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Old 04-06-2008, 10:15 PM
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supr2nr supr2nr is offline
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Default Mikuni VM Carb Question

I need to ask all of the people on this board who have or have had a mikuni carb. I am finding a dead spot on my bike in 4th gear while running at around 60-70MPH. The bike will just spudder loose power, backfires out of the carb, and then the bike picks back up.

Now, to add another level of complexity to this problem. The man that I bought the bike from ripped off the tach so I have no tach to guide what RPM I am at during idle to set the idle adjusting screw correctly. I would get a dwell tester but the only ones that I can find are for larger motors. So that being said maybe I can find out from you guys a few things.

1. What are the correct ways to tune these carbs. I can't really find any good documentation on it.
2. Can you give me an idea of how many turns your idle adjusting screw is at and how many turns your air jet screw is at. That might help me. Maybe if you can give me the amount of full revolutions you unscrewed both screws from being at the fully closed position.

Thanks!
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Old 04-06-2008, 11:49 PM
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piniongear piniongear is offline
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Default RE: Mikuni VM Carb Question

Somewhere around here in this mess I have the instruction sheet that came with my Mikuni 38mm carb that I bought in 1975. If I find it I will post it for you.
Your Mikuni is a round slide 38mm, correct?

The Mikuni is one of the finest carbs I have ever had. There are few adjustments to be made. The main jet is fixed.
The low speed uses a needle/seat.
The metering rod in the center of the slide has 5 notches as I recall. This needle blocks off the main jet when the bike is at idle. As you open the throttle the slide raises, letting in more air and the metering needle raises out of the main jet, letting in more fuel.
The clip on the needle should be in the middle position. Check yours to be certain it is set in the mid notch.

When the Sportster is idling and the engine is turning so slowly it is ready to die, that is about 850 rpm. When the idle speed increases to the point that it bothers you because it seems to be racing, that is around 1600 rpms.
So just adjust the idle to run between these 2 points. You do not need a tach for this because it is not critical.
After the idle speed is set to a nice slow but steady rpm, begin adjusting the low speed needle inward. Turn it in slowly until the engine starts to falter. Stop. Take note of where the needle is at this point.
Slowly back the needle out. The steady idle will come back. Continue to back out the needle until the engine starts to falter. Stop. Make note of where the needle is at.
Turn the needle back in to midway between the 2 points mentioned. You have just made the proper low speed adjustment to your carb! That is about all you can do with the carb. Some people change out the main jets to a larger jet. Wrong move!
The cabs are set up at the carb company to run well and start well. Leave them alone.
Your main jet should be a # 340E and your metering needle should be a # 6DP1.
All of the above is assuming you have a round slide 38mm Mikuni of course........pg
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Old 04-07-2008, 10:58 AM
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supr2nr supr2nr is offline
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Default RE: Mikuni VM Carb Question

Thank you piniongear. Very helpful. Mine is indeed a round slide 38mm carb. And has the main jet and metering needle you mention. My carb only has two adjusting screws one obviously being the idle adjust screw (which is a larger thumb screw) and the other is to what I thought was referenced as the air adjusting screw (which is a very small round screw closer to the air filter), but it appears that I have been wrong and from what you are saying it is the low speed needle.

Sorry, I have never really been a big carb. person. I know its bad but I actually prefer fuel injection, a lot less to go wrong but also a lot less to fine tune.

EDIT: After looking at your gallery pictures PG. Your old Mikuni is exactly like mine. The small screw I referenced as the air adjusting screw is right under the top air filter mount screw in this picture.
http://area51.tzo.com/bperry/gallery...60614_4?full=1


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Old 04-07-2008, 12:44 PM
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Default RE: Mikuni VM Carb Question

You are correct. Here is another shot of the carb I took this morning. The pencil points to the low speed adjustment screw. It is perhaps more accurate to call it the air adjustment screw.
I have always called it the low speed because that is what it adjust.......pg

edit note: Be careful with your parts for the carb. There have not been parts available for this fine old carb since before year 2000.


[IMG]local://upfiles/8331/FF06DF728CCD485DA7CD2D95AFE88790.jpg[/IMG]
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Old 04-07-2008, 01:11 PM
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Default RE: Mikuni VM Carb Question

Thank you very much for all the info piniongear.

Yah I started to notice the lack of parts for it this morning as I was searching for replacement gaskets, needles, etc. Really kind of worrisome since complete new carbs are not cheap.
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Old 04-07-2008, 01:51 PM
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Default RE: Mikuni VM Carb Question

I remember trying to buy a gasket kit in year 2000 for the Mikuni. I finally called Mikuni up on the phone and the guy said they no longer had any parts for it, not even a float bowl gasket. So, as I say....take good care of what you have. That Mikuni is still the best carb I have ever had myself. A good heavy slide. I used to replace Amal carb slides (on my British bikes) very often because they were made out of thin material and would vibrate inside the slide when idling . I would wear the slides paper thin, and then the slides started to dissappear. The result of course was the fuel/air mixture became unbalanced and a new slide was required......pg
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Old 04-13-2008, 10:12 PM
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Default RE: Mikuni VM Carb Question

PG (or anyone at that matter) one last question about this carb. I have noticed lately in the past few weeks that the carb appears to be taking on too much gas. What I mean by that is when the bike is off and the fuel supply at the petcock is turned off and after letting the bike sit for a while I can reach under and touch the bottom of the air filter and my hand has gas all over it.

To me this would mean that the floats are improperly adjusted and its allowing on to much gas and then coming out the filter. But on the mikuni vm38mm you can't really adjust the float level.

Any idea's on that one?
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Old 04-14-2008, 02:03 PM
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Default RE: Mikuni VM Carb Question

anyone?
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Old 04-14-2008, 02:26 PM
Racer_Rick Racer_Rick is offline
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Default RE: Mikuni VM Carb Question

It could be doing the same thing as my S&S. It pushes excess fuel out the carb when it is running & once it's shut off it leaks out the bottom of the breather onto the carb bowl. I usually pull the outer cover & clean it after a long ride.
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Old 04-14-2008, 02:26 PM
 
 
 
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38mm, adjust, adjustment, air, carb, filters, houston, jets, low, mikuni, round, slide, speed, tune, vm


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