How to Identify Mikuni Carbs, Jets, and Needles

Classic Norton Commando Motorcycles.

How to Identify Mikuni Carbs, Jets, and Needles

Postby Mark Cigainero » Mon Sep 13, 2010 8:56 pm

I recently purchased a good used Mikuni Carb on Ebay. It is a 34 mm carb but how do I know what I have. They said it had been on the shelf and was pulled from an 850 Commando?? It is in good condition.The flat square area on the side of the carb just says Z 4?? and printed close to the choke piston on the body is 1W2 with 00 under it?? I think I have the right carb. The needle jet has 159 P-4 printed on it, Main jet has 250 printed on it and the needle says 6DP5 with the clip in the third notch down from the top. The pilot jet is 35. Are these the right jets and needle for a Mikuni fitted for a 74 850 Commando??? I would appreciate any advise and as usual thanks very much for any help.
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Re: How to Identify Mikuni Carbs, Jets, and Needles

Postby geo46er » Mon Sep 13, 2010 9:54 pm

Hi Mark,
Log onto sudco.com and download their catalog (its in PDF form) and price list. They sell the manifold and any other thing you may need for your planned conversion. The jetting you list from your ebay carb is typical for 34mm Mikuni on an 850, although small changes may need to be made for your particular bike, good luck.

GB

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Re: How to Identify Mikuni Carbs, Jets, and Needles

Postby splatt » Mon Sep 13, 2010 10:58 pm

I'm not sure that sudco retail,
If the carb measures 34 in the bore thats what it will be, the needle is a strange number, 6DH3 or 4 is more std, 159 p4 is also a bit large but it may be right with that needle, 159p0 -p2 seems more common, main jet about right, 240 in mine, although one man on the forum uses a 300 in a 36mm carb, I tried a 260 and the engine went flat. Htghly recommend cable choke kit if you get it sorted.

The sudco site lists jets , needles ect, excellent reference.
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Re: How to Identify Mikuni Carbs, Jets, and Needles

Postby Coco » Tue Sep 14, 2010 7:17 am

splatt wrote:I'm not sure that sudco retail,
If the carb measures 34 in the bore thats what it will be, the needle is a strange number, 6DH3 or 4 is more std, 159 p4 is also a bit large but it may be right with that needle, 159p0 -p2 seems more common, main jet about right, 240 in mine, although one man on the forum uses a 300 in a 36mm carb, I tried a 260 and the engine went flat. Htghly recommend cable choke kit if you get it sorted.

The sudco site lists jets , needles ect, excellent reference.


I bought parts directly from Sudco as the retailer that sold me my new carb kit could help me with what I needed. Sudco was very helpful and shipping was fast.
1976 850 Commando Roadster

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Re: How to Identify Mikuni Carbs, Jets, and Needles

Postby pete.v » Tue Sep 14, 2010 8:44 am

Please validate your needle. I believe 6DP5 does not exsist (no such thing). Neither Sudco nor Mikuni nor this site, http://www.psep.biz/store/mikuni_vm_type_parts.htm show a 6dp5. Go to this site and get a 6DH3 and and a couple extra clips. When they go a flying, they're gone. While your at it, pick up a 159 Q2 needle jet. You just may find it useful.

The choke kit didn't work too good for me. The throttle adjusting screw is so easy to use. Once started, first or second kick always, push the choke back down, give the screw a little turn and at the first stop sign just turn it back.
Ker plunck Ker plunck Ker plunck Ker plunck Ker plunck Ker plunck Ker plunck Ker plunck 500 rpm all day.

By the way, I am the one that was using the 300 main on the 36mm VM on a 750. I use the 300 on my 34mmTM now. But that just me. You will find that their is no universal set up for these Mikuni's on a Norton(no two breathe identically, only similurely), but what's stated above should be concidered a starting point. The needle and needle jet setting is what sets these apart. When it's right, it's great.
Another interesting thing is once the needle and needle jet situation is cured, you can change the mains to your preference. As in 300+ for performance and 280 and below for economy. This may be unique to my machine, but I don't think so.
Rarely if ever will you need to be concerned about the pilot.
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Re: How to Identify Mikuni Carbs, Jets, and Needles

Postby RoadScholar » Tue Sep 14, 2010 3:56 pm

I sell these kits based on a 34mm VM carburetor, sounds like what you have, but look on the boss that carries the choke and you will see the the secondary venturi size stamped there (32 or 34--could be larger/smaller) in which case you should have:

Main jet: start at 240
Idle jet: start at 35
Needle: 6DH3
Needle jet: 159 P O8
Slide cutaway: #3

To start with at sea level to 5000 feet. I have sold a lot of these kits and hear very little from the purchasers, unless they live above 3500 feet.

RS
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Re: How to Identify Mikuni Carbs, Jets, and Needles

Postby Mark Cigainero » Tue Sep 14, 2010 9:58 pm

Thanks to all for the help. I will double check all the jet numbers and get back with you--Mark C.
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Re: How to Identify Mikuni Carbs, Jets, and Needles

Postby bpatton » Wed Sep 15, 2010 10:46 pm

6DH3and 6DH4 are the are the ones most people use. 6DH4 is more rich on the taper, slightly more lean before it.

For what it's worth, you might want to replace the float valve assembly on a carb of unknown vintage and use. I have had a VM34 for 15 years and around 35,000 miles. It was fine before the last rebuild but hasn't been right since. It was rich all over, especially on the pilot and cutaway. I was down to a 15 pilot to get the plugs somewhat clean. I checked the float lever tangs for height and checked the valve by blowing into a hose connected to the carb to see if it was sealing and it was. But that wasn't the problem A couple weeks ago I left the petcock open after a ride and found a puddle of gas under the bike the next day. The Mikuni has an overflow vent for the float bowl. So, I put in a new valve assembly. The float needle has a spring loaded pin in it. The spring on the old one was the same height as the new but it compressed with the slightest touch compared to the new, which is much more stout. Swapped it out, went back to the original jets and it reformed the bike to its old self, crisp. Mileage went from around 30mpg to 45mpg. I haven't checked it by leaving the petcock open overnight but it's only a matter of time :D



.........length.......tapers@.....dia@
......................................10mm------20mm------30m--------40mm-------50mm------60mm

#6DH3 62.3mm------22.0-------2.512-------2.512-------2.458-------2.286-------1.948-------1.607
#6DH4 62.3---------- 25.5-------2.520-------2.520-------2.440-------2.258-------1.915-------1.575
Bob

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