Vintage Dirt Bike Q & A

I am putting a 34MM Mikuni on a 1974 CR125 and I need an expert(s) advise. But first please note, if they wrote a book called "Carbs for Dummies" if would be dedicated to me !!
The only things I know about carbs are:
1. Gas goes in from the tank
2. Air goes in the Big end
3. It mixes the two and all goes out the small end into the engine
4. If it's not done correctly, it will ruin a brand new engine (don't ask how
I learned that)

However, I can disassemble and re-assemble one.

Now, with that info, can someone tell what all internals does this 34mm need? -- The carb was run on another bike (larger bore) so I am assuming about everything inside needs to be changed.

Oh, one other thing -- why am I using a 34mm? -- Because I have a 34MM and I'm too cheap to buy a 32mm.

All I want to do is get close -- maybe even a tad rich (see note 4) then I can start dialing it in.

Thanks

Terry

Terry,

I'd like to add one more item to your carb list:

5) Bigger isn't usually better.

I think I qualify as an expert in what not to do to a '74 CR125M, so maybe that will work for now. Everything below pertains to the original 125cc motor, not the 175cc unit that's in it now.

I'd suggest at least a 280 main jet, and the pilot jet that's in there now (whatever that is) to start. Perhaps someone else with actual 34mm Mikuni jetting info can get you closer than that, as I'm working from a conversion chart that may or may not be accurate. If your carb has a lot of hours on it, I'd replace the needle jet if it's still available, otherwise you may find yourself leaning out the main and pilot jets excessively while the motor still runs rich at 1/8 to 3/4 throttle (see #4 in your list). I'd also replace the L crank seal (behind the ignition) just to be safe, unless it's brand new.

It's possible to limit the slide travel to make your carb work pretty much like a smaller one, and allow you to use WOT below the powerband without the motor going to sleep.

Honda originally used a 28mm CR125M carb, a 29mm in the optional '74 hop-up kit, and a 30mm from '76 to '78. They did put a 34mm Mikuni on the MT125R road racer, with its fairly radical porting and intake timing; I think it may have come with a 300 main jet, but I couldn't find much else.

I put a 36mm Keihin on my CR, removed the intake bridge, and trimmed the rear piston skirt, resulting in zero low end, reduced midrange, about the same top end, and terrible throttle response and plug fouling as bonuses. I wouldn't recommend doing any of these things to yours. I wish I'd kept the 28mm.

Ray

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

I went and bought a VM 34mm carb for my 1979 cr125 back when I was getting the bike back in shape. Bike is stock with a freshly plated cylinder and new piston/rings. I was able to tune the carb enough to be rideable. Took it out on its first couple trail rides with the 34mm. I could tell that it could be better and the 32mm cured all the issues. It runs so much better and worth $100 or less.

That said, you can get it running good enough to ride with the 34mm. I wish I could remember the exact jetting sizes.

nola_fiddy