Mikuni Tm40 Tuning Manual10/12/2020
You must have JavaScript enabled in your browser to utilize the functionality of this website.The list of parts shown is specific to the carburetor listed, and should not be assumed to fit similar models, especially older carburetors, and Mikuni carburetors that came as an OE part.
Any product listed that is available in different sizes will allow a selection to be made on parent item page. If you are unsure of your selection, please contact us for assistance. Main system fuel flow is controlled by the needles taper from where it begins to lift out of the needle jet until about 34 throttle. For a better experience, please enable JavaScript in your browser before proceeding. Even though it is not a stock VOL carb, it sure seems the same. The stock VOL Mikuni carb is this: Carburetor type: Mikuni BDSR34 Bore size: 34 MM ID:41F1 Idle RPM: 1100 - 100 Main Jet: 132.5 Main Air Jet: 1.8 Jet Needle: 5E22-3 Needle Jet: P-0M Throttle Valve: 95 Pilot Jet: 27.5 Pilot Screw: 3.0 turns Here is something that may help others as it has helped me. There are three tunable parts in the idle circuit: 1) PILOT JET --- controls maximum fuel flow through the idle circuit. PILOT SCREW --- controls how much fuel is allowed to enter the carburetor venturi. The engine should have a smooth, steady idle with the screw between 14 and 3-12 turns out from fully bottomed (gently). If the engine requires more than three turns out, the pilot or pilot air jet may be too lean. As the throttle is opened the pilot screws position becomes less important than the sizes of the pilot and pilot air jets. A larger pilot jet richens the mixture from just off-idle to 14 throttle. A larger pilot air jet leans the mixture and a smaller one richens it. The pilot jet and pilot air jet have slightly different effects on mixture strengths. These effects are discussed in the General Tuning Procedure portion of this manual. The idle circuit can be adjusted by changing either the pilot or the pilot air jet. A one-size larger pilot jet will have nearly the same effect as a one-size smaller pilot air jet. It is generally easier to change the air jet since it is more accessible. After changing either jet, it is necessary to re-adjust the pilot screw for best idle. MAIN SYSTEM The main system dielivers fuel from 116 to full throttle. The main system becomes the important mixture control from about 14 throttle. The main system has three tunable parts: 1) NEEDLE JET --- controls mixture from 116 to approximately 14 throttle (varies with needle position). JET NEEDLE --- controls mixture from 14 to 34 throttle. MAIN JET --- controls mixture from 34 to full throtle The jet needle has a constant diameter section and a tapered section. The diameter of the needle and the inside diameter of the needle jet form an orifice through which all main system fuel must flow. Until about 14 throttle, the constant diameter section of the needle is within the needle jet and main system fuel flow is controlled by the size of the needle jet. A larger needle jet richens the mixture within its range of operation. Notches at the top of the needle allow it to be raised or lowered. Raising or lowering the needle determines the throttle setting at which the tapered part of the needle is raised out of the needle jet.
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