Round Slide Mikuni Carburetor Secrets

Jun. 17, 2015 By Rick Sieman
The round-slide Mikuni carb came on four generations of dirt bikes.

A huge number of vintage bikes are equipped with the good old round-slide Mikuni carb. In fact, just about every Japanese bike from the ‘60s and ‘70s came with this mixer.

All things considered, the round-slide Mikuni is an excellent carb, with a wide range of tuning capabilities. You can also find jets and parts easily.

Still, many Mikuni-equipped dirt bikes are running poorly, or they are hard to start. A basic understanding of this carb will make life a whole lot easier. And, if nothing else, making your vintage bike easier to start and less prone to fouling plugs will be worth the time to learn a few things.

When an engine is warmed up, it usually re-starts easily. Just open the throttle part way, boot it, and it starts. However, when it's cold, the problem is that gasoline drawn into the engine tends to remain in liquid form and not evaporate.

To create a combustible mixture in the cylinder, some of the fuel has to evaporate and make a vapor. The vapor mixes with the air, and when the ratios are correct it can be ignited with ignition spark.

If there is not enough fuel, or too much, you can kick until you're blue in the face, and the bike still may not start. When only a little bit of the fuel is vaporizing, you have to put in a great, excessive amount of liquid fuel just to get enough of it vaporized to start the engine. That's called choking or priming the engine.

Most Japanese carburetors (Mikunis and Keihins) and some models of the European brands provide starting mixture enrichment in a better way. They have a little lever on the carb body that provides a special starting circuit.

With the engine running, if you put your palm over the carb air inlet you will feel the rush of air through the carb and into the engine and you will notice that it tries to pull your hand firmly against the air inlet opening. The closer you get your hand to the opening the more the air is blocked by your hand and the more suction there is. If you allow your hand to close off the opening completely, the engine will quit running.

What this proves is that any obstruction in the air passage of an engine that is rotating will create a large suction (or vacuum) between the obstruction and the engine.

Carburetor designers make use of that suction to cause starting enrichment. One way is to use a separate little carburetor built right into the main body called a starting carburetor. It has an air inlet on the side of the bell mouth at the front of the carb. From the starting air inlet, air flows through a little passage in the body of the carburetor and passes by its own private fuel jet leading from the fuel bowl. There, the airflow joins with fuel flow through the jet, and both air and fuel flow through a passage toward the outlet of the carb at a point behind the throttle slide.

The primer control lever is called the choke, and when it's closed the starting circuit is shut off. Sometimes, when the temperature is very cold, you may have to use the choke lever for a few minutes while the bike warms up. However, if you leave it on too long, you might end up with that fouled plug we talked about earlier.

Air flowing into the starting carburetor makes fuel come out of the bowl to join the airflow, and this is caused by the high vacuum, or suction, because you are cranking the engine. The piston is moving up and down gasping for air, but it can't get very much so vacuum is high.

What makes high vacuum behind the throttle slide is the fact that you are cranking the engine with the THROTTLE SLIDE CLOSED. Leave the throttle completely closed until the engine starts. If you can't resist opening the throttle with a spastic reaction every time your kicking foot moves down, try putting your right thumb in your mouth.

If you insist on opening the throttle, the vacuum behind it disappears and the starting carb will not work. Most of the hard starting of Mikuni-equipped bikes is caused by this right-hand syndrome.

This is different than a choke primer, in that a choke mechanism is a mechanical slide that lives up in the top of the carb and can be moved down into the main air passage in front of the carb slide. Also, many chokes have a little trap door held closed by a spring. When you're cranking the engine with the choke closed, the door springs open just the right amount to allow the necessary amount of air to come in.

There aren't any special fuel or air passages with a choke of this type. When the choke plate is lowered into the air path, fuel will come up out of the needle jet and enrich the mixture for starting. The position of the throttle slide doesn't make much difference, because this system doesn't rely on a closed throttle slide to create high vacuum behind it.

Once you get the thing lit off, the bike now has to idle. The idle system on all conventional carburetors is exactly like the starting carb, except it doesn't have a lever to turn it on and off. It works anytime the throttle slide is closed (or nearly closed) and there is sufficient vacuum behind the slide to pull idle mixture out through a small hole, which is also just behind the slide - on the bottom of the air passage, usually.

