Petronix ignition issue

(Frank Andersen) #41

The idle setting is different with ‘ported’ and ‘manifold’ vacuum, Robert…

Remember that twhen the initial setting is done, the vacuum is reconnected. ‘Manifold’ vacuum is 18" Hg, which is added to the advance - while ‘ported’ vacuum have no vacuum in idle…

Basically, ‘manifold’ vacuum is fairly constant at steady throttle at all revs, varying with throttle movement - while ‘ported’ vacuum vary with throttle position…

The total advance of the Petronix at various revs and loads is of some interest - but only unloaded is readily tested. However, the engine’s demands for optimal effect is fixed by engine construction - it’s up to the distributor to meet those demands…

xj6 85 Sov Europe (UK/NZ)

(Robert King) #42

No, base timing is determined by mechanical advance degrees subtracted from 36-38 degrees with vacuum disconnected. The approximate 6 degrees is what you need.
Usually if your vehicle is a manual transmission, you can run manifold vacuum to your vacuum advance and still have a stable, repeatable idle; cars with automatic trans and/or a/c usually require ported vacuum for a repeatable idle.
Vacuum advance is strictly a low load, idle efficiency improver.

(Carl Hutchins, Jr. ) #43

Vacum is indeed a wondrous thing.

Early on, when emesion (sp) control became “fashionable”, the advance by vacum became a target.

  1. An after market system involving water temperature and vacuum advance was obligatory.

  2. My 69 Toyota FJ40 “sported” a simpler control. The metal line was cut and capped. No vacuum advance!!! I fixed that with a bit of tubing to reestablish vacuum advance.

Oddly, when it’s 6 spun it’s bearings, I built a composite engine. I used a “better” distributor.
No vacuum can. It’s built in curve all mechanical…


(j limongelli) #44

Frank, Cadjag1 is right…Your distributor is a vacuum RETARD.
If that’s the picture and is a 71 xj6 or Etype in the states with pollution control.
Either get the petronix distributor or Put in a series 3 xj6 with its spark box and coil and your done.
Ive done a million of these, I have no clue as to meshing the gears? The distributor has a ONE WAY key.
You cant screw it up.
The vacuum retard, was a VERY, VERY bad setup .
Time to move on to the 20 century and a petronix or Mallory or an xj6 they all work.
Good luck.
pm if you need help and another ps if yo go petronix you need THEIR solid core wires or the unit will short.

(Pete55Tbird) #45

I came across this in regard to English distributors and VACUUM advance/retard OR TOTAL LACK OF

I found it a short and at least to me interesting. Try it You will like it Pete

(j limongelli) #46

Yeah, Great design………Not.

(Frank Andersen) #47

No, I have the ‘European’ with ‘manifold’ vacuum advance…:slight_smile:

The reason for the variable advance is that combustion inside the cylinder takes time. As the mixture ignites at the spark plug, a flame front spreads at a constant speed of some 30 m/s.

There is an ideal position of the piston position at combustion completion for max power. As revs rise piston; speeds increase - and earlier ignition is required to complete combustion before the piston reaches the ideal position…

Compression pressures is a function of manifold pressure; the higher the pressure (the lower the vacuum) - the higher the compression pressures. As the mixture is compressed it heats up, adding to the heat and pressure generated by the burning mixture.

When the temperature reaches the ignition point of petrol, and there is still unburnt fuel - remaining fuel ignites explosively; the engine knocks. To avoid this the spark must be retarded, delaying ignition - particularly when high manifold pressure is detected.

Using ‘manifold’ advance this is achieved; high manifold pressure means no vacuum advance - the timing falls back to centrifugal advance. The ‘European’ philosophy, for historic reasons, aimed to extract maximum power from a given engine volume - maximum advance while avoiding knocking.

