Mikuni Motorcycle Carburetor Theory Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure.
Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch PSI. This means that atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow. Atmospheric pressure will force high pressure to low pressure. As the piston on a two stroke engine goes up or goes down on a four stroke enginea low pressure is formed inside the crankcase above the piston on a four stroke.
This low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and engine until the pressure is equalized. The moving air going through the carburetor will pick up fuel and mix with the air. Inside a carburetor is a venturi, fig 1.
The venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carb.
The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more.
The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor. Most motorcycle carburetor circuits are governed by throttle position and not by engine speed.
There are five main metering systems inside most motorcycle carburetors.Here's how to use it: Enter all the data in the light blue data boxes. If you hover the mouse pointer over a cell with a red corner then a message will pop up telling you about that cell.
1. TM28-418 28mm Mikuni TM Carburetor
Below the screenshot are the links to the needle and needle jet info that you will need to know, such as the needle jet ID of your carb. Starting at row 53 are all the VM needles and their data. Here's the sequence you need to follow: 1 First make sure your real life idle jet is correct.
If it is less than that then you need a bigger idle jet. If it is more than that then you need a smaller idle jet. The spark plug color should be light chocolate brown or medium grey. Too dark means the jet is too big. And the top of the piston should be black except close to the transfers and boost port. If it is all black then the main jet is too small, and if it is too clean then the jet is too big.
For small carbs you may need to lower the needle shroud if one main jet size is too rich and the next smaller size is too lean. In that case put in the lean jet and then lower the shroud 1. Also get the idle slide area to put at C Put 0 in the cell not used because the graph will crash if both jet 's are there. The ID is imprinted near the top of each needle. Data for a custom needles or ones not listed on this sheet can be entered at rows to You can use the manual method described on the previous page.
At D28 the slide height is calculated so you can use that. Tht depends on the needle specs as well as the data you enter at A32, B32, A34, BAir flows faster and smoother through the TM Series venturi due to the flat slide design and the jet blocks which help create smooth bore effect. The high velocity of the air flow means a stronger vacuum at the needle jet providing more precise metering and better throttle response. Performance features for the Mikuni TM Series Carburetors include a large selection of bore sizes for increased fuel mixture flow to match engine modifications.
And the TM Series offers a wide range of tuning components to allow precise fuel mixture metering in any application, at any throttle setting, under any riding conditions. These kits consist of the suggested jets for changing that are not matching those factory fitted in the TM, they include an extra one size up and one size down main jet for tuning the main circuit due to fuel quality variations, altitude and climate. Testing and conformation of correct main circuit jetting can only be done by physically changing jets and WOT testing.
These tuner jet sets have been determined from thirty years of experience, customer feedback, plus research and development as the Australian Mikuni distributor. From time to time these may not be exactly right for every engine, usually due to changes to porting, compression or numerous other things that contribute to the requirement for additional fine tuning but as a rule our kits are spot on or a very close and useful starting point, but we give no guarantees and sorry do not release kit details or jet size information.
If you have your own spec simply purchase the carb and buy any other required jets from the jet pages.
The picture with the cable shows the required length of inner cable protrusion and securing fitting at the carb end on basically a minimum length adjustment. The back board is divided into 1 and 10mm increments. All international buyers are responsible for the payment of their domestic customs duties and taxes charged on imported goods. Please check your tracking on all international shipments within 7 days to follow their progress and determine if it is being held by your local customs dept.
Your email address will not be published. This site uses Akismet to reduce spam. Learn how your comment data is processed. The back board is divided into 1 and 10mm increments All international buyers are responsible for the payment of their domestic customs duties and taxes charged on imported goods. Reviews There are no reviews yet. Menu Search.Skip to main content. Related: mikuni vm28 carburetor mikuni vm30 mikuni 28mm. Include description.
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Buying Format. All Listings. Buy It Now. Item Location. Canada Only. North America. Delivery Options. Free International Shipping. Free In-store Pickup. Free Local Pickup. Show only. Free Returns. Returns Accepted.Mikuni Carburetors should be replaced, repaired or tuned by the selling dealer, or a trained or qualified race mechanic.
This marking on a jetting part is your quality assurance guarantee it is a genuine Mikuni component. For tuning procedures we have a number of manuals available to the left. This is perhaps one of the best written book on carb tuning, just ignore the accelerator pump section if using a non pumper carb and remember that the mixture screw is an air screw where as on carbs with the screw located behind the bowl it will be a fuel screw in which case winding out is to make richer not leaner.
By following the procedures in the book you will determine if jets need changing and in what direction. Order required jets as your checks tell you. Please positively identify your jet type in the jets section, as all jet sales are final with no refunds.
Remember the tuning order is — idle circuitthen main jet and then finally mid range. If at all in doubt, see your local tuning shop. Then count the turns from this point to bottoming out, if you take less than 3 and more than 1 complete turns to achieve bottom, the jet is correct and you simply wind back to the centre point. Job done. If the jet is wrong, either having no stumble in one direction or the centre point being out of the range this will let you know that a jet change is required and weather you need to order richer or leaner jets.
Nothing else other than the pilot jet has an effect at idle. Some engines will actually have two size jets that may fall within the spec in which case see which feels better when riding, generally the richer one. At wide open throttle only the main jet is working so these are also very easy to tune with a few jets and a bit of spare time.
The performance changes can be detected at wide open throttle in low gears- we do not recommend the operation of motorcycles over the speed limit. Change and test with the next size jet both up or down to determine weather an improvement can be obtained and in what direction.
If you notice an improvement in one direction, keep changing to the next jet size in that direction till the improvement stops and she drops off the perchthen go back one.
Never drill jets and do remember that most aftermarket jets do not flow correctly. Generally if you have the right profile needle for your engine, a difference in performance and response can be detected by raising and lowering the needle. If improvements appear in one direction of adjustment but perfection is not achieved whilst having reached the end clip on the needle, then you will need to install a richer or leaner needle if available or else a corresponding needle jet.
Once having made a change its back to the middle clip position and start all over again.Idle mixture adjustment (air OR fuel screw adjustment) Carburetor
An inability to tune out a problem in the mid range will indicate that the needle profile is not matched to your engine or a slide change may be required. Never change main or pilot jets to try and correct a mid-range problem.
Menu Search. Good luck!TM Series Carburetors provide significant performance improvements over older design round slide carburetors. Air flows faster and smoother through the TM Series venturi due to the flat slide configuration and the jet blocks which help create a smoothbore effect.
The high velocity of the air flow means a stronger vacuum at the needle jet, providing more precise metering and better throttle response.
While the sku of this part is VM, it shares the typical characteristics of the TM series flat slides. Many people call this the TM28, but the sku is not a mistake.
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