Vintage Dirt Bike Q & A

Hey all. Been a while since last posted. 76 MR250 Bored out cyl.with new piston and rings, All new internal seals. Completely stock carb and jets. Needle on 2nd clip from the top. Rode it around the property this last year and it ran OK but not fantastic. Did not really worry about it . Took it to the mountains a few weeks ago, ran like crap. Would not run unless gas was almost wide open also would not idle very well. Left it in the mts. for a week or so and went to get it loaded and still ran like crap. Took carb off back at home and lowered needle and it ran a little better but would not rev at all.Took it off again and carb body was wet with gas. Am curious about jetting for altitude. Elev is @5000-6000. PS using gas:oil mix @40:1 Any suggestions would be great. Thanks so much Norski

The stock jetting for the 073A MR250 carb is: 100 main, 140 power, 48 pilot, 200 air, 130 starter, 2.5 throttle valve, 271006 needle. I'd verify at least the main and pilot values to see if the carb's been worked on.

From what I can tell, the MR250 was jetted very rich for sea level - it must be horrible up high, especially if worn.

I'd suggest: 95 main, 40 pilot to start (I'd also get a 45), with the needle clip at the #2 position (one position leaner than stock). This should wake up the motor a lot, although it's surely far from optimum. I use http://www.jetsrus.com, there are others too.

Check that the power jet O-ring is in good shape (it's a small one in the float bowl gasket area). If not, Honda P/N 91308-371-003 (4.1x1.5) should work.

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

You are not alone. And a lot of the time with these old worn carbs, it's the needle valve and needle. I gave up finding a replacement and stuck a new 36 mm Mikuni on it. No matter how I jetted it, it would just run smoking, blubbery rich. Wide open was good though.