Vintage Dirt Bike Q & A

i'm finaly starting the asembily of my 125, but i need a carburator. what size and brand would you all recommend, i'm going to make a custom manifold. this bike will be used for fast fire road and vintage mx riding thanks

Since no one else chimed in, here goes... the stock carb had a 40mm spigot, 58mm mouth, and 93mm overall length (I'm using the 30mm version on my 'CR175'). The Keihin PWK33 Air Stryker's dimensions are 42mm/59mm/91mm, which should be close enough. The TM32 Mikuni is 40mm/58mm/83mm, and ought to fit perfectly with the TMX/006 carb boot adaptor. The stock intake manifold and air boot should work with either of these carbs, and you might even get lucky with the throttle cable too.

Notice I'm not suggesting any 39mm carbs for this 70's 125cc motor. A 36mm just about killed the low end and half the midrange on my '74 CR125 motor, and I'm not sure it really improved the high-rpm power much, although it seemed good because the rest was so poor. :>) An oversized carb can be jetted for decent part-throttle response, but when you wick it wide open below the powerband and the motor goes to sleep, a cure can be hard to find.

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 ray, i do have a carb i got with the bike (it was a basket case) its a PE-05-RA. it looks like it could be the stock carb its very clean inside and the slide is real nice. its 34 id 40 od on the engine side and 58 od on the filter end. do the stock carbs run good ? or are there better ones that i could use? thanks jay

Jay,

If the ID is 34mm at the spigot end, I think yours is a 32mm carb (the venturi is just downstream of the slide - I measure it by sliding in different-size sockets and then measuring the closest one). My carb's outer dimensions are identical to yours, but the venturi is 30mm and the stamped number is 083A.

If it were my bike, I think I'd just clean the carb out real good and try it out. I blow out every passage, jet, and orifice I can think of, then follow up with spray carb cleaner. If you need jets, http://www.jetsrus.com has 99101-357 main, 99103-358 pilot, and N424-52 starter jets available, with good prices and fast shipping. I've had good luck with all my (undamaged) Keihins so far, and in your case, with the original carb on the bike, it shouldn't take much work if the carb is intact.

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

For whatever it is worth, my '79 125 carbs were basketcases too when I started - but were complete. I took them 100% apart and soaked the metal bits in some Berrymans engine cleaner for several hours. If you go that route, just make sure that you have removed ALL the rubber pieces.
After soaking in the cleaner, I took them out and cleaned them with some carb & choke cleaner then air dried everything.

I have a follow up question; have either of you ever messed with the jet settings on a '79 CR125? If so, any suggestions?

-timbo

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rides:
'01 Suzuki SV650 - not exactly stock.....
'79 Honda CR125 - running great, cosmetic restoration back on track!

I also have a 79 cr125r that needed a carb. After a little trial and error, the Mikuni VM 32MM round slide carb was my choice. It is a little longer front to back than the stock (a few mm if I recall). I had to shave the bell down to get it to fit inbetween the air intake and the air filter box (no big deal). Main Jet 300, pilot jet 40 with 2.5mm slide cut away. Runs great.

I tried the 34mm Mikuni but the bike lost too much low end for the sake of a little more top end. Not practicle on a small bore 125.