Vintage Dirt Bike Q & A

I bought a 1976 MR250 that sat for 2 years in a barn. I pulled carb and cleaned the jets I could get out. Couldn't get power jet and air jet out because of oxidation and corrosion. So my question is CR, MT, and MR carbs interchangeable? And if not what are the stock jet sizes so I can get a used carb and rebuild it. The bike is all original and I don't want to put a aftermarket mukuni on it. Thanks for any help anyone can give me.

I believe all the CR250M and MR250 carbs are physically interchangeable (43mm spigot, 62mm mouth, @ 96mm overall length). The later 36mm CR250M 010B carb looks nearly identical externally to your 073A MR carb, if that's important to you.

Here's some jet info etc.:

viewtopic.php?f=21&t=4222&p=14906&hilit=mr250+carb&sid=5ba89d573c719c2b98918528f46364bd#p14906

For what it's worth... the air jets are sometimes impossible to remove without destroying them, and as long as yours is clear it should be fine. If the power jet is clear it may be OK, but its O-ring and groove should be in good shape or it may not pull much fuel.

Ray

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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

Thanks for the jet specs. I don't know much about what jets do what. As I said I cleaned all I could get out and got the bike started it runs fine over half throttle but won't idle even with idle screw all the way in. The power jet would not come out and I didn't want to bugger it up so I just hit it with carb cleaner and compressed air seemed to be clear, but air jet in the back of carb doesn't even look like a jet carb is pitted real bad on the 3 holes back there. any suggestions this. Thanks

From the mouth (rear) of the MR250 carb looking in, there are four openings:

1) Air jet (bottom dead center); if you spray in carb cleaner and it spritzes out the needle jet, this jet is not completely plugged, although obviously it could still be dirty or corroded

2) Power air jet opening (left side center); there's normally no jet in this opening, but it's threaded for one. Carb cleaner sprayed in here should come out the power jet.

3) Choke air intake (right side center, recessed) only functional when choke activated

4) Idle / low-speed circuit air intake (bottom right corner, next to small brass plug); supplies air to idle air screw and pilot jet circuit

I'd try removing the idle air screw, then spraying carb cleaner into hole #4 with the float bowl off (it should spray mostly out the air screw hole). Now put the air screw back in maybe 2 or 3 turns (NOT fully seated) and spray hole #4 again; it should shoot out the pilot jet (tall jet near the main jet inside the carb) and also two small openings near the front bottom of the carb throat, near the throttle slide. My guess is all three of these things will not happen, indicating a plugged pilot jet, passages, or both. If your pilot jet is really plugged and/or corroded it may have to be replaced. Your bike will never run right below 1/2 throttle or idle well if the entire pilot circuit's not working correctly.

Ray

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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