Everything you need to know about a Mikuni

[mleach72]

After @Cruz Dog posted about tuning his Mikuni on his newly purchased bike, I figured I'll make a thread about getting a Mikuni dialed in perfectly. I'm not an expert on certain areas of these bikes. There are quite a few members here that know much more than I do. That's why I love this forum. An expert is just a question away. One thing I can say with a fairly high degree of certainly is that there are very few people that know these carbs better than I do. I have been using them for over 15 years. I have hundreds of hours of fiddling with them. I think I have read every article written by Joe Minton about them. Joe is widely recognized as the leading expert on these carbs. He worked for Mikuni for 20+ years and wrote the tuning manual. I highly recommend reading the manual. It is good for getting yourself familiar, but I do have some complaints about it though. First, it is about the size of a children's book. It contains a lot of information. This can be a bit overwhelming to first timers. Second, I feel it is a bit vague in certain places. I feel Joe could have gone into a little more detail to help you understand how the carb works. He does this in other articles that he has written. I'm not sure why he didn't do this in the manual. This is going to be a rather lengthy thread, so I'm going to go over each adjustment in a separate post. Please withhold any comments until I'm finished so the steps can stay together. Here we go! I'll start tuning in the next post.

 

[mleach72]

Start with the pilot jet. The job of the pilot jet is to supply fuel at idle and to deliver enough fuel to make the transition to the needle jet. The pilot jet will increase flow up to about 10% throttle. From there to WOT, it will continue to supply the same amount of fuel, but the effect rapidly declines as the throttle opening increases. When cruising at 10-20% throttle, the pilot is supplying a significant portion of the fuel going into the engine. The rest is coming from the needle jet. At WOT, the pilot is flowing the same amount of fuel, but it is a very small amount compared to what is flowing through the main jet. For any particular engine, there are usually 3 or 4 pilot jets that will work, but there is usually one certain jet that performs a little better than the others. No matter what pilot you have installed, the air/fuel ratio (AFR) will be basically the same after each jet is dialed in with the idle mixture screw. The difference between each jet is how the AFR changes as the throttle starts to open. If the pilot is bigger than it needs to be, the AFR will become rich. This will slow the engine response slightly. If the pilot is too small, the mixture will go lean. This also slows the engine response, and you will likely get a carb sneeze just above idle. When the correct pilot is installed, it will maintain the correct AFR for best response off of idle. To test the pilot jet, just ride around normally. Under light acceleration, try to pay attention to how the engine responds as you ease back into the throttle after shifting gears. You will notice that the engine will pick up a little bit faster with a certain size pilot jet. It may be subtle, but you will notice it. Every Mikuni comes from the factory with a 25 pilot jet installed. On a 107, I would try a 22.5 pilot. On a 117, I would go with a 27.5. Those are just suggestions. Every bike is different, but they should be correct or within one size of correct. Adjust as necessary.
Before testing the pilot jet, adjust the accelerator pump so there is a 2mm gap between the arm and rod. 2mm is about 3 full turns of the screw from 0 gap. This will create a delay on the ap. You can open the throttle slightly without engaging the pump. This will isolate the pilot jet and make it easier to judge the pilot without the ap adding extra fuel. This gap might cause a sneeze during quick blips of the throttle. Don't worry about that right now. Try to concentrate on the pilot jet. I will explain the ap adjustment later.

 

[mleach72]

When you think you have the right pilot jet installed, the next adjustment is the needle size. For each carb size (42, 45, and 48), there is a set of 4 needle sizes. All 3 carbs use the same jets, but the needles are specific to each carb size. Make sure you get the correct needle for your carb when changing needles. The 4 sizes are 95, 96, 97, and 98. 97 is the stock size. Each needle has two sections, a straight part and a tapered part. The straight part controls fuel from about 5-25% throttle. The tapered section controls fuel from 25-75% throttle. The only difference between the needles is the thickness of the straight part. The tapered section is exactly the same on all 4 needles. The 98 needle has a slightly thicker straight section. Less fuel is able to pass through the needle jet and the mixture becomes leaner in the 5-25% throttle range. The 96 needle has a thinner straight section. More fuel can pass through the needle jet resulting in a richer condition in that range. The 95 needle is even thinner resulting in an even richer mixture in that range. Changing needles only changes the mixture in the 5-25% throttle range. The mixture in the 25-75% throttle range will remain exactly the same as long as the needle clip position is the same. (I will explain that next). I have found that if you have a mostly stock engine with an EPA style cam, then the stock 97 needle is usually correct. Once you install a performance cam, the motor seems to respond well to the leaner 98 needle. I can't speak for every bike, but the 45mm on my pitbull and the 42mm on my harley both performed better with 98 needle after installing the 585 and Andrews 48's, respectively. I don't really have a good explanation for this, but I think it has something to do with the large overlap in performance cams. Throttle response and fuel mileage improved in both bikes with the 98 needle after the cam swaps. You will know if the 98 needle is too lean. Small throttle openings under 25% will feel unresponsive. Another sure sign that the needle is too lean is that the motor will take an unusually long time to warm up.
I have never tuned a Mikuni that needed a 96 or 95 needle. I suppose it's possible that a very high compression engine with an extremely long duration cam might need one of those needles. An engine like that produces very little vacuum at low rpms due to the long cam duration. It's possible one of the richer needles may be needed so the motor can run properly at low rpm. Otherwise, most street engines will use either the 97 or 98 needle.

