Vintage Dirt Bike Q & A

selecting a larger carb for 1974 cr125, which size is best 32 or 34 ?, also jetting and tuning info would be helpful , thanks for any advise.

IMO, the best carb size for a stock '74 four-transfer cylinder is 29mm, the same size carb Honda supplied with their optional '74 'GP' 5-transfer cylinder. I'm assuming you're using the OEM piston-port design.

I've run the numbers through my simulation software (port window chordal width / down angle / side flare etc.), and the stock '74 bridged intake port flows the same as a 29.8mm diameter tube. This is nowhere near big enough for a 32mm carb, let alone 34. To maintain a decent WOT vacuum at the venturi you need at least .5mm extra equivalent diameter everywhere downstream of the carb, and 1mm would be better.

The 28hp MT125R road racer only used a 32mm Mikuni, even with the HRC liquid-cooled hop-up kit. It can get away with this because it has a much bigger intake port that stays open longer, but it also has zero low end and cannot be kickstarted. Obviously if you put a 32-34mm on your bike it will still have some bottom end and will kickstart just fine, but it will still only flow 29.8mm worth of air/fuel mixture.

I'm well aware that everyone and their uncle was throwing bigger carbs on stock motors back in the day. Mine was a 36mm - I knew way more than any of those Honda engineers - which ran like crap despite months of jetting and rejetting, and caused me to finish way back in the pack in the only race I ever led (1st-turn pile-up). I then ported it and removed the intake bridge, giving an equivalent area of about 31.8mm (which I didn't know at the time) - it still wasn't enough. The huge drop in low end & midrange made me think I'd gained a lot on top, and the throttle response sucked unless the motor was screaming.

Just my @.02.

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

That's all well and good but.................
IF you boost port the cylinder and raise the compression and use a good pipe and a reed valve I was running a 32mm on tight tracks and a 34mm on fast open tracks.

Worked for me to get a National Championship.

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1994 AHRMA National Champion Sportsman 125 Intermediate.
1996 AHRMA National Champion Classic 125 Expert.

That's great - congrats! From my post:

A well-designed reed motor is a whole 'nother animal, taking long / deep pulls of mixture from the carb.

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

The OP wanted advice on carbs to use, I gave it to him.

Software simulators are ok but you can't ride them.
I worked for 7 years in R&D at SuperTrapp/kerker as a designer/dyno/flowbench/fuel re calibration specialist. Numbers that looked good on a dyno/flow bench did not always work in real world situations.

As for jetting and tuning, that will depend on your engine set up, location, ambient temp and fuel used.
IF you are going racing for a series or championship, I would HIGHLY suggest using race fuel, simply for quality and consistent batches.
IF you buy a carb from say Carb Parts Warehouse they are close jet/needle wise delivered, but I do suggest you tune as stated above.

There are a TON of mods the 125 needs to be the best, PM me and I will give you the experience I had with mine.

Good luck..

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1994 AHRMA National Champion Sportsman 125 Intermediate.
1996 AHRMA National Champion Classic 125 Expert.