That is the definition of AKI…..

 

HA, had that thought many times, and never got that brave. Whizzy bits at 3000rpm, in my face, and me with one eye, NOT HAPPENING.

Add to that, the timiing plates are not there anymore on any of them, another oil leak solved, duh.

More importantly, the engine is HAPPY, the owner is HAPPY, why mess with that.

 

I like your thinking!
I guess the point of this is what applied to a newly manufactured engine 35 years ago, before ethanol and oxygenated fuels, probably doesn’t apply now. Obviously best performance is determined by timing to the verge of spark knock, at the speeds normally operated at.
For a long time, before fuel prices shot up, I ran mid grade (89AKI) and premium (91AKI) and still had spark knock occasionally; I had drive timed it and don’t think it was much advanced from where it is now, perhaps a degree or two.
This car is driven daily, year round so fuel cost is a factor for me.

 

Fwiw, I use this website and try to go with ethanol-free, where possible, here in NY - there are supposedly several hundred gas stations in MI carrying ethanol-free:
https://www.pure-gas.org/

 

FWIW, I've been using 92AKI E10 for years and never have any knocking.

TBH, I can't remember where I set the timing; too many years ago.

Are you sure you don't have a "carbon knock" problem?

Cheers
DD

 

you are missing out on a good thing. Race cars use ethanol for additional power over Gasoline. Ask I’ll explain why that is.
They also use it to minimize deposits in the engine.

If you burn a small amount of gasoline in a glass bowl you will see the deposits it makes. Burn the same amount of Ethanol and there will be no deposits. In fact if you burn it in the same glass bowl you will notice a reduction in deposits.
Ethanol is also a cooling agent. Put a little ethanol on your arm and feel how cool it is as it evaporates.
Finally. You drink ethanol every time you drink a Beer , Glass of wine, or any alcoholic drink( Scotch, Bourbon, Gin etc.

 

Paul. Ethanol is the magic elixir.
For peak power racers use As much ethanol as they can. Indy 500 cars use 100% ethanol, NASCAR uses 15% ethanol. Other racing groups put cars running ethanol in faster classes.
Second Ethanol is a wonderful cleaning agent. That’s why it’s commonly used in hospitals
Third. Ethanol is a cooling agent.
4th you drink ethanol in every beer, wine or alcoholic beverage such as Scotch, Bourbon, Gin Etc.

 

Don’t believe it is carbon, but could be a myriad of other issues. I have some recent experience with the TPS signal in a Rover V8; changing the TPS reading changes which cell in the fueling map is used, thereby affecting mixture. That Lucas system (14CUX) isn’t far removed from the 16CU in the XJ-S in operation. Perhaps I have a lean condition? Maybe an air temperature or coolant temperature fault? Sure wish I had a method to display data in real time, like what RoverGauge provides.
Could be the vacuum capsule provides too much advance for a given vacuum, or the centrifugal advance springs are weak or wrong.

 

The best solution rather than blindly replacing parts is to put it on a chassis Dyno. ( I believe the British call them rolling roads. ).
I’ve stoped using them for the race cars. It’s $125 I no longer need to spend. Instead I put O2 sensors in every exhaust pipe ( I use 4 on the normally aspirated race car and 2 on the turbocharged race car.). and watch the fuel mixture. I go for 13.7-9 on gasoline and 9.0 using E85.
My distributors go on my Sun distributor machine. And I just check that they spin up and give a steady spark. Racing never goes below about 3800rpm. And that’s above the spark advance. Putting around n the pits I’m at a steady state in first gear and everything is a rolling start.

 

Idea:

Closing the enrichment switch on the throttle capstan or unplugging the air temp sensor will richen the mixture 8-10% or so. my be worth a try if you suspect you're running a tad lean?

Cheers
DD

 

I did check the enrichment switch (good) and the vacuum switch (closes at 2.5” hg). Good idea to try IAT unplug and shorting enrichment switch as a test.
Funny that if you are watching the travel computer instantaneous display, 75 mph, mileage shows 20 on flat ground- slight incline and the spark knock is continuous. Give it a little more throttle and spark knock disappears as traveler shows 10 mpg. This is 2700-3000 rpm, right at the knee in the centrifugal advance curve.
A buddy of mine used to have a Sun distributor machine….

 

Hi to all, I own a 1988 and a 1989 MY XJ-S V12 USA specs Jaguars, I have both DRIVERS HANDBOOKS which show the following, first the one on the 88 and next the one on the 89... They say almost the same thing, but the later a little bit more easy to get straight...


So I go with 87 octane...

Alfredo

 

My 83 HE runs fine on US 87 pump octane. There's no audible knock. Timing is over advanced a few degrees. Centrifugal and vacuum advance both are operational.
In the US, top tier fuel contains additional detergents and avoids additives that promote deposits, see the list at topteirgas.com. you don't have to buy higher octane premium to get better quality.

 

Well, I believe that I have solved the issue. I tried a couple of tanks of mid grade (89 AKI) without noticeable change. I decided to recheck the components in the vacuum advance system individually (advance capsule, vacuum reducer, and dump valve) and all seemed to operate correctly; I then checked them on the car and all seemed to work. As all the spark knock seemed triggered by throttle tip in, I started thinking about how the ported vacuum affected the vacuum advance…..
My ‘88 has 2 ports on top of each throttle body; on the right (where the advance connects) one of the ports is capped. The underhood vacuum diagrams and the V12 Focus manual do not show this. I connected vacuum gauges to each port and slowly opened the throttle, and noted a slight difference in how the gauges performed.
Switched the vacuum reducer connection to the rear most port (was on the front one), bumped the timing back to 18 degrees at 3000 rpm, and spark knock is mostly gone.

 

So do you suspect weak vacuum at the original port? Leaky port? Blocked port?

 

I think its position or diameter affects how much vacuum vs how far the throttle is opened. Sure made a difference. The way the reducer works is manifold vacuum is regulated to 11” until port vacuum exceeds that value, so the relation to throttle opening is critical.

 

***Edit**** The vacuum hose going to the vacuum reducer is now connected to the front port (was on the rear port).
The vacuum present at each port differs at what throttle position vacuum starts, and vacuum level up to 10”, where they start to have identical readings.

 

Here is where I am now; I have the distributor advance unit connected to the rear port on the right intake, and am not using the vacuum reducer or dump valve. I still had a moment of spark knock on throttle tip in, even with the timing retarded a degree or two. I believe that the vacuum was not dumping quickly enough with the reducer in the circuit. By connecting directly to the port, vacuum drops immediately, albeit there is no vacuum advance at idle. I just set the idle higher on the AAV to compensate.
I had the thought that vehicles equipped with the reducer and dump valves require a bleed hole in the advance unit, and I still had the original (JLM519?) which I dissected. No evidence of a bleed. Thought about a filtered bleed like was used on some Mopars, but could not locate one. Also remembered that when I had the current advance unit built, I requested a heavy duty diaphragm, which may slow the movement of the advance.
Anyway, timing set to 18@300, idle at 600, and got15mpg combined on the last tank- happy!

 

Well done.

I am SO happy I am down here. NO emissions stuff, NO vac reducer etc etc.

One vac line, capsule to RH underside port on engine side of throttle disc, and the car luvs me, what more can I ask??

 

Me too. This was recommended by David Marks (UK Jaguar specialist and JEC adviser) at an XJS seminar held by the JEC. Direct vac capsule to the under-TB port on A bank. It has worked perfectly for years and all that tube rubbish, timers; and funny little gizmos nobody understands are gone.