Vintage Dirt Bike Q & A

Just thought i'd throw some jetting specs out there for an engine build. MT250 with a mild port job, performance exhaust and 34mm Mikuni roundslide. I believe these would be in the ballpark for a factory setup (maybe a little rich). 2.5 slide cutaway, Q2 needle jet, 6FJ6 jet needle, 240 Main jet, and 35 Pilot. I know i've had my share of problems with the factory Keihen carb and i just got tired of trying to make it work. I did open my stock intake manifold up a little (w/ a dremel tool and carbide bit), but in the stock realm all you need to do is soak the rubber flange in very hot water (near boiling) for about 10 minutes in order to stuff the larger carb in place. Be careful,,,, they are prone to cracking with age. Do the same with the airbox boot; again be careful. I would imagine some effort to open up the stock airbox would be in order. UNI makes some handy 1 inch plug-in vents; they come in a six pack i believe. Use a 1" holesaw and drill some strategic holes in the airbox itself and pop in the vent covers to allow for more airflow. I'm using a performance breather i designed to replace the factory airbox- no issues with airflow but it's off-road only. The thing carburates MUCH better, starts right up and is very clean running. More faster (grin). If you think the 34mm is too big a jump from the 28mm stock, consider this. The physical diminsions (outer) of the 32mm Mikuni are the same as the 34mm. The inside opening is smaller of course. So,,,the 32mm might be a better carb for a bone stock MT250. Also, that 28 Keihin is what came out stock on the MR175 and early CR125. I paid less than a $100.00 for mine from Speed and Sport. They figured the jetting very close the first time. Awesome folks!
I'd post a pic but i'm struggling with the upload. Hope this helps anyone out there looking for a better carb setup. Joe

Thanks for the info! This is the first time I've seen specific info about putting a bigger carb on the MT250. The stock 28mm is just too small for even a mildly-tuned 250, and in my opinion it's marginal even for the MR175 motor.

The MT250, MR175, and '74-'78 CR125 intake manifolds all have the same 2.58"/66mm bolt spacing, although the openings differ. The '76-'78 CR125 intake manifold (Honda P/N 16211-400-000) starts with a 40mm I.D. and can probably be easily stretched to 45mm, and the Suzuki '72-'75 TM250 intake manifold (Suzuki P/N 13110-16500) has a 43mm I.D. and the correct bolt spacing, although the bolt holes are bigger. I'm also investigating putting the rubber section of an MR250 intake manifold on a different flange, just waiting for a reasonably-priced MR250/CR250M intake manifold to show up on eBay. At some point I want to try a 32mm or 34mm carb on the 175, and I'd like to mate it to a large rubber-mount intake.

I did the boiling water thing too, it worked fine with the 30mm Keihin (40mm spigot O.D., 5mm larger than the 28) I stuffed into the MR175 manifold. I opened up the inside of the rubber tube section as much as I dared, but I didn't want to leave the rubber too thin. The MT250 cylinder opening is 30 x 41mm and the MR's is 27 x 34mm, so I think we'll both be OK. The carb spigot I.D. is slightly larger than the venturi, so even if the intake manifold I.D. is slightly smaller than the spigot it should still allow full rated flow from the carb, or close to it.

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

Info sharing is a good thing. I knew i wasn't the only one eyeballing these manifolds and wondering what could be done. Thats some good manifold intel Ray. Let me amend the jet package a little. I settled on a 45 Pilot jet. At first the bike fired right up but relied on the choke circuit to run clean at low throttle openings. So that was an easy diagnosis. Pilot jet lean. Pulled out the 35 and went to a 45 Pilot jet and all is right with the world. Nothing worse than a foul running 2 stroke due to jetting; not good for engine life either. Joe

Joe,

Yeah, it's great to have this site to share this stuff at. It only took a few minutes to type, but that manifold info was collected over the course of about 16 months. Maybe now someone will read the results here and get what they need right from the git-go, possibly saving a ton of time and money.

It's great running a Mikuni, and being able to get everything you need easily from multiple sources. Having stuck with the Keihins for various reasons, my situation is somewhat different. The toughest problems I've had with them usually involved replacing one or more of the many discontinued parts they use, which can take forever to find on eBay etc. Serves me right for being a Keihin guy.

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

I replaced my 1979 cr125r's carb with a mikuni 32mm round slide vm. I had to shave the bell down a few mm to get it to fit between the intake and the air filter boot. I eventually found this chart which verified I was jetted in the ball park (wish I would have found it sooner):
Keihin Mikuni equivalent
90 95
92.2 100
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
* indicates you can use 5 higher or lower than the indicated number (Mikuni doesn't have half sizes)


For example, my 1979 CR125R came with a 155 main jet in the Keihin carb. According to the chart, I would have to use either a 290 or 300 mikuni main jet (no half sizes so I have to pick higher or lower). I went with the 290 main and it runs great. I still want to try the 300 to see the difference.

Your situation will be a little more complicated by the fact you went from a 28mm to a 34mm. I tried both a 32mm vm and a 34mm vm in my 125 and noticed a big difference with the same size main jet. The 34mm carb ran richer with the same size main jet and moved all my power to the top end of the power band at the expense of low-midrange power.

good luck and let us know!
nola_fiddy

That's the first accurate Keihin/Mikuni jet conversion table I've seen - thanks! I have two others I found on the Web; one is somewhat usable, but the other's way off.

Here's some info I found this morning online:

Keihin - OEM main jets - Jets are measured using an unknown convention, in steps of 2 or 3.

Mikuni - Made for Mikuni Carburetors, but will also fit Keihin. Available in 2.5 increments. Mikuni uses yet another measurement convention, reportedly based on flow rate. The result is that Mikuni's sizes are vastly different from the other makers'.

---

"We are often asked for a "cross reference" sheet that compares our main jets to Mikuni or Keihin. The fact is you cannot directly interchange the jets for a given size. Many things affect fuel flow through a jet of the same orifice size. The entry and exit tapers of our main jets differ from those of other companies. Due to this, the fuel flow changes as a function of velocity through the carburetor venturi. This means that two jets of equal orifice size will have a different fuel flow curve relative to intake air speed. At a certain speed the two may flow equally, but a change in velocity changes the flow characteristics.

The Dynojet main jet hole size is measured in millimeters. For example, a DJ142 has a 1.42mm hole.

Michael Cory
Dynojet Research & Development"
---

I remember the time I hogged out my CR125's intake for a 36mm Keihin, raised all the ports 1mm, and chopped 4mm off the rear piston skirt. I ended up with a motor that had almost no power below 7,000 rpm.

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

Keihin jets are fuel flow calibrated in the opposite direction (from the threaded end through the orifice and out the fuel inlet end). The number on the jet corresponds to the number of cubic centimeters of gas that flow through the jet when poured in the calibration direction from a height of 50 centimeters. Therefore the operational flow rate does not correspond to the number stamped on the jet.

The flow rates do correspond on Mikuni jets.

Fun stuff!

Are there any updates to this thread?

I'm in the process of trying to wake up an MT250 & would appreciate any advise.

Thanks
~JM~

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