Vintage Dirt Bike Q & A

hello every body.I just finished my 73 cr250m resto,rebored cylinder,all new bearings and seals and the bike goes so rich with the stock 34mm keihin that has been rebuilded with new stock jets,needle,and float valve,everithing is new except the needle jet that i didnt find anywere.is possible that the carb is worn out?, wich kind of mikuni will fit same intake and filter boot diameters?
Im running the stock expansion chamber too.
Thanks and sorry for my poor english.
Daniel

Daniel,

After many hours spent working on my Keihins, I understand now why many people just replace them. i feel they were good carbs when new, but now they are worn in so many places, and most of the parts are almost impossible to find. I've been able to get mine working well, but it would have been so much easier to just get new ones.

I believe your carb's dimensions are 43mm (spigot/cylinder end), 62mm (mouth/inlet) and 98mm long. The Mikuni TMX38-27 with air boot adaptor is 44mm / 62mm / 94mm, and the VM36 is 43mm / 116mm / 62mm. I don't know if the OEM throttle cable will work with these.

Yes, needle jets wear, and make the carb run richer. I found new replacement needle jets and needles for my 28mm and 30mm Keihin carbs, and they made a real difference. The 28mm unit had many hours on it, and a 1/4 - 1/2 throttle richness I couldn't stop just went away, like magic. The 30mm carb had very low hours, but I still noticed better mid-throttle performance.

Increasing the throttle slide cutaway can really help with low-midrange richness. I just had my machinist convert the 2.5 slide on my 30mm to 3.0mm, I'll be trying it out today. Changing from a 3.0 to a 3.5 made a big difference on my 28mm, too.

Ray

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

Thanks a lot Ray, now im sure that is going to be better to give up with the old keihin. I have just ordered a new mikuni vm36mm wich has the same manifold and air boot diameters and a reasonable price, so i will apreciate any body who is running the same carb to tell me about a starting point for jetting setings at about 1900 ft over sea level.
Thanks again.
Daniel

Daniel,

If the Keihin used a 145 main jet and 55 pilot jets, maybe start with 230 main and 50 pilot jets on the Mikuni. It will probably take time to get the jetting correct for your elevation, bot not too much.

Here's a Keihin-Mikuni conversion table that may actually be correct:

KEIHIN___MIKUNI

92.5______100
95________105
97.5______110
100_______115
102.5_____120
105_______125
107.5_____135
110_______140
112.5_____145
115_______155
117.5_____165
120_______170
122.5_____175
125_______185
127.5_____195
130______ 200
132.5_____210
135_______215*
137.5_____225*
140_______230
142.5_____245*
145_______250
147.5_____265*
150_______275*
152.5_____285*
155_______295*
157.5_____310
160_______320
162.5_____335*
165_______345*
167.5_____360
170_______370

Ray

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