Basic Carb Adjusting Procedures/Guidelines

- Timing can affect performance.  Know where your timing is set before making carb adjustments.  A couple of degrees can make a big difference in acceleration characteristics.  Properly setting timing may cure ills without getting into carbs.  Gearing can also be a significant factor. 

- Exhaust pipes/baffles MUST be clean.  Clogged pipes can kill top end.

- Excess carbon on pistons can cloud tuning issues.

- Air leaks and/or bad crank seals can affect tuning.  A leakdown test is strongly recommended.

- Low compression can't be compensated for with tuning.

- Change in fuel grades or blends can make tuning requirements change.  It is best to use premium fuel.

- Before making any adjustments be sure the carbs and jets are clean.  That means ALL jets... pilot, needle jet, and main, are removed and inspected/cleaned.  Carb body passageways should be blown out with brake cleaner or soaked in carb cleaner for several hours and blown out with compressed air.  There is a very small air passage in the bell of the carb that clogs very easily and is a major cause of jetting problems. Disassembly video Part 1  Disassembly video Part 2

- Mikuni VM carbs are designed with three main circuits..... idle, mid-range, full open.  Tuning and diagnostics of these circuits is based on throttle position, not RPM.  It is a good idea to mark the throttle with tape at various positions when diagnosing problem areas to make it easier to know which circuit requires attention.


1- Set float level per spec.  This is usually done by inverting the carb and measuring the "float height" in the "at rest" position in reference to the gasket surface with gasket removed.  Precise setting is made when the float tang just touches the needle plunger.  "Fuel Level" can be used when using a special float bowl.  If any carb is leaking from the overflow after assembly this problem must be addressed before continuing with adjustments.

2- Mount carbs and assure there are no air leaks from missing cap gaskets, loose fit, etc.  Set all needle clips to the same position.   ref:  Be aware that some carbs (S series) will allow slides to be fitted backwards and the clamp space may need to be cut wider. 

3- Set carb sync. 

4- Adjust air screws (one carb at a time) for highest idle RPM with a warm engine.  It will require that idle screw is turned to increase RPM for the cylinder to be adjusted.  Air screw adjustment should be made at >2000 rpm.  Adjustment should be made in slow, small increments.  If air screw is out more than 1 3/4 turns for highest RPM, a smaller pilot jet should be fitted.  If air screw is out less than 1 1/4 turns, a larger pilot jet should be fitted.  Return idle to normal and repeat for next cylinder.  Idle RPM should be 1250-1400 RPM. 

The airscrew is a fine-tuning adjustment designed to allow the carburetor to be slightly adjusted for variances in atmospheric conditions. The air screw works with the pilot jet, mainly affecting the engines initial starting, idling and initial power delivery. Proper adjustment of the airscrew can offer direct feed back on pilot jet sizing.  When adjusting the air screw, tightening the screw is richer and loosening it is leaner.

If you get no RPM fluctuation when adjusting the air screw there is a good chance that there is something clogging the pilot system or the wrong size pilot jet is installed. In most cases, the pilot jet should be the same as original stock, ref:  Clean the system thoroughly with carb cleaner and blow out with compressed air.

A lean setting will cause your engine to surge at very low RPM’s, bog or cut-out when the throttle is opened quickly and have trouble idling down.

A rich pilot setting will result in hard starting, plug fouling at low RPM’s, sputtering as the throttle is cracked opened.

    - Note: Kaw specs for air screw setting is just a starting point.  Adjustment is required for best performance.
Each carb must be ADJUSTED for optimum idle. That is done via AIR SCREW adjustment.... seeking the point where idle rpm for that cyl is highest. That is the point where the fuel/air mixture is optimum at idle rpm. Starting from scratch, unless you're extremely lucky, there is no "balance" between cyls or carbs.... one cyl will be pulling the other two. When this condition exists ONLY the carb on the pulling cyl will respond to adjustment. Setting the idle stop to insure that the "pulling" cyl carb is in control of idle rpm will then allow adjustment of that carb to be seen in rpm changes. An alternative is to pull the plugs of the cyls not being adjusted so it would be apparent which cyl is "pulling". If, using this method, an air screw has no effect on idle speed, something is wrong. On a new stock bike setting air screws X number of turns is "close". Changes in altitude, humidity, pipes, or filters and all bets are off.

