Tuning Info

Discussion in 'Performance Mods' started by jaguar, Dec 7, 2013.

  1. jaguar

    jaguar Well-Known Member

    good info from http://www.groupk.com/tec-carbs97.htm

    Reading spark plugs - Determining proper fuel mixture by inspecting the color and condition of the spark plugs can be very helpful in situations where the engine is being operated constantly at full rpm under full load. "Reading plugs" for perfect fuel mixture is very common in high speed auto and motorcycle racing where the engines are nearly always run at full rpm and full load. Closed course personal water craft racing, however, requires as much "partial throttle" operation as full throttle. Furthermore a pwc racing engine seldom experiences full steady loads because of the rough water conditions. This means that spark plug readings, done on a pwc that is being ridden on a rough water course, has very questionable accuracy.

    To get an accurate plug reading on a pwc, a fresh set of spark plugs should be run in the machine for 3-5 minutes at full throttle/full rpm on relatively smooth water. At the end of the full throttle running, the throttle should be chopped and the kill button pushed simultaneously (called a "plug chop"). If the engine is run at partial throttle for even 3 seconds after the full throttle run, the plug reading will be invalid.

    After the full throttle running, and the plug chop, a combination flashlight/magnifying glass must be used to view the carbon deposit at the base of the porcelain (down inside the spark plug where the porcelain insulator and outer steel spark plug casing meet. A ring of dark brown at the base of the porcelain denotes ideal fuel mixture, light brown is lean, and a ring of black is over rich. This is the only area of the spark plug that accurately indicates fuel mixture. Furthermore, this reading only indicates full throttle fuel mixture. No part of the spark plug can indicate low speed or mid range fuel mixture. The upper part of the spark plug porcelain (by the electrodes) is often very light or white in color, however this coloring is mostly affected by additives in the gasoline and oil. The coloring of the end of the porcelain in no way indicates appropriate fuel mixtures of any throttle range. The cosmetic appearance of the spark plugs can defiantly help a pwc mechanic to quickly diagnose the symptoms of a major operational problem. But as far as carb fine tuning for personal water crafts is concerned...reading plugs qualifies as a very questionably accurate way to fine tune the carbs. Very few professional PWC engine builders recommend their customers to do carb fine tuning based on plug readings...and even fewer engine builders do it themselves.

    The weather - Weather and altitude can defiantly be a factor during fine tuning. The factors that will require you to go leaner are, higher altitude (changes of 1000 ft. or more), higher temperatures (changes of 20F or more), and higher humidity (changes of 20% or more). It seems that the big changes in weather are what actually affect the mixture.

    Air leaks - The lower end of a two cycle engine must be air tight to about 10 psi. If there are any minute air leaks at a crank seal or a gasket surface, tiny amounts of air will intermittently leak into the lower end and cause a temporary lean condition. As a matter of reality, about 50% of the engines on an average race lineup have an air leak. Most of those leaks are not big enough to cause chronic hard-starting or piston seizures, however they are usually big enough to cause on-going jetting problems.

    As the castings of an engine expand and contract with heat, so too can the air leaks change to admit greater and lesser amounts of outside air during operation.

    Group K offers an inexpensive pressure test kit that allows you to quickly check for, and locate, any potential air leaks your engine may have. An engine with a small air leak will never carburate consistently. Remember...air leaks never get smaller.

    Reeds - If your reed petals are chipped or frayed in a way that does not permit perfect sealing, the low speed and mid range circuits will be very difficult, if not impossible, to set accurately. Damaged reed petals will cause a false low speed rich condition, not to mention hesitations in mid range that you will not be able to carburate out. Installing aftermarket reeds will often require significant changes in cab adjustment.
    SANDSA likes this.