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The cooling system on a Lagonda is under real pressure, in our experience it is very difficult to get this system working really well as the heat can't 'escape' from the underbonnet very easily, this causes lots of problems with fuel vapourization & overheated electrics, coil & dynamo, etc.
Below is a little advice that can help alleviate this, within this section are parts that can help.
Solutions: Firstly, get the radiator reconditioned with a high performance core - not flushed out or flow checked - that doesn't work, never has & never will !,
or have a new radiator made in alloy which is much more efficient, this transfers more heat to the same amount of air being pushed or pulled (at or near idle speed) through the radiator,
Once you have sorted the radiator make sure that all the area in front of the radiator is 'ducted' to force the all of the air coming through the grill through the radiator with no air escaping around the sides, over the top or under the rad.
2nd, Is to make sure your thermostat is working properly, the thermostat (stat) works in 2 ways, when cold the thermostat bypass is open but the stat is closed thereby bypassing the most of the coolant around the engine but not the radiator, as the engine warms the stat opens once it gets to its predetermined temperature, normally between 75° & 88° depending on climate, the stat blocks off the bypass at the same time allowing all or most of the coolant to flow through the radiator & then cools the engine properly.
If you remove the stat without blocking the bypass pipe the coolant flow is probably about 50% through the rad & 50% around the engine only, with the rear 2 cylinders getting very hot indeed,
The original type, correct thermostat for both 2.6 & 3 ltr's is the same as a Rolls Royce & Bentley part, these are between £147 & £174 each, however we make an adaptor that easily fits in place of the original stat, this is a one-off purchase which uses a Jaguar stat for an early XJ6, this is about £5 with 3 different temperatures.
3rd, Is to flush the block of all rubbish that has accumulated in the last few years, there are several products on the market to do this, be careful though as its easy to damage the delicate old alloy components on the engine / cooling system.
4th, Is to increase the efficiency of the cooling system & fit an internally shaped alloy water pump cover / fan boss, this knocks off about 10°c off the running temperature without a thermostat & restrictor plate- so a lower setting stat will then work, say 75°c, whilst the pump cover is off make sure the impellor is only 0.015" to 0.020" from the timing cover or even better, fit one of our high performance impellors, these work better at the slower engine speeds a Lagonda engine should run at (than an AM / DB engine) as the 'blades' are shaped to pull more coolant through the engine than the STD part.
Whilst on cooling systems, Don't use one of the Performance Coolant mixes, these will damage the delicate aluminium components in the engine, which are already over 55 years old, they may also release collected debris from around the cooling system & move it into the radiator core. Just use soft or distilled water & a good quality ethylene glycol antifreeze with inhibitor & change it every 2 to 3 years giving the system a good flush at the same time.
5th, Overhaul the distributor & change the distributor advance curve for a more modern setting (less advance at upper rpm), & acurately set the ignition timing at 750 rpm idle to be about 15° BTDC & to 38° BTDC max advance at 4000 rpm, with no vacuum advance connected as this can lead to piston or valve failure.
6th, Make sure that the engine is at its peak efficiency with carbs set up correctly, well balanced with no wear in the spindles, jets, needles, etc.
7th, Make sure that the brakes aren't binding & are releasing properly, especially a problem when using silicone brake fluid.
8th, Just make sure that the transmission isn't seizing or something odd.
Lastly, you need to get the air out of the engine compartment or have a flowing airflow, to allow some air out of the engine bay remove every other piece of cloth tape from the rear bulkhead behind the bonnet, use good super glue to locate the tape that is left.