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Gower & Lee

S.U Carburettors

  1. Checking Mixture
  2. Setting Float level
  3. Jet-gland Replacement & Jet Centering
  4. Pump Contact Breaker Gap

Dellorto Carburettors

  1. Balancing DHLA

Checking the Mixture on S.U Carburettors

Checking Mixture

A. Check for correct mixture by gently pushing the lifting pin up about 1/32 in. ( .8mm )

B. The graph illustrates the effect on engine r.p.m when the lifting pin raises the piston, indicating the mixture strength.

RICH MIXTURE:- r.p.m increase considerably.

CORRECT MIXTURE:- r.p.m increase very slightly

WEAK MIXTURE:- r.p.m immediately decrease


S.U Float Level Setting

If you have any problems with setting the float levels, check all parts for wear and replace where necessary.

H & HD Type (T1 & T2)

The correct way to set the float level is to hold the float chamber lid inverted with needle valve held in the shut-off position by the float arm only .In this position a 7/16" rod or drill bit should be able to be slide between the float arm and the top of the lid.

float lid Fork arm

HS Type (Brass float)

For HS type carburettor's with brass floats carry out the same procedure as above but us a 5/16" diameter rod to check the gap between the float and the lid.

HStype lid

HS Type (Plastic float)

Hold the float lid assembly in the inverted position and measure the gap between the float and the lid. For the early type of plastic float with a metal lever arm the gap should be between 0.125" and 0.187". Small adjustments can be made by bending the metal lever arms. For later all plastic floats the clearance should be between 0.062" and 0.187" but this type is not adjustable.

HS type lid .............................................HS type lid

Early metal lever arm .................................................... Later all plastic float

HIF Type

Hold the carburettor in an inverted position so that the needle valve is held in the shut position by the weight of the float. Check that the point indicated on the float is 1mm +/-0.5mm below the level of the float chamber face.To adjust the float level bend the brass arm which rest on the needle valve. Recheck level and check for free movement of the float.

HIF float lid


Jet-gland Replacement

Jet AssemblyIf a persistent slow leakage is observed at the base of the jet unit ( a mere surface dampness can be generally be disregarded ) it is probable that the sealing glands and large sealing cork require replacement. The jet lever or cap should first be detached from the jet head , the locking screw removed, then the entire jet unit can be withdrawn. All cork seals should be soaked in oil to prevent splitting on refittment. After the jet assembly has been refitted it must be correctly centred on the tapered needle to prevent the piston from sicking during operation.

Jet Centering

When the suction piston is lifted by hand (engine not running ) it should fall freely and hit the " jet bridge " with a soft metallic click - that is, with the jet adjusting nut in its topmost position. If this click is not audible, but is so when the test is repeated with the jet in the fully lowered position , then the jet unit requires recentering on the needle.

  1. Remove the jet lever arm or jet cap depending on type of SU carburettor and adjust the jet to it topmost position.
  2. Slacken off the large jet locking nut until the jet bearing is just free to rotate by finger pressure.
  3. With the damper or oil cap removed, and using a pencil on top of the piston rod, gently press the piston and needle down to the jet bridge.
  4. Tighten jet locking nut.
  5. Lift the piston and check that it falls freely and evenly, hitting the jet bridge with a soft metallic click. Then fully lower the jet and recheck to see if there is any difference in the impact sound, and if there is, and the second test produces a sharper impact sound, the operation will have to be repeated until successful.

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Contact breaker gap

Contact points The blade should rest against the projection on the bakelite moulding when the points are open. The points should make contact when the rocker is mid position. the tension of the blade must not hinder the rocker action. To check the gap 0.030in . hold the blade against the moulding and insert a feeler gauge between pump body and rollers of outer rocker U.

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Balancing DHLA

  1. Disconnect control rod from lever (1) of rear carburettor.Dellorto carburettors
  2. Unscrew throttle stop screw (2) clear of throttle arm (3).
  3. Ensure all throttle plates are just shut by turning balance screw (4) counterclockwise.
  4. Srew throttle screw (2) IN one complete turn. Screw mixture screw (6) Out two complete tuns from the fully closed postion.
  5. Reconnect conrtol rod lever.
  6. Run engine until operating temperature is reached
  7. Adjustthreottle screw (2)to give recommended idle speed.
  8. Adjust each mixture screw (6) in turn to give highest staedy r.p.m., IN to weaken, OUT to richen.
  9. Reset idle speed with screw (2) if necessary.

Use a sychrometer if availble to confirm that the carburettors are balanced by volume. Slight adjustment to balance screw (4) can be made if necessary.
A vacuum reading from each choke tube can be obtained by removing the blanking screws (8). Connections can then be made to a suitable manometer or mercury columns for carburettor synchronisation.

This procedure may also be used on Weber DCOE carburettors

Weber Tuning manual available £14.95 + p&p
It covers operation principles, adjustment settings, installation checks, special application, auxiliary components, exploded diagrams & part numbers for weber performance carburettors.

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