How to set up a Mikuni Carburetor

Moderators: WILD BILL, livewire, 2kidsracin, Pool Man, chili man

How to set up a Mikuni Carburetor

Postby Honey » Sat Jan 10, 2009 9:57 pm

How to jet your Mikuni Carb
HOW-TO Jet Your Carb
This information is from December '96 issue of DIRT WHEELS and

WHAT MEAN JET? : Carb jetting can be easily understood if we understand the basic principles of carb and engine operation. A carb mixes fuel with air before it goes into the engine. When the mixture is correct the engine runs well. The bottom line is a carb must be adjusted to deliver fuel and air to the engine at a precise ratio. This precise ratio can be affected by a number of outside and inside influences. If you are aware of these influences you can re-jet your carb to compensate for the changes. I'm going to show you some examples of how you can change your jetting for better performance and in some cases increased engine life. As with any engine work be sure you have good tools the correct parts and a good manual before you get your hands dirty!

ALTITUDE COMPENSATION: For our first example let's say we find a new riding area WAY up in the mountains. Our jetting is dialed in for our usual riding area which ranges from sea level to 1500 feet. Our NEW riding area starts at 4000 feet and goes up from there. Going to a higher elevation will require will require a jetting change but which way? Like our fuel density, air density can also change. Higher elevations have less air density then lower ones. At high elevations our engines are getting less air, so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down 1 main jet size for every 1750 to 2000 feet of elevation you go up (info for Mikuni carbs). If you normally run a 260 main jet at sea level you would drop down to a 240 at 4000 feet. Something else goes down as you go up in elevation is horse power. You can figure on losing about 3% or your power for every 1000 feet you go up. At 4000 feet your power will be down about 12%-even though you re-jetted!

For our second example let's say we are still at our new 4000-feet elevation riding area and a storm comes in. We head back to camp and ride it out overnight. The next day there's a foot of snow on the ground the skies are clear and it's COLD!. aside from getting the campfire going and making some coffee you should be thinking about jetting again! Cold air is dense air and dense air requires bigger jets. If the 240 jet ran good the day before you will need a bigger jet to run properly today. If the temperature is 50 degrees colder than it was the day before you can actually go back to your sea level jetting, a 260 main jet! If you don't re-jet you can kiss your assets goodbye when you rebuild the seized engine. Air temperature makes that much difference!

Our final example will deal with something often overlooked. We are still up in the hills enjoying our NEW riding area when we notice the old fuel supply getting shorter. No biggie; there's a little store/gas station just down the road. A short trip a few bucks change hands and we are ready to go again. Out on the trail the bikes are running funny, sometimes "pinging" and running HOT. What happened?!?! When we changed jets to compensate for altitude and temperature we were still using SEA LEVEL gasoline. Gasoline sold at higher elevations have a different blend of additives to compensate for the altitude. Generally high elevation gasoline is less dense to compensate for less available air going into the engine and to aid starting. The lighter specific gravity of the high elevation fuel actually "leaned out" our mixture! A 1 to 2 sizes bigger main jet will get us back into the hunt.

If you ride in vastly different areas try to bring enough or your normal fuel along to last the entire ride. It will save you hassles and gray hair in the long run!

PILOTS,NEEDLES,MAINS: So far we have only talked about main jet changes to compensate for altitude, temperature and fuel density. As most of you know there is a pile of jets in a carb. While main jets are the most critical for ensuring full power operation and engine longevity, the other jets are equally as important for a good running engine. Let's run through them quickly. Pilot jets control the low-speed and idle mixtures. Many times an adjustable jet is used in conjunction with the pilot jet. The adjustable jet allows a precise setting of the idle mixture. If the adjustable jet is located to the rear of the carb and usually on one side it is a AIR adjustment. It controls the amount of air that mixes with the fuel coming from the pilot jet. If the adjustable jet is to the front of the carb, on the side or bottom, it controls the amount of air/fuel mixture going into the engine. In either case if adjusting the mixture screw won't improve the low-end running speed it's time for a different pilot jet.

Throttle valves (the slide) control the off idle, to 1-quarter open, mixture. Some aftermarket carbs have replacement slides available with different "cutaways". Changing the cutaway changes the mixture. More cutaway is lean, less cutaway is rich. Some carbs do not have different slides available, so you have to compensate by changing the mixture on the idle circuit or needle circuit. Partial throttle hesitation or rough running can be caused by the slide cutaway. Needle jets control the amount of fuel going by the needle and into the engine at low to mid throttle. There are 2 types of needle jets used in a carb. One is a primary type that has a very precise hole drilled through the middle of it, along it's length. The size of the hole relative to the size of the needle determines how much fuel goes into the engine.

