When I was learning about carburetors some 45 years ago, the order of
preference when checking for faulty operation was as follows:
- If in doubt do not move anything whose use you do not understand - read
books and get skilled help: start on the outside and work your way in
- Locate your nearest stockist before undertaking any work - things can
break unexpectedly that worked perfectly well before!
- Fuel level in float chamber - too high gives rich mixture - check float is
actually "floating" and has not gone porous or holed if hollow (Common with
SU floats in the late 60's! - I had four in succession on a Spitfire in 1968
inside 6 months). For many years, Japanese carburetors had float chamber
windows to check that the fuel level was correct.
- Float valve not seating - the other part of high fuel level, normally
accompanied by a smell of petrol if the fuel is overflowing
- Fuel pressure too high - will overcome the float valve closing pressure.
Normally caused by use of the wrong supply pump or return line being
blocked. Many people fitted Filter King Regulators as both a filter and
pressure regulator to overcome such instances. Check if the pressure varies
with engine speed if the engine is fitted with a mechanical pump as faults
here could give a problem at high engine speed but be fine at idle
- Crud level in float chamber - any sign of crud in the bottom of the
chamber indicates that there will be more deposited in the jets so a good
blow through with an airline is most likely required, preferably with the
jets out - do not poke the jets with bits of wire or drills!
- Brown staining in the float chamber - this is most likely a stain/deposit
from the fuel and is probably so thin that it is insignificant and can
usually be removed with a dose of Redex or similar in the tank.
- Adjustment - on fixed jet carburetors the adjustment only operates at the
idle condition as it is usually a bypass control on the idle jet. On
variable orifice carburetors such as SU's, moving the main jet affects the
whole of the operating range
- If plugs come out black then there is an excess of HC in the mixture but
is it fuel or oil? Give the engine a run at high load, preferably up a hill
but not necessarily desperately fast then check the plugs as this overcomes
to some extent any influence of oil. If it is oil then you would notice a
high oil consumption if it is burning on the plugs and normally high blow
bye
- Wear in spindles that run in the carburetor normally cause a weak mixture
- Carburetors fitted with accelerator pumps could see a rich mixture if the
diaphragm has failed - pressure test if possible at a lowish pressure, not
an airline
I will see if I can drag some more from my memory but any Haynes manual for
a carbureted car has a good checklist
Good luck,
Hamish
----- Original Message -----
From: "taximark" <[email protected]>
To: "Quantum Owners Group" <[email protected]>
Sent: Saturday, October 23, 2010 10:21 AM
Subject: [Quantum Owners] mixture adjustment.
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