Fitment of Correct Needle, Barrel Spring and Carburettor Installation

Preamble

Before the new Skinners Union (SU) carburettor can be installed, the needle and piston spring need to be exchanged. The size (and use) of the engine determines the needle and spring to be used. Making changes to these components in effect, adjusts the feeling of the motor.

I’m not in any way versed in the subtle art of needle selection but fortunately, HNJ Engineering recommend:

  • Needle: BDL
  • Spring: Yellow

I’d highly recommend reading “SU Carburettors Tuning Tips & Techniques” before dismantling anything to change the needle and piston spring. (Brooklands Books: ISBN 9781855202559)

An SU is a delicate instrument and will not respond well to ham fisted or ill-informed tinkering

G.R. Wade

The Parts

In addition to the carb, the only additional parts for this are the new needle and piston spring. I also took the liberty of painting the HNJ Engineering air intake elbow black.

  • SU HIF 44
  • Red (supplied) and Yellow stings
  • New BDL needle and the one supplied. Spot the difference?
  • Intake and fittings
  • Masked off
  • Painted

Installation

Changing the spring and needle is best done on the bench, although it can be done with the carb fitted to the engine.

First, the suction chamber assembly needs to be removed by loosening the 3 machine screws
Once the suction chamber is removed, the circlip on the end of the piston damper can be CAREFULLY removed allowing the piston to slide out of the housing
Needle, piston and piston spring

Now the needle can be exchanged. The HIF44 has a spring loaded jet needle that ensures the correct biased position is obtained. See photo below. Earlier models did not appear to have this spring and manual biasing was required…. when mechanics were mechanics.

Note the needle is biased (off centre) to the needle retainer. The retainer is removed by unscrewing the grub screw on the slide of the piston
Grub screw removed (lower) allows the needle, spring and retainer to be withdrawn. The picture is a bit misleading as the spring should be on the left of the needle holder. The needle passes through the needle holder.
Spring, retainer and correct needle are refitted to the piston rod
The grub screw ensures the brass retainer sits flush with the piston rod (note the bias
Now the correct piston spring can be offered up…
… and fitted back into the suction chamber (not forgetting the tiny circlip)
Needle needs to engage with the main jet when refitting the suction chamber
Fortunately, there’s a guide on the side of the piston to facilitate this alignment
Now the suction chamber can be refitted and the dashpot filled with the damping oil.

With the modifications complete, the unit can be fitted to the engine.

The carb bolts neatly to the HNJ Engineering adaptor using the M8 nuts and washers supplied
Next up, the intake elbow can be attached. Supplied in the kit is a hose tail and blanking plug. The hose tail is for vehicles requiring a vacuum take off for the brake servo. LGL doesn’t have a brake servo so the blanking plug is used.
HIF 44 carburettor now installed on the engine

Closing comments

The carburettor is installed and I’m very pleased with the quality of the new unit as well as the installation kit, but there’s still a few jobs to do before the engine can be started:

  • Modify the throttle pedal
  • Install the throttle cable
  • Modify the cold start cable
  • Make a new air intake duct
  • Install new petrol lines
  • Install new vacuum advance