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1979 Honda CR125 with Mikuni VM34 jetting Questions
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Author:  chrisg19 [ Mon Dec 15, 2014 6:56 pm ]
Post subject:  1979 Honda CR125 with Mikuni VM34 jetting Questions
I just picked up a 1979 Cr125 with a Mikuni VM34 carb on it.

The bike originally came with a Keihin Carb, and somewhere along the line someone switched it to a VM34.

Pulled the carb apart, to clean it, and saw that it has a 260 Main jet installed in it, and a 52 Pilot. (Main Jet appears to be a MIKUNI HEX MAIN JETS 4/042 type)

I am trying to get a decent starting point jetting for 40 degrees Farenheight, and 5500 elevation, but not sure where to begin.

The bike was located in Michigan, and I am in Denver. The cylinder was toast when I got the bike, and I just had it replated. I have no idea how the bike ran prior, so the jets listed above may be way off.

I have the original manual for the bike, and the Keihin that came stock on the bike came with a # 155 Main, and # 60 Pilot jet. Using the Correction chart in the manual, the correction factor for 40 degrees, and 5000 feet is .75. According to that the original Keihin, should have a # 116 Main, and a # 45 Pilot.

I don't know where to find a conversion chart for the VM34 that is accurate to the Keihin's. Most charts I see show a 155 Keihin being equal to a 133 Mikuni jet. But this carb has a 260 in it, that's like double the "conversion charts" online. Yet the VM34's Mikuni sells seem to come standard with the 260 Main I reference above. I must be doing something wrong.

I know the VM 34 is a popular swap for the 79 CR125, so am hoping someone can get me to a rough starting point that I can fine tune from there.

Chris

(I am going to post this same question to several different forums in case you see it at more than one forum)
Author:  rayivers [ Mon Dec 15, 2014 7:52 pm ]
Post subject:  Re: 1979 Honda CR125 with Mikuni VM34 jetting Questions
It seems no one knows for sure exactly what Mikuni jet is equivalent to which Keihin jet. The relationship appears non-linear, with the differences smaller at the lower end of the scale and reversing direction at some point below 100.

I think the chart below is the most accurate, but then I really don't know either. :) I also don't know what the asterisks mean.

Just FYI... an '82 RM250 (Mikuni TM 38) uses 290 main and 55 pilot jets stock, and an '82 CR250 (Keihin PE? 38) runs a 160 main and 72 pilot.

Ray

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
Author:  spannerman167 [ Wed Jul 08, 2015 2:33 pm ]
Post subject:  Re: 1979 Honda CR125 with Mikuni VM34 jetting Questions
This should help

http://www.allensperformance.co.uk/jetsizeconv.html
Author:  evelmike [ Wed Jul 08, 2015 6:18 pm ]
Post subject:  Re: 1979 Honda CR125 with Mikuni VM34 jetting Questions
How does it run? I ran a 260 on a 1975 cr-250.pilot sounds way big
Author:  spannerman167 [ Mon Sep 28, 2015 1:42 pm ]
Post subject:  Re: 1979 Honda CR125 with Mikuni VM34 jetting Questions
260 is way to small for an air cooled motor it won't take long to seize if full throttle is used for extended periods.

I also have a 79 CR125 fitted witha new 34mm Mikuni VM carb
I'm starting with a 340 main and a 32.5 pilot.

I had a 80 CR125 fitted with a Mikuni VM and I had it jetted on a dyno so I'm using those jetting spec's
as a starting point.

Keihin 1.547 (marked 154)= Mikuni 320
Keihin .611 (marked 61) = Mikuni 50

Remember Keihin figures are bore size, Mikuni are based on flow.
Author:  chrisg19 [ Fri Oct 02, 2015 6:08 pm ]
Post subject:  Re: 1979 Honda CR125 with Mikuni VM34 jetting Questions
spannerman167 wrote:
260 is way to small for an air cooled motor it won't take long to seize if full throttle is used for extended periods.

I also have a 79 CR125 fitted witha new 34mm Mikuni VM carb
I'm starting with a 340 main and a 32.5 pilot.

I had a 80 CR125 fitted with a Mikuni VM and I had it jetted on a dyno so I'm using those jetting spec's
as a starting point.

Keihin 1.547 (marked 154)= Mikuni 320
Keihin .611 (marked 61) = Mikuni 50

Remember Keihin figures are bore size, Mikuni are based on flow.


This post was extremely helpful. Thank you. I got away from riding the 79 for awhile as I picked up a 1980, and have been working on rebuilding that.

Anyway I am back to the 79 jetting now, and your post was very informative. If you don't mind me asking what elevation were those settings for, and what temperatures. I am at 5500 feet above sea level, so will need to adjust for that. And just to be sure we are on the same page, the 340 main jet you mentioned was a 4/042 Genuine Mikuni jet, correct?

Thanks again for the information, I really appreciate it.
Author:  spannerman167 [ Sat Oct 03, 2015 2:51 am ]
Post subject:  Re: 1979 Honda CR125 with Mikuni VM34 jetting Questions
Glad to be of some help,
It was purchased as a genuine Mikuni part number 4/042 item and the Jet was stamped 340 ( a little difficult to see, a magnifying glass would be wise ).

Like I say I believe this jetting would be a damn site safer starting point than the supplied jetting.

I don't know the elevation the setting was set at but I'm in the UK so obviously the temperature wasn't that warm lol.

My 1980 was set on a dyno to these spec's and never changed over 2 seasons.

I'm trying to get out and ride my 79 with these settings, hopefully within the next few weeks I will manage to get out.
Author:  spannerman167 [ Mon Oct 05, 2015 1:05 pm ]
Post subject:  Re: 1979 Honda CR125 with Mikuni VM34 jetting Questions
I rode my bike with those jetting spec's and it went really well, its not perfect but not far off.
Author:  chrisg19 [ Wed Oct 07, 2015 1:27 pm ]
Post subject:  Re: 1979 Honda CR125 with Mikuni VM34 jetting Questions
spannerman167, I rode the 79 over the weekend with your settings as a base (adjusted for altitude). I ran a 310 Main, and a 32.5 Pilot, and these settings were nearly spot on. I am going to go up a little on the pilot, as it seemed just a tad lean down low, but the power was great throughout the range, and the plug came out a nice tan color. I am very grateful for the feedback provided, it saved me a ton of time, and now it is just fine tuning from here. Thanks Again for the insight.
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