5- Idle screw balance should be set so that any clockwise movement of the idle screw from any carb increases RPM.

    - Note: Once each carb is adjusted they then need to be SYNCed to deliver power equally at the same time. This is done in two steps... slide position and idle stop screw setting.

SLIDE POSITION must be adjusted so at any given throttle position the slide opening is the same on all carbs. It is accomplished with the cable adjusters on the carbs and should never change once set. It is a static adjustment.

The other SYNC adjustment is BALANCING the carbs at idle so each cyl has equal "pull". That is done by setting the IDLE STOP screws so that any clockwise rotation of any stop screw increases rpm.

Use of a mercury indicator is another way to setup carbs, but the same principles are applicable.

- The aforementioned settings will primarily affect idle and performance thru 1/4 throttle position.

6- Adjust the needle clip position to affect performance in the 1/4 to 3/4 throttle position.  Raising the clip will lean out the mixture.  Lowering the clip will richen the mixture.

When the clip setting is lean the bike will be very zingy sounding . Lean in the midrange will rob  power and cause the machine to run hot and seize easily or even hole a piston.

When the clip setting is rich the bike will have a lazy feeling in the midrange. Exhaust note will be a little flat sounding. In more extreme cases of richness the engine will sputter or kind of crap out in the midrange.

The safest way to set the clip position is to richen up the clip position setting until the machine loses a little power (feels lazy/unresponsive) then lean it back one position. Ideally you like to run the needle setting in either the 3rd or 4th clip position, if possible. The needle clip jetting is especially critical to your bike's reliability because on average more time is spent in the midrange than any other part of the throttle. Most triples pull very hard in the midrange, putting quite a load on the engine. This makes a lean condition very detrimental to your reliability.

7- Main jet size affects 3/4 to full throttle performance.  Plug chops can help determine correct jetting.

The main jet does not effect the jetting for starting and idling. It plays no part on low RPM or mid RPM jetting either. The main jet is very important to your bike's overall tuning, but should never be over emphasized at the expense of needle tuning or other facets of your carburetion tuning.

When the main jet is lean the engine will experience detonation or "pinging". Exhaust note will be of a higher, tinier type note. Engine will over heat easy and can be down on horsepower. A moderately lean main jet can cause engine seizures. A severely lean main jet can cause the engine to burn a piston (hole in top).

When the main jet is rich the engine will be a bit flat or lazy at ¾ to full throttle, giving off a flat, dead sounding exhaust note. When the main jet is severely rich the engine will sputter in the high RPM’s and have a lot of trouble making power up top.

The safest way to get the main jet setting as near correct as possible is to richen the main jet setting up until the engine begins to lose power and sputters at WOT. Then, reduce jet size until sputtering disappears and WOT is clean and crisp.

A plug chop can be done to fine tune main jet size.



- Giving a little "choke" at various throttle positions can help determine if you need to be richer at that throttle position.

- Altitude, humidity, type fuel, premix, filters and pipes can alter jetting requirements.

- Note that use of "kits" will many times cause problems.  Usually, gaskets and float valves are the only parts worthwhile in the "kits".  Jets may or may not be what you need and needle taper is not stock. 

- The float controls fuel level in the float bowl at the bottom of the carburetor. It has no effect on jetting but can cause some symptoms that can be easily confused with a jetting problem. If the fuel level is too low for example, it can cause a bog similar to a lean condition.

- Make sure engine has passed a pressure test. It can be virtually impossible to tune an engine with an air leak. It is imperative that you establish a solid baseline with proper assembly and an air leak free engine. *Note: On a 2-Stroke engine there is absolutely no way to be sure your engine DOES NOT have an air leak without properly performing a pressure test.

- Establish that the engines compression is not weakened. Any loss of compression for whatever reason will give off signs that engines jetting is rich.  Any loss of compression on a 2-stroke engine should be followed by a top end disassembly and inspection of piston, rings, cylinder liner etc.

