Vintage Dirt Bike Q & A

I bought the bike last summer. It was in good shape, still had the original tires on the bike. It also ran but ran poorly. It was running rich and blowing smoke like crazy. I put in a new piston and rings and changed the main jet from a 160 to a 155. I fired it up and it ran better but still rich. I then lowered the carb needle to the lowest setting and bamb! It's now running pretty good - still need to tweek it a little.

I don't have much experience with the 2 stokes and have not had to make such drastic changes to any of my 4 stroke projects. I am in New Orleans, so obviously sea level at best!! Do you think my needle is worn or there is some other issue with the carb? I set the floats to the proper level but they are old, are they prone to failure with age? Is there a good replacement carb being made today that will work on this bike?

Any insite would be appreciated.

thanks,
nola_fiddy

Try 34mm VM round slide Mikuni carbs. I have had great luck running these on the 79 CR125s. I could never get the stock carbs to run good.

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One is not enough.

Thanks spuncrazy for the info. I found a 34MM Mikuni VM round side on the web. It comes with a 240 main jet, 40 pilot jet and Q-6 needle jet. Do you know if these are the correct jets?

thanks,
nola_fiddy

Sounds to lean to me.
I've a 1980 CR125 admittedly with afew motor mods also running a VM34 carb.
As I couldn't find out a base setting to start with I had it setup on a dyno.
340 main 32.5 pilot.

Thanks spannerman, I found a chart that matches the Mikuni and Keihin jet sizes. The Mikuni 340 jet is a 160 keihin jet. So I should be good somewhere between a 310 and 330 Mikuni main jet. The Keihin 55 pilot jet would equate to a Mikuni 40. I will buy a few of each in different sizes to dial it in.

Nola,
For the 1979s i used the stock jetting that came with the new carbs. I only had to drop the clip one notch on the needle. I live in San Diego and it works fine around here.
My bikes engine is stock except for a DG head and Mikuni carb.

Spanner,
I also have a couple 1980s that don't run as good as the 79s with this carb. I will try the 340 main jet and lower the pilot to a 32.5 and see if the 80 runs better.
Thanks

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One is not enough.

Thanks for all the good info guys

I am going to give up on the stock carb that came with the bike - it had been messed with in the past. I will get the Mikuni VM 34mm stock and take it from there.

I'm having the best time working on these old bikes! I started out a few years ago getting an xr75 that I got for xmax in 1977 running again. It sat untouched for 20+ years in my dad's garage with just a few missing parts. It is in great original shape - no restore necessary - running great.

Since then i've picked up a couple minis. My first bike ever was a 1972 Z50. I got it used in 1976 and it had been treated rough. So I found a newer 1997 z50r and tricked out the frame and engine. I also picked up a xr70 that was in pieces for cheap and put it together. It's a great bike for the kids and the neighbors who pester you to have a ride!

When I out grew my 75, I was eyeing up the cr125. I had a few friends who had them and I thought it was the next step. I never did get the bike back then and once I got back into the bikes, I knew I wanted one. I really like the 79-80 models and when I found a local deal, I picked it up!!!

My first 2 smoke - having fun figuring them out.

later

Check the physical size of the 34 mikuni before you buy. I found the size too small to use existing boots without modification to the carb or boots. The 36 mikuni is very close in diameter to original and will normally bolt right on.

Where did you find the jet conversion chart for kehin to mikuni it sounds very usefull and I want it.

http:\\www.allensperformance.co.uk\jetsizeconv.html

I measured my stock Keihin PE carb on the 79 CR125R, the spigot od is 40mm and the air od is 58mm. These match the Mikuni VM 34. I should be good. With this carb on the 79 model.

I got a Mikuni VM 34MM carburetor the other day. I installed it last night. The size was almost perfect. The air intake boot is a smidge too small for the carb. I had to lube it with 2-stoke oil and it slipped in - No big deal. Everything else worked fine, even the throttle cable worked out. It came with a 240 main jet, 40 piolet jet, Q-6 needle jet. I didn't make any adjustments to the needle or anything, just bolted it on. I gave it a run this morning and it was running nice. I didn't have much time to play around with it, but may have to lean it out a little by lowering the needle or playing with the air screw. I will play with it and report the findings. All in all, the Mikuni VM 34MM is a great replacement for the 1979 CR125R.

The only issue I have now is this rattle that is coming from the top end. I just replaced the piston, wrist pin and wrist pin bearing with Wiseco parts. The cylinder chrome is in exellent shape. The rod seemed nice and tight. Is this just a normal sound for these old CR's? Could I have bearing issues (this is a low hours bike - still had the original piston when I tore it down)? Also, it doesn't seem to be comming from the crank case. I will break out my scope and listen to it tomorrow.

thanks,
nola_fiddy

Be careful with that 240 jet. Chances are it will be way too lean. I bet you end up some where around 310-320.

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Terry Moody #48f
3x 74 Honda CR250M Elsinore
74 Honda XR75
73 Honda QA50
78 Yamaha YZ125E
78 Yamaha YZ250E
78 Yamaha YZ400E
79 Kawasaki KX250
79 Honda CR250R
79 Suzuki RM400
80 Honda CR125
2x 83 Honda CR480R
98 Kawasaki KX250

Does anybody out there know the correct jetting for the 1979 cr125r with a mikuni vm 34mm carb????

If I get a larger main jet, will I have to swap out any of the other jets (pilot etc)?

thanks
nola_fiddy

I went ahead and ordered 320 and 330 main jets.