Yes, thanks, Glyn, for your insights and explanations. More valuable information for future browsers, as well as current readers. SAE20 in the UK seems to be exclusively mineral, but I seem to be topping mine up fairly frequently, so probably not too much of an issue.

I did a more thorough road test yesterday and can now say with confidence that Curve 0 works fine on my car. I guess there might be a little more advance to be tolerated higher up the rev range by trying curve 1 or 2, but I'm not looking for the last drop of performance from the car, so I'm happy with it as it is.

Thanks for your help, everyone, on this.

 

Glyn, Thanks for the clarity on why we stick with the jag damper oil recommendation. Thinking of my experience with ZS CD175 on XKE, also calling for 20 wt., Still have the original qt. can, as can find any other use for 20 wt, not even the lawn mower.
Given the 20 wt. is constant and the carbs are by nature self-cooling. I assume the different dampener spring rates compensate for quicker accelerating 4 cylinder engines with same carb.

Glad to see the "1965 Jaguar 3.8S RHD DG Auto" is back and racing from round-about to round-about as spring weather has arrived. Let us know what is screeching at 3000 RPM, when you get it sorted out.

Rgds David

 

David. I'm no SU Carb expert. As others know from previous posts here I'm on a steep learning curve with constant depression carbs. I'm starting to get a handle on the things. I'm a twin sidedraught Weber & Dellorto man. (Long string of Alfa's in my youth & then fuel injection). I'm an oil company man & tend to talk from base principles.

So I guess so & it also depends on how much air the engine is pumping/shifting for a specific carb size & mixture modified by the needles.

You can stick the 20W in your engine in cooler weather. It will do no harm.

 

David, Glyn is correct in saying the damper spring weight is dependent on the air flow and fuel of the specific engine setup.

A race tuned engine would run with no spring at all and no damper oil for max acceleration, but an XK engine will not run like that without modified needles, a race engine is not expected to accelerate from low revs, and they are pretty rough running down low in the rev range.

When the accelerator is floored, there is a rapid change in pressure either side of the carb, if the damper rises too quickly the air / fuel ratio's go out the window as the volumetric efficiency of the intake is not good enough to allow enough air to flow into the cylinder, at higher revs the volumetric efficiency increases.

You can run a standard engine with no spring or oil and providing the accelerator is pressed only as far as the engine will breathe and accelerate you would not notice, when the carb is opened past the point when the engine can keep up then the air/fuel would be off.

If the intake tract is ported increasing air flow etc. the damper spring can be lighter, but the needles need to be matched to the engine (richer needle), this was my point regarding the pinking issue on this thread. If the compression increases the the needles should be a little richer this allows for a little more advance without pinking. The straight port heads on later Mk2's the needles went to a CL which is a bit richer than the TL this is due to the increased efficiency of the head itself primarily.

So hopefully to answer the question, a lighter spring works with a faster accelerating engine, but the lighter spring does not allow the engine to accelerate faster.

 

Hello Bruce, I have recently bought this 123 Bluetooth. Am I not right in saying that the graphs are based on crankshaft advance, not including the static setting of for example 10 BTDC. Therefore is not you suggested curve far too advanced at 28.7 which is in fact 38.7 BTDC ?
i am trying to find the best curve for my 3.4, with 9/1 pistons, 2 HD8’s and UN needles.

 

John

One of the current specialty distributors overhaul shops, recommend jaguar XK engines distributors be calibrated and curved for maximum performance, cooling and mileage, to the XKE distributor #41060..spec, with a mechanical advance of 18-20 degrees at the crank", with initial timing set at 10 degrees BTDC, for a totalmax- advance of 28 to 30 degrees at crank. Using the OEM vacuum unit #54415894…. 7-14-8..for optimum smoothness and economy.



IMO: Given the lower octane fuels of today, going to a max of 34 degrees at 4000 RPM, will still give you a safe margin. This is the XKE distributor #40617A curve, with an OEM 7-14-8..vacuum advance curve.

Engines that are setup for racing, can tolerate more mechanical advance coming in sooner, than a good all-around street performance engine.

See comments from a 2012 post, detailing optional Jaguar distributors. The data points from these spreadsheets are still good for detailed one-off mapping of a 123 program-able distributor.

Rgds
David