CARBURETOR AND JETTING ISSUES
See our Mikuni jetting chart here
MIKUNI JETTING - GENERAL
First of all before doing any jetting adjustments, you must make sure that your ignition timing is correct, that your compression is good, and that your engine has no air leaks. You can make sure your engine is air tight by spraying carb cleaner where the carb meets the manifold and at all cylinder gasket joints with the engine idling. If the idle changes you have found an air leak. Once you know that your engine meets all of these creteria, make some temporary marks on the hand grip and the handlebar casting to indicate closed throttle, 1/4, 1/2, 3/4, and full throttle. Install the largest main jet included in the carb kit and ride the bike once through the gears at full throttle. If it stutters and smokes you need to fit a smaller main jet. If the main jet is much too large the engine won't even rev out at full throttle. Keep working your way down in jet size until you have found the largest jet that will let it rev out cleanly. Now do a "plug chop" to test your main jet (check your plug color after a full throttle blast through the gears followed by an instant killing of the motor). You want a milk chocolate color if the engine is being broken in, and a coffee with cream color after breaking in. A short burst through the gears won't hurt an engine that isn't broken in, but don't overdo it, and let it cool somewhat between tests to prevent overheating and possible seizure.
After getting the main jet dialed in, set up the needle by driving the bike at a constant cruising speed and rolling the throttle on and off between 1/4 and 3/4 throttle. Stuttering and a "rat-a-tat-tat" sound at around in this throttle range indicates a needle setting that is too rich, in which case you should lower the needle by raising the clip one notch. Momentary bogging (or in extreme cases stalling out or even backfiring) in this range indicates a lean condition, which is improved by dropping the clip one notch to raise the needle. It is the gap between the needle and the atomizer which determines how rich or lean the engine is running at part throttle, so it stands to reason that a thinner needle will be richer than a thicker one. Furthermore, some needles are thicker or thinner at various points, providing a variety of tuning options for different engine setups. It follows that a needle which is offering good performance at 1/4-1/2 throttle but is too rich at 3/4 throttle can be replaced with one which is thicker near the bottom in order to lean out the 3/4 area. The needles we provide with our kits are generally well suited for most Lambrettas, but occasionally a different taper is required and we stock many other variations. If the proper jetting cannot be achieved with ajustment of the needle, you may need to change the atomizer (NOTE: TMX carbs don't have a separate atomizer, it is cast into the carb body). These are available in a range from lean to rich with a numbering system that typically goes up through the alphabet from N through Q, followed by an even number for even more accuracy (for example P-6, P-8, Q-0, Q-2, and so on). Changing the atomizer has an even effect on the whole working range of the needle.
After dialing the needle and atomizer you now only need to find the correct pilot jet (or "idle" jet). Your goal here is to find a size of jet which will give you a smooth, even idle with the air screw 1 to 1-1/2 turns out, with a minimum of smoking. The pilot jet also has some effect on the way your engine behaves when you are decelerating with the throttle closed. If it bucks on deceleration and hesitates or coughs (or in extreme cases even backfires) when you then try to accelerate, it is too lean. If it stutters and spits and gets "loaded up" when you try to accelerate then it is too rich. This rich symptom is very similar to how your engine feels when you forget to turn off the choke after the engine has warmed up.
JETTING THE 28mm MIKUNI ON PISTON PORTED ENGINES (NO REED VALVE)
It is very important on piston ported cylinders with the TM28 carb that the proper atomizer is being used. It should be of the 169 series (usually 169-P0), and the shield at the top should be trimmed down to 3 to 4mm (the lower it is trimmed the leaner your carburation will be in the 1/4 throttle range). These carbs come stock with a 175 series atomizer, which is only suitable for reed valve engines such those fitted with Imola or TS1 cylinder kits. We always supply the 169 series atomizer with the carb kit when we sell it for piston port use. It is also often necessary to raise the cutaway on the front side of the slide to about 4mm to clean up the jetting in the 1/4 throttle range. This can be done with a rotary flap wheel on a die grinder, being careful to mimick the arc of the original cutaway.
