Early XK120s: Where to place a test vacuum gauge?

Fellow Jag Addicts –

I’ve been slowly dialing in the timing, points adjustments (distributor and plugs) followed by tweaking the carb synchronization and mixtures of 670236. I have a Colourtune, digital timing light, compression gauge, carb sync meter, etc. – and have been using them all – but the one test I haven’t performed so far is simply checking the motor with a vacuum gauge. They can be so useful for things like diagnosing a sticking valve, mixture strength, and so on. But you experts already know that. My problem with this car is… where do I attach the hose for said test gauge? The early type inlet manifold on this car has no vacuum port like for the windscreen washers. The one on the front carb that goes to the distributor looks to be forward of the venturi so, I doubt that will work. I saw on the bottom of the inlet manifold – about 3" aft of the starting carb blow back valve – that there is a 3/8 BSPP brass plug. Can I use that? Or does that provide access to a water passage? Any thoughts?


That old of a car should have a vacuum advance on the distributor, and that is the advance the engine ‘sees’, You can use a ‘T’ fitting in line, and keep the tube on the distributor and read your gauge. That way, you can see how much advance the engine is getting.

I finally found myself a Redex vacuum gauge (the set also included a compression gauge) on Ebay. My intention is to use it to dial in the mixture on each carb. This is quite a good way of doing it, apparently. A vacuum gaugecan be used for all sorts of engine diagnostic tests. The set included some of the small brass barbed and threaded nozzles which can be threaded in to the side of the carb heat spacers. This can be tricky (I split one) because the 1/4" thick spacer has only just enough thickness to allow a hole to be drilled and threaded. I was lucky to find a NOS Speedograph Richfield one with the nozzle already fitted. Pity the plastic is blue, but it’s fairly well hidden on my car, so is a bit hard to see behind the coil. I’m looking forward to trying it out!

Hi Chris. If you have the C.4953 manifold, it has a brass plug at the rear end which is indeed a vacuum source. It was added when the Mark VII production began, for the brake vacuum booster. The earlier C.2377 manifold did not have one there.

The 140 manifold has one there as well.

Rob and others –

No, there is no brass plug per your picture. And the one that I mentioned aft of the blowback valve (see below – sorry it’s not such a good picture) is a coolant drain plug. So, I’m thinking, the only way to truly access the interior of the fuel/ air part of the manifold is to either drill small holes on each carb spacer or, temporarily remove the blowback valve banjo bolt plus valve, thread and tap for a barbed fitting, then re-install, hook up a gauge, restart the car and take my readings. In practice I’ll probably turn a second banjo bolt on the lathe that incorporates a barbed fitting as I’m loath to modify the original part.