Basic Carb Adjusting

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.

- Mikuni VM carbs are designed with three main circuits..... idle, mid-range, full open. http://www.geocities.com/mraxl/carb/mikunicircuit.html 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.

- 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.

1- Set float level per spec. http://kawtriple.com/mraxl/carbspec.html 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. http://kawtriple.com/mraxl/tips/floatgage.htm "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. http://kawtriple.com/mraxl/carb/carbleak.htm

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: http://kawtriple.com/mraxl/carbspec.html Be aware that some carbs (S series) will allow slides to be fitted backwards.

3- Set carb sync. http://kawtriple.com/mraxl/carbsync.htm

4- Adjust air screws (one carb at a time) for highest idle RPM. It may require that idle screws are backed out to reduce RPM. If so, readjust air screws. Idle RPM should be 1250-1400 RPM. 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.

- 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". Change 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 priciples 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.

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

- 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. http://www.geocities.com/mraxl/carb/jetting.html

- 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.

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.