The other type of needle jet is constructed essentially the same except for a bunch of holes drilled into the side of the jet. These holes allow air to mix with the fuel before it's metered into the engine. Either type of needle jet works well in most cases but there is power to be gained on HIGH PERFORMANCE 4-STROKES by going to the needle with the holes in the side. These are called "bleed" type needle jets and produce more midrange power in a 4 stroke. In any engine going to a leaner (smaller) needle jet is the easiest way to re-jet the midrange running when going to higher elevations. Changing the needle jet leans out the mixture evenly at all the midrange throttle settings moving the needle clip doesn't. Jet needles more commonly know as the "needle" control the fuel mixture throughout the midrange. The shape or taper of the needle dictates how much fuel goes into the engine at a given throttle opening. The needle must work in conjunction with the fueling requirements of the engine relative to slide position.

If you have an engine with a strong hit in the midrange the needle will probable have a noticeable reduction in size the slide is half open. Remember it takes fuel to make power and when the engine makes power it needs fuel NOW! If it doesn't get the right amount of fuel it pings or misses. You many have cleared up a little midrange pinging by moving the needle up a notch but at the same time you may have over-richened some other areas. If the problem isn't to bad you won't even notice the rich condition. If the machine stutters before it comes on the power that part of the needle's taper is too small and the only way to cure it is to get a needle with a different taper. Finding the right needle can be difficult so hopefully moving the clip will do the job. Finally the good old main jet comes into play at 3-quarters open to full throttle conditions. Most of you already know a bigger main jet has a bigger hole so it lets more gas into the engine! Pretty simple!!

As simple as it is the main jet is absolutely CRITICAL to high-speed engine operation. Not only does it meter the gas into the engine, it can aid in cooling the engine as well. A properly sized main jet will let the engine make good power for a long time. A 1 size smaller main jet may make greater power for awhile. A slightly rich mixture burns cooler than a lean one so be sure the main jet is big enough! One final note on jets. All of them and the carburetion functions then perform tend to overlap into some other jet's territory. If you mess with one jet, you may have to mess with a few of the others .My best advice is to not change more than one jet at a time. Slowly work out the correct jetting and keep notes on what you are doing. If you get totally fouled up at least you can go back to where you started.

SIGN, SYMPTOMS AND CAUSES: How would you know if there was something wrong with your jetting? If you listen, your engine will tell you! All you need is an interpreter. Since I speak and understand several different engine dialects, I will give you a hand. Let's start with lean conditions because they can cause the most damage. In a lean condition the engine will surge and sometimes ping under acceleration. The engine will also be "cold-blooded" (hard to start and keep running) but will run better when hot. The spark plug will look bone white or burned in extreme cases. The engine may spit back or sneeze thought the carb once in awhile too. If the engine is running rich the throttle response will be fuzzy and not too quick. The engine will burble, miss and blow black smoke. It will start easy but will run funny when fully warmed up. The plug will be dark, wet or fouled (possible all 3!).

Ok so what do you do first to cure the problem? The very first thing is to check and adjust the float level. If it's off one way or another it can throw the jetting off too. Set the float to the specs and retest the running. The next item is to determine a rich or lean condition. Let's say the engine gets hot and doesn't pull well. This is a lean condition so the engine wants more fuel. Stick in at least a 2 size bigger main jet and try it again. If it's better but still not right go even bigger on the jet and try it again. Bear in mind that drastic or sudden changes in jetting usually mean an air leak has developed somewhere in the engine. Find it and FIX IT!! When the engine burbles on the top end come down 1 jet size at a time until it winds all the way down. Don't drop and more sizes! If the engine seems sluggish and lumpy or want to load up on the bottom end the mixture is TOO RICH. Adjusting the low speed mixture screw helps a little but doesn't cure the problem completely. What you need now is a new pilot jet. Go 1 size smaller and try the adjustment again. When the engine runs smooth with the adjustment screw about 1 and a half turns out from the seat you have it!!

IS THAT ALL THERE IS TO JETTING??: There's a lot more to jetting than just stuffing jets in holes and hoping the problem goes away. If you can understand what your engine is trying to tell you when it runs funny you will have a better chance or correcting the problem than someone who doesn't have a clue. When you re-jet, go slowly and carefully until the problem is solved. As a final thought let me remind you that jetting is a lot like life, if you have a choice it's always better to be a little rich!!
User avatar
Posts: 1839
Joined: Thu Dec 25, 2003 5:13 pm

Return to Informational Pages

Who is online

Users browsing this forum: No registered users and 1 guest