- Compression Test How To: Always use a quality name brand gauge (SNAP-ON Best Choice). Note length of threaded probe in relation to spark plug length. Perform test with engine cold, throttle wide open. Kick machine over until needle quits moving (10-15 kicks normal). Perform 3 separate tests, record results.  It is always a wise idea to perform a compression test on a fresh engine right after break-in to use for future reference.

- Spark Plug: To properly tune a 2 Stroke engine it is imperative to have a good quality spark plug that is functioning properly. This means that the plug cannot be too old or partially fouled. Spark plug gap is essential; an improperly gapped plug (or partially fouled one) can cause the engine to run poorly, sometimes appearing to be a jetting problem when in reality the problem originates at the spark plug. Also make sure you are using the correct heat range.

- Weak Spark: Weak spark is not only detrimental to your engines performance but can make tuning your engine a nightmare. Weak spark will make the engine run rich. When an engine is running rich due to weak or poor spark the machines performance will slowly continue to deteriorate. Some common causes of weak or poor spark can be a failing or faulty coil winding, faulty plug cap or plug wire, failing coil, improper ground etc.

- Plug Color: Ideal plug color is a chocolate brown. A rich plug will be a darkish brown or black. May be oily and sooty. A lean plug can be a light brown, or gray (some severe cases can be white). Plug color must be checked after a plug check has been run. To perform plug check run engine at specific RPM that reading is desired for at least 5 seconds, then turn off engine without letting RPM’s fall. This test is very difficult to perform at any RPM other than wide-open throttle.

It can take years to learn how to accurately and precisely read spark plugs. For amateurs, plug color should not be the only piece of evidence used to adjust jetting. Plug reading should be evaluated with other jetting evidence to achieve a proper conclusion on tuning.

- Fuel: The fuel used in your triple is very influential in tuning your engine. Premium grade fuel is recommended.  Various grades, brands, and ethanol levels can cause inconsistent jetting feedback and make your engine run hotter than normal.

- Baffles: 2 Stroke baffles must be functioning properly. A clogged, poorly maintained, baffle can cause the engine to run funny making tuning difficult. An excessively packed baffle can also cause a tuning difficulty, making the engine lose power and run rich.

- Exhaust Pipe: Check your pipe for severe dents in head pipe section or end cone area, dents in these areas may affect tuning. Also check for carbon build up inside pipe. Any carbon build up at all is not desirable. Heavy build up will hamper engine tuning and rob power. Pipes with these problems should be replaced.  Pipes must be used that are designed for the engine.  Just because they fit does not mean they are "tuned" for your displacement, etc.

- Air Filter: It is highly recommended to do your initial engine tuning/jetting with a new air filter. A used filter will never quite work as perfect as new one. Old, dirty, improperly serviced filters will severely hamper proper engine performance and tuning. Some aftermarket clamp-on filters can make tuning difficult.  When performance is critical use a new filter.

- One Change at a time: When tuning a carburetor, only make one adjustment at a time. This is a good rule to follow so the tuner will not get confused or mislead from false information. Keeping to one adjustment at a time will help ensure that you will always know what effect each adjustment had on the engine.

- Unresponsive?: If during the jetting/tuning process you have made a number of adjustments (especially main jet or needle clip) to the carburetor and the engine has not changed. There is a strong possibility that something other than carburetion jetting is causing your engine to perform incorrectly. Jetting is a constant. When adjustments are made to a machine with all components working properly the engine will respond in some way. Depending on the adjustment made the bike will either run better or worse, but there is almost always some form of change. When changes are made without any response it is a sign of other problems. Things to check out would include; low compression, weak spark, fouled plug, failing reeds, air leak, clogged air filter, clogged or over packed baffle, etc.

- Proper Slide Throw: It is always a good idea to confirm that the carburetor slide opens and closes all the way. This should be done with carburetor hooked up but not attached, as to physically see slide travel thru its complete stroke.

Fine Tuning

Here is a link to a more detailed tuning procedure not written specifically for triples in pdf format.

Listed below are some tips that may be helpful to fine tune and/or correct carb tuning issues.

* Surging is a common problem with H2's.  Surging is a symptom of a lean condition.  Lots of folks try to cure it with oversized pilot jets but the best fix is to use 2.0 cutaway slides and careful slide sync.

