Good work Dave thumbs

Being involved in restoration in the early 90's I was brought up on decoking engines, the old boys I worked with used to say....

"There's only one way to decoke an engine properly, the head off way!"

However, they did advocate one other method that if completed regularly from new, would significantly extend the period before the head needed to come off for the first time.

The technique was to take a plant sprayer, the type that can be adjusted to give a really fine mist, it was then filled with tap water. Yes, you hear correctly just pain old tap water! Importantly, before you execute the 'plant sprayer decoke' the engine needs to be properly hot, not just idled in the workshop until the temp gauge says it's hot, but driven and put under load until it's properly what we used to call 'Italian Service' hot.

You then remove the air cleaner, hold the engine at 3,000rpm and spray your fine mist of tap water into the carburetor choke, the skill is not to overdose as the risk is you may hydraulic the engine. In practice it's not hard to get the dosing right, what you're listening for is a drop in engine speed, as soon as you hear the revs drop slightly stop spraying and blip the throttle to save the stall, it's definitely a two handed process.

What amazed me when I first saw the 'plant sprayer decoke' being demonstrated was just how much cr*p came out of the exhaust, the method definitely works as we had the head off the old 70,000 mile Mk2 Zephyr more or less as soon as the engine had cooled, our vacuum gauge already showed a burnt valve, the purpose of the 'plant sprayer decoke' was just to show me (the apprentice) how it worked and how effective the method could be.

I was told ideally it should be done every 3,000 miles and from new, this was back in the 1950's and 60's when the old mechanics I worked with learnt their craft. The practice of decoking engines largely disappeared from the mid 70's on as fuel and engine oil quality had improved massively, piston rings, bore honing techniques, piston and valve guide materials/machining were all way way better too so the combustion chambers started see a lot less oil making decoking a thing of the past.

Then along came the practice of force feeding the engine with its own blow-by gasses, this wasn't great but we largely got away with it especially after the advent of multipoint port injection, however, ironically by the early 2000s the need to decoke an engine returned with direct injection.

So perhaps it's time for the return of the 'plant sprayer decoke' thinking

BTW, if anyone is still doubting how effective the method is, they clearly haven't seen an engine with a head gasket failure where the seal between the cylinder and a coolant passageway has been breached, when you lift the head off an engine with this particular type of head gasket failure the cylinder seeing the coolant will be completely steam cleaned, while the others are all as dirty as each other.

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This is why the valve saver kit should work, but it's possible for the purposes of keeping the cylinders carbon free it may actually work better if it's just filled with good old tap water wink

Last edited by Montegue; 13/11/23 04:55 PM.