Here’s a chart that can give you some clues about Rich vs. Lean...
Rich Symptoms
- Poor power output
- Starts too easily when cold – requiring little or no choke. Responds readily to the throttle when cold. Runs worse as it warms up
- Hard starting when hot
- Ragged idle …due to “loading up” of unburnt fuel
- Idle that tends toward lower rpms than expected based on the setting of the curb idle screw
- Easily settles to idle, but has tendency to dip below set idle speed then recover …sometimes stalls
- Strong pungent odor of unburned fuel
- Buildup of black, dry, sooty carbon deposits on spark plugs. Bad cases of this buildup will foul the plug completely and kill that cylinder.
- Excessive buildup of dry sooty deposits in the exhaust system
- Responds to throttle, but sluggish
- When fully warmed up, runs much worse when choke is applied.
- Poor fuel mileage
- Black “puffy” smoke during hard acceleration
- Temporarily removing air filter element makes the engine run better.
- Runs worse as you climb to higher elevations
- Fresh engine oil quickly turns black from excess fuel dilution
Lean Symptoms
- Poor power output
- Hard to start when cold …requires excessive choking.
- Lengthy warm-up required.
- Runs better (but not good as it warms up)
- Spark plugs overly clean …with no deposits or slight glazed appearance
- Rough, erratic idle that drifts toward higher RPMs than expected based on the setting of the curb idle screw. Sometimes a lean condition will cause a “hanging” idle that is slow to settle down to set idle speed.
- Backfiring
- Sluggish …hesitates when the throttle is opened, then recovers (often accompanied by a slight backfire)
- Vague throttle response
- Surging at steady throttle cruise operation
- When fully warmed up, runs better when choke is applied.
- Engine runs hotter than normal. Headers can turn cherry red in extreme cases!
- Temporarily removing air filter element makes the engine run worse.
- Unusual “sucking” noises in the intake area.
- Runs better as you climb to higher elevations
- Slight back-firing on deceleration. This normal tendency is controlled by the air cut-off valve(s) operation. If the air cutoff valve(s) is working properly and you still have backfiring on deceleration, something is causing a lean condition.