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.
