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Scoot F.A.Q.


Q. What are some symptoms of rich and lean conditions?


Rich :
  • Sputtering or blubbering often occurs with excessively rich conditions.

  • The engine may have a flat or lower pitched sound.

  • Performance may improve in cold and dry conditions or as elevation decreases.

  • Performance may degrade in hot and humid conditions or as elevation increases.

  • Performance may improve if restrictions are removed from the inlet path, such as removing the air filter or part of the air box to allow more flow. This is only a test procedure. The carburetor should be adjusted to correct the condition.

  • Performance may degrade if restriction is added to the inlet path, such as partially covering the airbox inlet or snorkel or taping over a section of an open air filter. This is only a test procedure. The carburetor should be adjusted to correct the condition.

  • Pre-mixed two-strokes may smoke excessively or produce lots of residue (this could also be a symptom of pre-mixing too much oil with the fuel). Even four-strokes experiencing very rich conditions may produce some black smoke.

  • In very extreme cases the cylinder walls may become washed down with fuel and you may find that oil in the crankcase smells of fuel and becomes thinner from fuel dilution.

  • Cooler than normal engine temperature. This is especially easy to notice with the use of a cylinder head temperature (CHT) gauge.

  • Poor fuel economy.

  • Dark / black or wet spark plugs.


Lean :
  • Bogging with a sort of "buhwaaa" sound may occur upon application of the throttle.

  • Very lean conditions may cause surging RPM, even to the point that the scooter may not have enough power to move in some cases.

  • The engine may take on a more tinny or hollow tone.

  • RPM may hang, remaining higher than it should be after the throttle is released.

  • Performance may improve in hot and humid conditions or as elevation increases.

  • Performance may degrade in cold and dry conditions or as elevation decreases.

  • Performance may degrade if restrictions are removed from the inlet path, such as removing the air filter or part of the air box to allow more flow. This is only a test procedure. The carburetor should be adjusted to correct the condition.

  • Performance may improve if restriction is added to the inlet path, such as partially covering the airbox inlet or snorkel or taping over a section of an open air filter. This is only a test procedure. The carburetor should be adjusted to correct the condition.

  • Elevated engine temperatures. This is especially easy to notice with the use of a cylinder head temperature (CHT) gauge. Spark knock may begin as temperatures increase.

  • Two-strokes may seize or soft-seize due to the heat from running lean or it could even cause the piston crown to cave in or melt. Four-strokes can seize, but it's not a common occurrence because they are more forgiving and often display much more obvious symptoms than a two-stroke prior to seizing.

  • Popping or backfiring.

  • While riding wide open throttle, quickly roll off of the throttle to about 7/8 throttle. This should create a slightly richer than normal mixture briefly. If performance improves during this period, jetting may be too lean.
  • Light / white or clean spark plugs.


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