 

[mleach72]

The next adjustment is the needle clip position. At the top of each needle, there are 5 slots. An E-clip will snap into one of these slots. The stock position is the middle slot. Moving the clip up allows the needle to drop slightly farther down into the carb. The needle taper is now slightly thicker at any given throttle position in the 25-75% throttle range. Less fuel can pass and the mixture becomes leaner in that throttle range. Moving the clip down has the opposite effect. That lifts the needle. The needle taper is now thinner and the mixture becomes richer in the 25-75% throttle range. The stock middle position is almost always correct. However, if you feel that you are running rich or lean in that area, moving the clip should take care of the problem.

 

[mleach72]

Next is the main jet. It screws into the bottom of the needle jet and is easily accessed by removing the bowl drain plug. At about 75% throttle, the needle lifts out of the needle jet and no longer has any effect on fuel flow. From 75%-WOT, the only thing controlling fuel is the size of the main jet. There are a couple tests in the manual to set the main jet. I never seem to have much luck with them. I just use the "butt dyno". It may take some trial and error, but you can usually feel when the main jet is correct. You can go to a dyno to set the main jet. Any competent operator can set the main jet by seeing which jet makes the most power. If you take your mikuni in for a dyno tune, this is probably all you are going to get anyways. Most operators don't know the subtle nuances of a mikuni to get the rest of the carb dialed in. They will set the main jet, and if the bike runs well, which it probably will, they leave the rest of the carb alone. That works, but throttle response and fuel mileage may not be as good as it can be.

 

[mleach72]

The final adjustments are on the accelerator pump. There are 3 adjustments - nozzle size, pump starting point, and pump ending point. Before starting adjustments, close the pump gap back down to 0 gap. There are 3 nozzle sizes - 50, 60, and 70. Mikuni errs on the side of caution and puts a 70 in all of their carbs. The 70 is meant for bigger engines over 100 ci, so no adjustment should be needed on a big dog. On harley TC88's and 80" evos, drop down to a 60. On sportster engines, drop down to a 50.
The pump ending point adjusts how long the stroke lasts. This is basically the same ap adjustment on an S&S carb. It is located on the top of the carb. It is the headless screw with a slot in the top of it. Unfortunately, on Harleys and pro-street big dogs, this can't be adjusted with the carb mounted. There isn't enough room to get a screwdriver between the tank and carb. I usually just turn the screw down until it is flush with the top of the carb before mounting the carb. That will end the stroke at about 1/2 throttle. With the motor warm, I'll give the throttle a snap. If the engine responds well, I'll leave it there. On chopper style bikes, there should be enough room to make the adjustment with the carb mounted. It will be just like the adjustment on an S&S carb.You will need a very small screwdriver. Turn the screw all the way down into the carb until it seats. This shuts off the pump. Snap the throttle. The motor should stumble. Keep turning the screw out until you get a swift response from the engine.
The pump starting point is a great feature on the Mikuni that other carbs don't have. Adjusting this creates a gap between the pump arm and rod. This gap creates a delay on the pump. You can open the throttle slightly without engaging the pump. This saves fuel. When you are cruising and climbing and descending hills, you are constantly easing on and off the throttle. With a gap in the pump, you aren't engaging the pump every time you get back in the throttle. On an S&S carb, every time you open the throttle, no matter how slightly, you are squirting fuel into the engine whether it needs it or not. From 0 gap, turn the screw in 3 full turns. That is about 2mm. If you are getting a sneeze when blipping the throttle, like when downshifting, turn the screw out 1/4 turn at a time until the sneeze disappears. Only make an adjustment if you are getting a sneeze with the engine completely warmed up. A perfectly tuned Mikuni is going to run a bit leaner than an S&S carb. A sneeze during the warm up period can be considered normal.

Well, that's it! That's everything I know about a mikuni. I swear by them. I think they are a genius design, and I don't think carburetor technology can be advanced any further.

 

[Mickmorris]

After reading all this awesome information & it was awesome my friend! I’m glad I have a S&S super G shorty on all my bikes.

 

[HMAN]

@woodbutcher. This should be a sticky.

 

[woodbutcher]

DONE

 

[liferider]

Apparently you have never seen my Bigdog chopper with a Mikuni 48. Best add on I did to the bike. It absolutely woke this 107 up!

 

[mleach72]

Apparently you have never seen my Bigdog chopper with a Mikuni 48. Best add on I did to the bike. It absolutely woke this 107 up!View attachment 124375

Jonathan, are you sure that is a 48? The cap on the 48 doesn't have the "step" on the left edge. They had to redesign the 48 cap with a bigger recess because the slide sticks out of the carb at WOT. If you put a 42/45 cap on a 48, the throttle won't open completely.

 

[Cruz Dog]

Man that’s a lot of info. I hope I can find someone who can get it right.

 

[mleach72]

Man that’s a lot of info. I hope I can find someone who can get it right.

You can do it. I have no doubt. Like I said before, the mikuni is designed so well that things can be off and the bike will still run fairly well. All you may have to do is a idle screw adjustment for the altitude. If you have any questions, don't be afraid to PM me.

 

[liferider]

Yes I’m positive it’s a 48