Tuning Wiki
WikiCarburettor tuningHistory

Carburettor tuning

Revision 1

by llandowtuninguk@gmail.com
Publish carburettor tuning for the performance section.

This is a historical revision. Read the current page or return to its history.

Page organisation in this revision
Page type
Technical article
Subject
Performance
Parent
Section level
Applies to
No ECU-specific applicability
Tool groups
None
Manufacturers
None

The carburettor is the mixture control

On an engine without electronic injection, the carburettor is what turns inlet airflow into a fuel quantity. A performance change that increases airflow, camshaft overlap, exhaust scavenging or engine speed will usually make the original needles or jets wrong somewhere in the range. The symptom is a flat spot, a rich stumble, or a lean full-throttle run. The cure is a metering change for that carburettor and that engine, not a jet size copied from a different engine.

Two families cover most of the engines this applies to. A variable-choke carburettor, of which the SU is the usual example, changes the size of the air passage. A fixed-choke carburettor keeps the venturi size and uses more than one fuel circuit because the air speed through that venturi changes with throttle and engine speed.

Variable choke

The 1958 SU workshop manual describes the carburettor as an automatically expanding choke. The piston rises and falls with manifold suction, which itself depends on how far the throttle is open and on the load. As the piston rises it opens a larger air passage over the single jet, and the tapered needle fixed to the piston opens a larger fuel area at the same time. SU's stated reason is to keep the air speed over the jet roughly constant, so the fuel still atomises at small throttle openings without choking the engine at full power. Because that speed stays roughly constant, the manual says a separate idle jet is unnecessary. Mixture is the needle profile, plus the jet height used for idle and for cold starting.

The needle is the main tuning part. SU treats the profile as something chosen for the engine, the climate and the fuel. A stronger or weaker needle changes the mixture across the lift of the piston, not only at idle. Lowering the jet away from the needle richens the mixture; the manual describes that movement as the cold-start enrichment. On a multi-carburettor engine the throttles and the pistons have to move together. Balancing the air before chasing a needle stops one cylinder correcting for another.

Fixed choke

A fixed-choke carburettor has a venturi of set size. Air speeds up in the venturi, the pressure falls, and that depression draws fuel from a jet. At small airflow the depression in the main venturi is too weak to meter cleanly, which is why these carburettors add circuits the SU manual can do without:

  • Idle and progression. Fuel is drawn from small holes near the throttle plate, where the local air speed is high even though the main venturi flow is small.
  • Main circuit. Once airflow is high enough, the main jet and its emulsion tube supply the fuel. Air bled into the emulsion tube helps break the fuel up and stops the mixture going rich as the depression rises.
  • Enrichment. A power valve or a similar circuit adds fuel at large throttle, where a mixture nearer the safe full-load side is wanted.
  • Accelerator pump. A squirt covers the lag when the throttle opens faster than the main circuit can follow.
  • Choke. Cold starting restricts air or adds fuel, in the same role as lowering an SU jet.

Jet, emulsion tube, air corrector and pump size are specified for a carburettor type and an engine. A larger main jet is not a general upgrade. It richens the top end and does not fix a lean idle or a flat spot that belongs to the progression holes or the pump.

When the engine changes

These airflow changes usually require a mixture check across the range, not only at idle.

  • A freer air filter, a larger throttle body or a different inlet manifold.
  • A camshaft with more overlap, which weakens manifold vacuum at idle and can pull the mixture off the idle circuit.
  • An exhaust change that alters scavenging at one engine speed and not at another.
  • Forced induction on a carburettor engine, which reverses the pressure problem: the float chamber has to be referenced to the boosted air, or the carburettor goes lean as soon as boost arrives. That is a different installation from a naturally aspirated jet change.
  • A change of fuel. The SU manual treats the fuel as one of the reasons to change the needle.

Set the ignition and the valve clearances before judging the mixture. A weak or uneven cylinder looks like a carburettor fault. Where a meter is available, an air-fuel or CO reading at idle, at a cruise and at full load tells you which circuit is wrong. The needle charts and jet tables published for a given carburettor are the starting catalogue. They are not a measurement of this engine.

Sources