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Lean vs. Rich?

Experienced wrenches sometimes forget that we operate with a set of understandings that becomes so ingrained as to become second nature. I don't normally rely on gizmos like the K&N Air Fuel Monitor shown on my supercharged GL1000 (above). But since things happen so quickly on that bike, it's a useful and entertaining tuning aid.

Normally I can tell immediately if a bike is running rich or lean by clues that are obvious...at least to me. I forget (sometimes), that others operate with different levels of experience on such matters.

When you are chasing carb gremlins, rich vs. lean is a very important diagnostic determination that can help send you down the correct path to a solution. This purpose of this tech tip is to help readers make this determination with certainty.

What are we taking about here? Briefly, rich vs. lean refers to the relative proportion of fuel and air that is combusted inside an engine...compared to the practical "ideal" proportion.

The simplistic ideal proportion for our purposes is 14.7:1 This is known by scientists as stoichiometric combustion - the ideal combustion process during which a fuel is burned completely with no pesky leftovers. This means 14.7 parts of air for each 1 part of fuel. Ignore for now complexities like air density, altitude, barometric pressure, fuel vaporization, relative humidity, ambient air temperature, how much "non-gasoline" is in gasoline, how we measure "parts" of air and fuel, etc., etc.

Slightly confusing - but the scientific ratio (e.g. 14.7:1) is always expressed as parts of air to 1 part of fuel.

Described from the point of view of AIR:

When you have more air relative to fuel than ideal (ratios greater 14.7:1)...that is operation in the lean range.

When you have less air relative to fuel than ideal (ratios less than 14.7:1)...that is operation in the rich range.

or...

Described from the point of view of FUEL:

When you have more fuel relative to air than ideal (ratios less than 14.7:1)...that is operation in the rich range.

When you have less fuel relative to air than ideal (ratios more than 14.7:1)...that is operation in the lean range.

In the real world, practical considerations like ease of starting, engine heat management, fuel economy, throttle response, drivability, max power output, emissions control, etc. necessitate departures from theoretically ideal mixture ratios. Carbureted engines usually have a range of fuel and air circuits and other strategies to vary the proportions of fuel and air throughout a wide variety of engine speed and load conditions. Electronic fuel injection is of course more capable and clever, but carburetor engineers have developed some very elegant solutions that work quite well!

A good example is the choke (or starting circuits on GL1200 carbs). The choke provides a temporary super rich condition (approaching a ratio of 10:1 or so) which promotes easy starting of a cold engine. The richness is necessary because cold fuel in a cold engine does not vaporize as readily...so the effective richness of a given mixture is reduced.

Incidentally, chokes work not by cutting off air as commonly explained. Rather, they reduce the effective size of the main carb venturi. This generates an increased vacuum signal which can draw more fuel through the applicable fuel circuits.

Here's a chart that can give you some clues of 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.

Notice that the first item on each list above is "poor power output." Any calibration other than ideal...rich or lean will reduce the efficiency and power!

Once you're on the right trail, then you need to verify the rpm range where the problem occurs: idle, transition off-idle, mid-range, full power, or deceleration. This will point you to the circuit(s) or issues which are causing the problem.

Remember:

Rich conditions can be caused by too much fuel and/or too little air.
Lean conditions can be caused by too little fuel and/or too much air.
So, it is imperative to verify both fuel and air circuits (including emulsion tubes).
On early 'Wings, each cylinder is individually carbureted. Thus, you can have 1 cylinder that is over-rich for one reason and another cylinder that is over-lean for another reason!

One example: a very simple and common problem is dislocated or pinched intake runner-to-cylinder head o-rings. This causes a big vacuum leak that permits extra "false air" to enter that cylinder resulting in a very lean condition.

Here's a list of the typical reasons for lean and rich situations.

Keep in mind that ignition problems can mimic certain carb problems. Always verify ignition function thoroughly before delving into carburetion!

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