* Slide cutaway- changing slides one size can lean/richen mixtures at 1/8 - 1/2 throttle settings.  The smaller the number, the richer the mixture, i.e. 2.0 is richer than 2.5.  Slides can be built up to reduce cutaway with JB Weld to richen mixture or by removing 0.020" from the bottom of the slide.

* Needles can have washers added beneath the clip to raise the needle less than one full clip length.  Sudco carries special washers for this purpose.

* Sparkplug gap can be varied to optimize power output.

* Air intake to carb body can influence tuning.  Short air filters with blocked ends may cause tuning problems.  Foam UNI Filters have been known to cure some flat spot tuning ills.  Stock inlet tubes smooth air flow and are sometimes better than aftermarket pods.

* UFO's have reportedly improved tuning performance.

* Polishing intake ports is NOT a good thing to do.

* Machining intake adapter ID to match cylinder intake port IS a good thing to do.



John Aylor wrote:

The pilot system is the pilot, air screw and a bit of the slide cut out. At very low air flow through the carb (about to 1/8th throttle). The fuel is fed in front of the slide (engine side, high vacuum side) when there is not enough vacuum to pull fuel up through the needle jet. The airscrew adds air to the fuel to get the correct fuel ratio for idle and just off idle combustion.

Then you get into 1/8th to 1/4 throttle. The pilot is supplying all the fuel it can flow still, but now there is too much air flowing through the carb for the fuel the pilot can supply and the mixture goes lean.  Here is where the slide cut out comes in.  The angle at the back effects the speed of air over the main jet. The higher the cut out the slower the air. A 3.0 cut out, will be leaner than a smaller 2.5 cut out.  Now there is enough air flow over the main jet to pull fuel. So at 1/8th to 1/4 the cut out effects the mixture.  (The "dam" on the back of the needle jet also has an effect, but that's a total different ball of wax and shouldn't be messed with unless you really know how to fine tune carbs).

The pilot system is the easiest to get in parameters.  Get the motor to operating temp, turn in and out the the air screws one at a time to get the highest idle and count the turns out.  Then adjust the pilot size to get the air screw in the proper range and that's it (unless you have a drastic temp or altitude change).

The needle jet and the needle just work on a very simple math "area" principle.  When the diameter of the needle is close to the inside diameter of the needle jet, there is little room for the fuel to flow by the needle. The needles are tapered, so as the slides are raised, the needle diameter gets smaller, increasing the area between the needle and needle jet diameter, allowing more fuel through the needle jet.  And of course when raising or lowering the needle in the slide, this adjusts the fuel through the needle jet at that particular throttle setting.

Then leaving the pilot system alone, you get the 1/4 throttle correct. (basically taking off from a stop) If it burbles or "goes flat" you are looking at needle position and slide cut out.  The needle adjustments are a bit coarse in this throttle area, and of course you are effecting up to 3/4 throttle moving the needle.  When you get the "take off" correct, then you check the rest of the midrange up to 3/4 throttle.

If you have to adjust the needle, then it will throw off the "take off" and a slide cut out adjustment is in order.  Now this gets a bit complicated, getting into different needle tapers, but it seems you are close enough without having to change needles.  So when you get the slide cut out correct for "take off" this also effects the pilot system, so you have to check the pilot system again and adjust again if necessary to get the air screw in the correct range.  All this should be done with a huge main jet installed, to take that out of the system.

And this (on properly set up carbs) takes us up to 3/4 throttle.

By the time you are to 3/4 throttle, the needle is still in the main jet, BUT if you take the diameter (area) of the needle and subtract it from the diameter area of the needle jet it needs to be LARGER than the diameter area of basically a way too rich main jet. That way, when you are jetting the main jet (3/4 to full throttle) the restriction in the fuel delivery system is ONLY the main jet.  You can Lean "seize" a motor with a huge main jet, because the restriction is in the needle/needle jet and not the main jet.

This is why in the Sudco manual they have you start with the pilot system, then mid range and then main jet. When properly tuning a new carb set up, you put in a huge main jet. Then get your bottom and midrange perfect (up to 3/4) throttle and anything higher is just burbly fat. This way you KNOW the needle/needle jet is NOT the 3/4 to full throttle fuel restriction. Then you jet down the main.