Q. What are some symptoms of rich and lean conditions?
Rich :
Lean :
- 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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