To test mixture weakening at higher RPM, increase the engine speed in increments
of approximately 300 RPM (using a tachometer if available) up to a maximum of 2500 to
3000 rpm, observing the reading between each adjustment. (Remember that the
instrument may take 15 seconds to respond to a particular adjustment, and a further 1
minute tostabilise).
TheCO level should fall progressively and stay low during the speed
increase.
(7)
The mixture enrichment for acceleration (accelerator pumpfair valve damper)
can be tested by rapid opening and immediate release of the accelerator. Opening to
half throttle should be sufficient. Within a few seconds of thisoperation the Gastester CO
indication should increase before returning to its previous setting. The degree of
increase will vary according to how this procedure is carried out and also with the type of
fuel system. A fixed choke carburettor with accelerator pump will usually give a more
pronounced increase than a variable choke carburettor or fuel injection system.
7.
COMMON PROBLEMS
(1 )The car does not drive well with thecorrect idle mixture setting. This is a common
complaint (it may even occur toa small extent on a new vehicle when the mixture is set to
the lowest of the manufacturers stated CO tolerance). On older vehicles the cause is
likely to be a fuel system fault which creates a weak mixture just above idle speed.
Remedy: Clean the idle jet and idle air bleed jet on fixed choke carburettors. Check
for
needlefjet wear on variable choke carburettors (above 40,000 miles). These are
available as replacement parts. Check operation of acceleration enrichment device.
(2) The correct mixture setting cannot be achieved
Setting is continually too rich.
Remedy: Clean the idle air bleed jet and air passage on fixed choke carburettors.
Check for severe needle jet wear on variable choke carburettors. Check for high fuel
level in the float chamber. Check cold start device.
Setting is continually too weak.
Remedy: Clean the idle jet on fixed choke carburettors. Check needle and jet for
disengagement from adjusting device or sticking on variablechokecarburettors. Check
for air leaks.
(3) The engine misfires at idle with the correct mixture setting.
Remedy: Check for general engine
condition
-
compression pressures, sparking
plugs etc. Check for air leaks, these may cause severe variation in mixture between
cylinders. Investigate mixturequality
i.e. fuel air mixture should be provided to theengine
in a finely atomised form. Faults may be due to partially blocked air jets, prematurely
feeding main jet system due to high float chamber level etc.
(4) The mixture setting drifts
Remedy: Check for leaking float chamber needle valve if CO level steadily
increases with prolonged idle. Check for high float chamber level. Check Gastester
CALIBRA'IION in air, slight drift will occur during extended operation. Good stability
should be obtained over a period of five minutes or more. A variation of, for example 0.5%
CO at 2.0% CO is not uncommon .on an engine which is in good working order.
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