Sometimes there are two holes to deliver idle mixture: one just behind the back edge of the throttle slide and one just under it. As the throttle slide is lifted up, the one under it also starts discharging mixture. The one under it is often called a bypass orifice. Sometimes the two are called primary and secondary idle discharge orifices.

The idle circuit is from an air hole in the mouth of the carb, past a screw that sticks into the air passage, to adjust the amount of airflow past a special jet that draws fuel from the float bowl and then through the discharge hole - or holes - in the main bore of the carb.

The screw that sticks into the idle air passage is called the pilot air screw, or the idle air screw. The jet that limits fuel flow into the idle circuit is usually called the pilot jet or idle jet. Sometimes the air screw and associated jet are called slow-running screw and jet.

Exploded view of a round slide Mikuni carb.

The idle mixture is controlled by three adjustments:

A throttle stop screw which goes into the side of the carb right beside the throttle slide. It limits how much the slide can close. The slide is never fully closed, so some air ducks under it and flows into the engine when you think the throttle is closed.

You adjust the throttle stop screw for the idle speed you want and the idle air screw for mixture strength at that idle speed. These two controls interact and a good adjustment procedure is as follows:

Turn the idle speed up a little higher than you want it to be, using the throttle stop screw. Now turn the idle air or pilot air screw inwards until the engine runs poorly.

Usually turning this screw inwards makes the idle mixture richer, and when the engine begins to stumble it's because of a too-rich mixture. Stop turning the screw inward when the engine acts like it is about to quit running. Start backing it out and count half-turns of the screw as you do it.

The engine should speed up as you back the idle air screw out and then start slowing down as you continue backing it out. When it runs poorly, stop backing it out. Count the number of half-turns you made while backing the screw out from run-poorly to run-poorly. Set the idle air screw halfway between these extremes, or at a nearby setting where the engine runs fastest.

Back out the throttle stop screw to reduce the idle speed to what you want. Then re-check the idle air screw adjustment and reset to the place where the engine idles fastest or smoothest.

The pilot or idle jet should be the right size to allow you to perform the idle adjustment. If you can't get it to idle properly, you might have the wrong size jet. Here's how to tell: let's say you're turning the idle air screw in and it should be making the mixture too rich. The engine speed keeps increasing until finally you have the screw turned all the way in and it's running best right there. By the time you get the screw turned all the way in, the mixture should have been much too rich. So you can figure that the idle jet is too small and needs the next size larger.

If you back out the idle air screw and the engine just keeps running faster and faster until finally the screw falls out of the carb it tells you the idle jet is too big and is letting too much fuel through. Install a smaller (leaner) one.

Some manuals tell you to turn the idle air screw all the way in and then back it out a specified number of turns for a good idle. Usually this is around one or two turns out. If you can't get a good idle with this setting, or close to it, the idle jet is the wrong size.

The idle system is part of the carb's entire operating system. By opening the throttle slide, the main system is supposed to take over and the idle system is supposed to decrease delivery of mixture. The transition between these two systems is critical to good performance when you first crack the throttle, and on up to about one-eighth open.

There are two factors that aid transition from the idle system to the main system. One is the setting of the idle system itself. Some mechanics set the idle as described and then test throttle response by whacking open the throttle. They try slightly different idle air screw settings to see what gives the best throttle response off idle.

The part that affects throttle response coming off idle is the throttle slide cutaway. Most slides have a V-shaped section removed from the bottom of the throttle slide, with the open end of the V facing toward the mouth of the carb. The amount of cutaway is indicated by a number stamped on the slide. A higher number means more cutaway.

More cutaway makes the mixture lean. Less cutaway makes it richer. If your engine hesitates when you open the throttle part way and seems to hesitate briefly before giving you a surge of power, the off-idle mixture may be too lean. Try a throttle slide with less cutaway.

If it stumbles when you crack the throttle wide open, it isn't throttle slide cutaway. Why? Because when the slide is quickly pulled up, cutaway doesn't have a chance to affect the transition.

Now you have most everything you need to know.  There’s no longer an excuse to have your old bike run poorly. Newsletter
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