for a little while, a long time ago. I cut 10mm off the bottom of the rear piston skirt looking for more top end power, and at idle the carb would "chuff" small puffs of mixture out the carb mouth on each crank rotation, sometimes slightly wetting the inside of the air filter with gas. I tried not to let it idle too long while this piston was in it, and all subsequent pistons were stock.

This is a very unusual thing to have happen, and in a reed-valve bike with no intake issues it really cannot happen, as dogger mentioned. The only times I could see it occurring would be in a piston-port roadracing motor around idle, or in a motor with a top end so worn out that blowby was pressurizing the crankcase. In a stock MT250 it should never happen at all unless something's defective or modified - so on your bike, in my opinion it's definitely time for a compression check (I'd want to see well over 100 psi) and maybe a top-end teardown if there's any question at all about its condition.

Ray

Here's a float level adjustment diagram:

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric