Vacuum Drop Test - SU Carburettors

SU carburettor suction chambers and pistons are furnished as matched assemblies from the factory. There is a controlled clearance and thus controlled air leakage between the piston and suction chamber bore.  A convenient means of checking this is a vacuum drop test. The proper damper should be in place. Chamber and piston should be clean and dry, check piston for any points of drag or interference over full travel. Plug holes in the bottom of piston with windshield caulking (commonly referred to as Dum-Dum) - any other suitable compound which is light can be used. Spray piston rod lightly with WD-40.
Turn Assembly Upside Down with piston against top of chamber, carefully holding needle allow the chamber to slide down and off the chamber as you measure the time this action takes.
Specified Times Are:
1 ¼" and smaller carbs
3 to 5 seconds
1 ½" to 1 ¾" carbs
5 to 7 seconds
2" carbs
7 to 10 seconds
If vacuum drop time is too fast, the carb will tend to run rich, if drop time is too slow carb will tend to run lean. If both carbs are too fast there is nothing you can do to correct, chances are someone has sanded the inside of chambers to clean them (this is a 'NONO'). If they are too slow you can very carefully polish the piston or chambers or both (but be very careful) and recheck frequently for compliance. If one carb is fast and the other slow, there is a good chance the pistons have been interchanged. Try switching them.
It is ideal if both are alike and right in the middle of the spec, but that doesn't happen very often these days. If you have carbies that are just below the spec don't fret, tuning adjustments can account for this - but if they are say 1 second or so, I would hunt around for some different chambers.
Of special note if you do interchange pistons, it will be necessary to re-centre both jet assemblies.
Ideally this type of check should be done during complete carby rebuild.

Changing Polarity of a Lucas Dynamo Circuit

  1. Disconnect the battery
  2. Swap the connection of the ammeter (if fitted)
  3. Check that the electric fuel pump can operate with negative earth (some later models of SU pumps have polarity sensitive circuits.)
  4. Disconnect voltage regulator terminals 'D' and 'F'
  5. Reconnect the battery - positive to supply and negative to chassis
  6. Stroke the field terminal wire (disconnected from terminal 'F') across the regulator terminal 'A' at least 5 times creating a reasonable spark at about 1 second intervals - this action ?flashes? the field windings and changes the dynamo polarity.
  7. Reconnect the 'D' and 'F' regulator terminals
  8. Start the engine, check operation of charge light, and ammeter (if fitted)
  9. Enjoy your driving.

Austin Healey "C" Series Engine Oil Traces in Cooling System

The Problem

Even the most youthful 6 cylinder Austin-Healeys are now well over 30 years old, as time passes a weakness in the design of the engine blocks is becoming more apparent. There is an area within the block where the oil galleries and the water cooling passages are very close to each other. If at any time in the past, the engine has been neglected or not set up for long time storeage, it is possible for the block to  corrode to the extent that this area becomes porous, allowing the oil, being at higher pressure to enter the coolant passages. The problem usually manifests itself as an oily sludge in the radiator. Unfortunately freshly rebuilt engines, in which all the scale and corrosion has been removed from the coolant passages ( and the oil pressure restored) are particularly prone to this occurrence. In the past this has meant that an otherwise sound engine block has had to be scrapped.

The Solution

The solution to this problem is to reinforce the areas of the block that are susceptible to this corrosion with seamless stainless steel tubing. This then allows the repair of otherwise unusable blocks and will prevent this problem occurring in blocks which currently appear sound.

A Repair Kit

An oil gallery repair kit usually contains the following items.
Stainless Steel Tube 9/16" O.D x 4 1/8"
Stainless Steel Tube 7/16" O.D x 5 3/8"
Press Tools
3/8 BSP plug
Copper Washer

Fitting the Block Repair Kit

This kit requires access to equipment that is capable of drilling relatively deep holes within fine limits and as such DIY enthusiasts are advised to entrust this work to an experienced engineer. The two locations that require attention are the main oil gallery feed drilling and the cylinder head oil feed.

Preparing the Block

Once the engine is stripped it is recommended that the block be thoroughly degreased and that the waterways are de-scaled using an acid solution.


Using a 7/16? (11.11mm) diameter drill with a working length of at least 5 ½? (140mm) open out the cylinder head oil feed to a depth of 5.375? +/- 0.020 (136.53mm +/- 0.25) taking care that this remains concentric with the existing hole and perpendicular to the head gasket face.  Using a 9/16? (14.29mm) diameter drill, again with a working length of at least 5 ½? (140mm). Open out the main oil gallery feed to a depth of 5.375? +/- 0.020 (136.53mm +/- 0.25). Care should be taken to ensure that this hole remains parallel to the head gasket face
Note - It is important to drill to these tolerances in order to avoid breaking through to the main oil gallery and to ensure that the orifice to the centre main bearing remains unrestricted.

Re-forming the thread for the oil gallery plug

The thread for the replacement oil gallery plug is 3/8? BSP. The hole will need to be opened out to a diameter of 0.60? (15.25mm) to a depth of 5/8? (16mm) prior to tapping.
Once these operations have been completed it is VITAL to ensure all traces of swarf have been removed from the oilways. It is recommended that a high pressure air gun be used for this purpose.

Fitting the Stainless Steel Tubes

It is recommended that before any sealing compound is employed, the tubes are pressed a little way into their holes in order to check the fit. The tubes should be a reasonable press fit. If a great deal of effort is required it is recommended that their diameter is reduced a little with emery cloth until a reasonable fit is achieved.
Both tubes should be fitted using the same technique. The sealing compound (Loctite 640 in this case) should be applied sparingly (according to the manufacturer's specs.) to the tube, leaving the last 1/8? (3mm) of each end dry. The tube should then be driven home either with a hammer or press using the tool supplied. Once the tool comes up against the face of the block the tube will be in the correct position. Any excess sealing compound should then be removed and the new plug and washer can be fitted in the normal way.
It is also recommended that once the kit has been fitted and the sealing compound allowed to fully cure, the cylinder block oilways be pressure tested at about 150 psi (10 Bar) to ensure the repair has been completely successful and that there are no other flaws in the block.