Vintage Dirt Bike Q & A

Hello! I had a 1975 CR125 that I purchased new. The 125 class was loaded at my local track. My first race was two rows deep. It was just too jam packed for learning to race so I sleeved my 125 to 100cc and raced the 100cc class. While cleaning my garage, I came upon my hardly used carb for the 125cc. The ID number 012A. I heard some rumors that the 012A carb was a rare 29mm carb. The throat measures 29mm as best as I can measure. Can anyone substantiate this rumor?

I believed the 012A may have been the '75 29mm optional carb - hopefully I didn't start that rumor - but one appeared on eBay recently and the seller measured the venturi at 28mm (but see below). The two genuine 29mm carbs I've seen pictures of were both factory-bored 005A '74 carbs; they're easy to recognize from the spigot end, as the carb throat is offset downward and the bottom of the spigot looks really thin. One thing's for sure; you don't see many 012A's.

Re the eBay measurement above... it's very hard to measure the venturi diameter accurately in these carbs. The only way I've been able to do it is to find a socket that's very close to the venturi diameter (carb throat between slide tunnel and intake bellmouth), then wrap it with aluminum duct tape until it just fits and then measure it. You might want to measure your 012A again using this method and see what you come up with. For it to perform well as a 29mm at WOT you'll need to see 29mm in the venturi and more than that (probably 29.5mm +) in the spigot; the 28mm carbs I've measured have all been 28mm venturi and @ 28.7mm in the spigot.

I've bored two 28's to 29mm; an 005A and an 060A MR175 carb. The venturi side is easy; just set the bottom of the cutter even with the carb floor and run it in till you're as close as possible to the needle jet shroud. The other side takes a lot more work; you need at least 29.5 to 30mm downstream of the slide while maintaining spigot strength, and the 28mm spigot has hardly any metal to spare. We ended up milling in 3 directions to get it done, but the carbs worked great - they had more power in every throttle position, not just wide open.

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. That carb on ebay might have been mine. I posted it and then received three inquiries about it being a 29mm. I had never heard of such a carb. I did my best to measure with a bore mike but to be honest, I wasn't really sure exactly where on the carb I should be measuring. Your description helps. I'll measure again and see what I come up with. I want to be as accurate and knowledgeable as I can before I put it back up. Thanks for your help.

I bet that was your carb on eBay - thanks very much for measuring it for me. Some eBayers don't even respond to questions at all.

I've been pretty lucky with socket measurements - I've used them for carbs, intake/exhaust manifolds, pipe inlet flanges, etc. There's also lots of other things lying around the average house that can be used - small bottles, coins, vacuum cleaner attachments, maybe even a ping-pong ball. If the spigot-end measurement (easy to do) is close to 30mm, it's likely your 012A is a 29mm.

I'm assuming the 012A is the carb that came with the bike? For a while I thought the 056A carb on most '75's was a 29mm, but I finally got one and it was a 28. The '75 CR125M uses a different pipe, cylinder, exhaust manifold, intake manifold, and carb than the '74. Since the '74 hop-up kit replaces these same items and apparently the '75 cylinder is the '74 optional one, I figured all the other things might be the '74 hop-up kit pieces or similar. Maybe the 012A was only used for the early '75's and then they switched to the 056A- it would be interesting to know.

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