Derek's Tech Info- Carbs, Jetting, and How They Work

A useful technical article from the UpNorth Tech Guru, Derek

Jetting and carb work, how I do it, and what the goal is. This article concerns the Mikuni 38mm Cv's common to Ducati's Monster, and SS, as well as most other bikes with these carburetors.

The carb has several parts, broken down into several main circuits, the pilot, needle, fuel screws, main, and the slide. Float height will affect the fuel delivery as well.

The Pilot circuit bleeds fuel at virtually all throttle settings and is responsible for most of the evils found in carb tuning. The fuel screws are used to fine-tune this circuit. This circuits tuning will effect all systems above it (main and needle) It basically bleeds in fuel all the time, and helps on over run by supplying fuel to help cool the piston. The Pilot circuit will cause the part throttle surging (too lean) and plug fouling (too rich).

The needle effects mostly the acceleration and the transition from the pilot to the main jet . The needle is contained in the slide, and goes up and down in response to vacuum form the engine. The spring is used to regulate the rate at which the spring rises and feeds fuel. As the needle rises, its diameter is reduced, and the amount of fuel allowed through is increased. The taper and diameter of the needle effect the fuel delivery as well, although they are not something we can easily change, without changing needles. The material the needle will effect the metering over time, so stick with Titanium for v-twins, due to their big vacuum pulses that pull the needle against the seats, and increase wear. The metering of the needle will affect the main circuit, and will cause a "soft" midrange, when mis-jetted. Either lean or rich, the bike will feel like it slows down in the midrange, and then picks up again as you come onto the main jet (assuming the main is correct).

The slide controls airflow into the engine. The butterfly in the back of the carbs allows the air in, and the slide regulates this acceleration, and prevents the air from stagnating. If the air slows too much (the slide "pops" up) the bike will hesitate, and then recover. The idea is to choose a spring that will control the slide, allowing in enough air, without slowing the airflow by opening the orifice to fast. As a last resort, you can drill a SMALL hole in the slide to slow its rise (by equalizing the pressure a bit), although your best bet is selecting a spring that works for your application.

The Main jets deliver fuel at full throttle and higher throttle positions. It will make the bike pull strongly to redline, or it can cause the bike to hesitate, and refuse to pull strongly to redline. Main jet selection will determine the outright horsepower without really affecting the ride-ability. The main does not have the ability to effect the jetting on the other circuits.

So we now know that:
• The "lower" circuits affect each level above them.
• The needle and pilot, combined with the slide spring effect real world riding
• The main will determine outright HP, but not have much of an effect on part throttle cruise, and acceleration.

What do we do with all this?
• You must sync the carbs
• You must make sure that there are no vacuum leaks (o-rings or manifolds)
• You must select the pilot jet and needle position, and then work on the main jet.
To test each circuit, the bike must be fully warm, and needs to be ridden hard for a minimum of 10 min.

As a brief guideline:

Full throttle
Select the main jet that gives you the best top speed.

Set the needle based on best acceleration between 4 and 6500 rpm.
Part throttle
Pilot jets and fuel screws. 3500rpm part throttle cruise should be smooth, with no surging, softness.

Too lean will cause surging, counter this by turning in the fuel screws out in half turn increments. More than 5 turns out, and you need the next richer pilot jet.

Too rich and the engine will feel soggy and soft. Counter this by turning in the fuel screws, and make the engine become "crisper". If the throttle is "blipped" and the throttle hangs up, and then drops back to normal idle, the pilot is to lean, turn the fuel screw out half a turn. If the throttle is "blipped" and the idle drops below idle and then comes back up, the engine is too rich, and you need to turn the screws in half a turn.

Rules of thumb:
* In general, a rich problem gets worse as the engine heats up, and the converse is true.
* Select the main jet FIRST

* The fuel level in the float bowls will effect the idel and part throttle running, so set them evenly!

* If the engine is flat between 2500 and 3500, raise the needle.
* If the engine is wet or soft between 2500 and 3500, and the problem gets worse as the bike gets    hotter, lower the needle. (if the needle is left to high, you will foul out plugs on the street.)
* The airbox lid should be trimmed (900ss) or removed (Monsters) This affects the intake resonance,    and where the positive and negative intake pulses collide.

Long story short: it will affect your jetting.


Please note that cannot accept any liability for the accuracy or content of this section. Visitors who rely on this information do so at their own risk. If you are unsure it's worth contacting your local Ducati dealer who will be able to help. Do not attempt a repair or modification if you do not have the correct tools or knowledge to do so.