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HS40 Carburetors

(General Tuning Procedures)

Your Mikuni HS40 is itted with the tuning parts we found to

work with a majority of engine tuning combinations. However, the

tremendous number of differing exhaust systems and cams available

for Harley engines make it impossible to accommodate all possible

combinations with one carburetor set-up. You will probably ind that

the HS40 will run perfectly on your engine without exchanging any

parts. But if it doesn’t, you may alter its tuning to suit your engines

needs by following this guide.

There are many more replaceable parts that affect tuning in

the HS40

than in the stock Harley carburetor. With these parts you will

be able to precisely tailor the HS40 to your engine’s requirements

throughout it rpm and throttle setting range.

Each tuning system is easy to modify and diagnose, but only

when you understand what each system does and how it works. before

making any alterations to the HS40, if any are needed at all, read the

section of this manual describing the various tuning components and

their functional range. There is simply no point in attempting to tune

any carburetor unless the engine is completely sound. Valve and rings

must seal properly, the ignition timing must be correct and the spark

plug clean and gapped. Some exhaust system may also make

carburetor tuning dificult. For instance, it is almost impossible to get

smooth responsive carburetion with straight and open pipes. If you

have nay doubts about the condition of your engine, tune and test it

before beginning what could be frustrating and unproductive effort to

ix another problem with the carburetor.

Tuning the Idle Circuit (Pilot System)

The irst step in tuning any carburetor is to get the idle circuit

correctly adjusted. And the irst step in this procedure is to adjust the

pilot screw position for best idle. Mikuni sets the pilot screw at three

turns open during assembly. This is the position we have found to be

right most of the time. If the screw position has been altered, gently

bottom it and re-open to three turns out form the fully closed position.

Next, ride the bike until the engine is at its normal operating

temperature. This may require several miles at highway speeds. If you

have an oil temperature gauge, ride until the oil temperature is at or

near 150 degrees.

With the machine vertical and the engine idling near 1,000

rpm, adjust the pilot screw in one half turn at a time until the idle either

slows or becomes irregular. The pilot screw is now too far and the idle

mixture is too lean. Pause for a few seconds at each half turn to allow

the engine to settle down and give a clear indication of the mixture


Now, begin turning the screws out in half turn intervals until

the engine again slows or begins an irregular ride. Count the turns

between the lean and too rich position.

Set the pilot screw mi-way between the too-lean and too-rich

positions. You may further reine the pilot screw position with further

riding experience but this will be very close to the perfect idle mixture


If you allow the engine to get too hot during the pilot screw

adjustment procedure, the resulting adjustment will probably be on the

lean side of correct. If you have a large fan use it while adjusting the

idle mixture. If you do not have one, you may need to take time out for

a short ride to cool the engine back to normal temperature.

Remember, if the best ride is achieved with the pilot screw

less than 1/4 turn out, the pilot jet is too large or the pilot air jet is too

small. One or the other will need to be changed.

On the other hand, the pilot screw must be more than three

and a half turns out for best idle, the pilot jet is too small (or the pilot air

jet is too large). One or the other will need to be changed.

If the pilot screw’s best idle position is outside the 1/4 to

3-1/2 turn range., the carburetor’s mixture will be either too lean or too

rich as the throttle is just being raised off the idle position. Once you

have a good idle with the screw within this range, you may proceed

to the next stage: turning the needle jet. The pilot air jet is selected

by riding at 15 to 30 mph. If the engine surges or detonated (lean),

the pilot air jet should be reduced in size. Example: 1.1 to 1.0. If on

the other hand the engine misires or there is after burn from the

exhaust (rich), it would indicate the pilot air jet is too small. If the air

jet is changed, the pilot screws should be re-checked and adjusted if


As mentioned earlier, the idle circuit has an important effect

on mixtures up to quarter throttle. However the idle system’s effect on

mixture strength overlaps the effect of the needle jet in this range.

If the idle circuit is incorrectly adjusted, it will not be possible to get

the needle jet correct.

Tuning the Jet Needle

(Tapered portion)

It is unlikely that you will need to change the needle jet from

the one supplied in your Mikuni HS40. However, in case you do, you

should be aware of how it works and how to tell if the one you have is

too large (richer) or too small for your particular engine set up.

The needle jet’s effect on mixture is limited from about 1/16

throttle, where the main system begins to deliver fuel, to 1/4 throttle,

when the tapered section of the needle begins to emerge from the

mouth of the needle jet.

Lean Condition

If the needle jet is too (lean) small, part throttle acceleration

will be lat. There may also be some detonation during part throttle

acceleration, although this can be caused by other factors. A lean

needle jet will result in a slow warm-up.

If part throttle acceleration is lat, install one size larger

needle jet and compare the performance. If acceleration is improved,

leave the larger jet in and take a fairly long ride at steady speeds to

give the spark plugs time to color evenly.

Take a spark plug wrench with you and after a few miles at

steady speeds, stop and remove a plug for inspection. Be careful as

you stop not to operate the throttle. The extra fuel from the accelerator

pump can cause a false plug reading. The body of a spark plug can

be from light gray to brown to dark gray. If the plug body is black and

has a sooty appearance the needle jet is probably too rich and a

smaller one will need to be itted.

Rich Condition

While a black sooty spark plug is a sure sign of richness,

there are others that are a bit more subtle. If your engine responds

crisply, at low throttle when it is cold, chances are the jet needle is

one size larger than it needs to be. Assuming, of course, that the idle

circuit is correctly tuned and adjusted.

Poor fuel mileage is another sign of richness and because

of the way most of us ride our Harleys, the richness is usually the

result of a needle jet that is too large. The color of the end of the

exhausts is a sign of a mixture strength. If the exhaust color is black,

chances are that you can reduce the size of the needle jet.