Engine Fault Finding With Colortune - Gunson COLORTUNE G4074 Mode D'emploi

8. Engine fault finding with Colortune

As Colortune allows you to view the burning of the fuel air mixture, most of the faults
observed during its use will be mixture / fuel system related. However, there are other
faults which affect efficient combustion and these will also be easier to diagnose. For
example, an engine misfire which occurs despite the mixture being correct can be
identified as an ignition or compression fault.
The following information gives a basic guide but operator experience and other
published information will be invaluable in identifying the causes of observed
combustion problems. These will vary with specific fuel systems and other engine
equipment.
Always determine the extent of the problem, for example:
Is the fault affecting all cylinders, multiple cylinders or just one cylinder?
In the case of multiple cylinders what is common to these?
(Two together may have a head gasket leak between them or be fed by one of twin
carburettors).
Is the fault mainly at idle speed, low throttle or wider throttle opening?
AT IDLE SPEED
Symptoms
Blue combustion flame is
not achievable when
adjusting idle mixture
Yellow combustion flame is
not available when adjusting
idle mixture
Blue flame turns yellow
after a prolonged idle
Blue combustion flame is
not constant, intermittent
yellow at all or most
settings
Blue combustion flame is
not visible (non adjustable
systems)
Fault details for different fuel systems
Fixed Choke - Blocked idle air jet / idle jet loose
Variable Choke - Wear on needle / jet
Injection System -Senses high air flow, air flow device faulty / vane
sticking, throttle position sensor / switch. Cold start device or
temp sensor
All systems - Manifold air leak. Check other cylinders to help
pinpoint position of leak.
Fixed Choke -Blocked idle jet / idle air jet loose
Variable Choke - Needle loose or shoulder set low, sticking air
valve.
Injection System - Air leak or idle air valve, MAP sensor fault
(detects low air / manifold pressure).
Fixed Choke and Variable Choke - Leaking float chamber needle
valve.
Variable choke - overheating causes rich idle. Check inlet manifold
insulator block fitting.
Fixed Choke -Blocked idle air jet gives erratic idle fuel discharge.
Float level high gives main circuit "drip" feed
Variable Choke - Float level high. Advanced ignition timing
producing engine instability.
Injection System - Oxygen sensor response slow
Injection System - Oxygen sensor / connection or control failure.
Air leak to exhaust manifold or to inlet manifold if confined to
most cylinders but not all cylinders.
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