MIKUNI TM24 DETAILS
The 24mm Mikuni has many advantages over cheaper carbs such as a PWK knockoff, including genuine Japanese quality and a full range of readily available genuine Mikuni spare parts. Note that some Italian cylinder shrouds have a little scoop on top, right underneath where the carb sits, and this scoop partially obstructs the Mikuni from fitting correctly. A notch about the size of a quarter must be cut out of the scoop, which will have no effect on cooling. The stock airbox flows plenty of air for the larger carb, but the main restriction is in the air scoop under the seat. For maximum performance we recommend drilling four 3/8" holes in the bottom of the air box and cutting out the dividing wall inside the air scoop. Alternatively you could remove the whole airbox and run the GP hose and UNI filter using connector #0490.F.
JETTING A MIKUNI 35mm TMX CARB:
The main jet is easy-just start big (for most engines around 380) and work your way down until the motor revs out all the way with full throttle applied. From there you can fine tune by checking plug color until you get a nice tan color. You will probably end up using a 260 to 300, but every motor is different!
Adjusting the needle is very easy on the TMX because there is no atomizer to change, just different needles. The larger the suffix number on the needle, the leaner the needle is (a 6EN11-58 is leaner than a 6EN11-57). Most engines wiill use a 57 or 58, then adjust the clip position until you get the smoothest carburation at mid-throttle.
TMX carbs often use small pilot jets, in the 20 to 30 range. Use a jet that gives you a clean idle with the mixture screw turned out 1/2 to 2 turns out. If the engine bucks and knocks when decelerating, the idle circuit is too lean and a larger jet should be fitted.
JETTING THE MIKUNI TM24 FLAT SLIDE CARB
Start by installing the carb with the following settings: Needle: 4DH7, middle clip position Main Jet: Largest provided in kit Pilot Jet: Largest provided in kit With these settings the scooter should start and run fairly cleanly at low to mid revs. If necessary adjust the idle speed screw (the knob-like screw on the left side of the carb as you are looking at installed on the scooter) until you get a steady idle, and you're ready to start the fine tuning:
Take the scooter for a spin and see what it does at full throttle through the gears. No need to wind it out all the way in 4th, just a quick blast up through the lower gears.
If doesn't want to rev out cleanly at full throttle, take out the main jet and substitute the next smaller one.
Repeat Step #1. Keep this up until you find a main jet that allows the engine to rev out cleanly at full throttle. NOTE: If the engine does rev out cleanly at full throttle with the largest jet we provided, you should acquire and install a larger range of jets to make sure that it's not too lean. It is much easier (and safer for the engine) to detect a rich situation than a lean one. We provide jets which in our experience cover the range needed for your particular engine setup, but sometimes the required jets fall outside of this range.
After you have found the proper main jet for full throttle operation, take the scooter for another ride, but this time just cruise around using 1/4 to 3/4 of throttle opening. Roll the throttle back and forth within this range and check for surging and knocking on deceleration, followed by bogging on acceleration. These are signs of leanness, and you should raise the needle by dropping the clip down a notch at a time until the problem goes away. On the other hand, if the scooter decelerates smoothly but then stutters and hiccups on acceleration and only clears out after holding the throttle open for a while, the needle needs to be leaned out by raising the clip. This rich situation will also be evidenced by a lot of wet black residue coming from the exhaust. Occasionally the range of five clip positions will not give satisfactory results. If you cannot get the midrange lean enough, a leaner atomizer (also known as the "needle jet" or "nozzle") is needed. Conversely if you cannot get the midrange rich enough, a richer atomizer can be fitted. Contact us for further assistance if this occurs. (NOTE: TMX carbs have no atomizer, but a leaner or richer needle is all that is needed.)
Once the needle and atomizer are dialed in, all that is left is dial in the pilot jet (or "idle jet") and the idle mixture screw. The process is similar to dialing in the needle, but you cruise around slowly with the throttle barely cracked open, rolling back and forth between about 1/8 throttle and throttle closed. The same symp toms for rich or lean conditions will arise with the throttle closed as they did when you were checking the needle. When you have found the right pilot jet the scooter will carburate smoothly when the throttle is just cracked open from idle, and the engine will idle smoothly with the idle mixture screw opened up 1/2 to 1-1/2 turns (the idle mixture screw is just behind the mouth of the carb on the right, inside a hollow opening which is at an upward angle). The final idle mixture setting can be achieved by backing out the mixture screw until the engine starts to falter, and then turning it back in until it smooths out.
Bearbeitet von marc74, 3. March 2008 - 15:29.