Other advance methods described has other priorities, but does not aim for maximum performance. ‘Ported’ vacuum is taken from a pickup port at the carb - the air speed past the port decides the vacuum; the higher the air speed, the lower the vacuum. It can be used for both ‘advance’ and ‘retard’ - but in all cases; the lacing of the port is material…

Complicating matters is the analogue, mechanical distributors used. They are imprecise and cannot give optimal timing throughout load and rev range - so safety margins are required to avoid knocking. Modern ignition systems are digital - and are also equipped with antiknock sensors…

The main point is that the advance influences engine performance and indeed emissions. But altering timing cannot alter the engine itself; you cannot force the engine against its will…

xj6 85 Sov Europe (UK/NZ)

(j limongelli) #48

Whoa let’s go slow

It took me twelve years to graduate the third grade😁


If what I have described is “ported” vacuum then all the pre - 1975 UK XK engines I have known have had “ported” and not “manifold” vacuum. I always thought that manifold vacuum was for the more stringent emissions controls in the US market, so surprised you have a European but manifold system, Frank.

I too am running to catch up with Franks technical knowledge (which far outstrips mine) but I think I have grasped the basics of timing as follows (for ported vacuum and my 1971 4.2 litre). Static timing is 8 crankshaft degrees BTDC (according to Jaguar); this is the base setting at which the engine should be set. Dynamic timing is 20 crankshaft degrees BTDC at 1,700 crankshaft rpm (according to Jaguar). Therefore the distributor needs to advance 12 crankshaft degrees. The graph shows 1,700 cs rpm as 850 dist rpm and reading off the vertical axis this gives 4.5 dist degrees of advance which is 9 crankshaft degrees ie 3 degrees too little. This is all mechanical advance and all that the basic workshop (and I) can use to time the engine.

For comparison the Jaguar workshop manual gives a chart showing centrifugal timing advance degrees with the dist mounted in a test rig. I assume that the values given are dist degrees and dist rpm. At 800 dist rpm (1,600 cs rpm) the advance should be 5-7 dist degrees (10 - 14 cs degrees). So again the actual mechanical dist advance is a little less than “spec”.

Enough theory - time for a little practical application!


(Pete55Tbird) #50

Frankie. Obviously you are not confused enough.
My first car was a 1929 Ford Model A. The distributor was STRAIGHT MECHANICAL ADVANCE.
No vacuum of ANY KIND. There was a lever on the left of the steering wheel when placed UP retarded the
timing to allow the engine to easily turn over to start. After starting you lowered the lever to advance the
timing for more power.
It was not until later car got vacuum advance. This requires Manifold vacuum OF SOME KIND. Before start
and at IDLE engine vacuum is LOW or Non Existent so the effect on timing is ZERO.
Whatever timing advance is built into the distributor is what the engine SEES and uses Until manifold vacuum
builds UP to overcome the springs in the vacuum advance can.
End of Rant. Pete

PS. Race cars or high performance cars of any type DO NOT USE VACUUM advance.
Vacuum advance is to increase MPG at low/part throttle HIGH Manifold vacuum cruise
OR to reduce SMOG ( NOX ) for emissions.

(Frank Andersen) #51

Either(!) ‘static’, engine stationary, or(!) ‘dynamic’, engine running, is used, Frankie - they mutually cancel each other…

The initial setting is the reference starting point for the dist - the internals of the dist then gives the advance appropriate for the engine at other revs and loads. That’s ‘what the engine sees and uses’ - to quote Pete…:slight_smile:

In your case; the dynamic setting spec is 20 deg at 1700 rpms - as read at the crankshaft. All(!) advance settings refers to the crankshaft scale - which reflects engine position when ignition occurs; the plug spark…

To illustrate; with mechanical points setting the engine static was straight forward. Turn the engine to the spec advance (like 8 deg). The place a cigarette paper between the points, and rotate the dist body clockwise till the paper falls - indicating that the points just starts to open. Which is when the coil fires - tighten dist and your static timing is set.

With electronic ignition it is a bit more cumbersome, and ‘static’ specs are therefor usually not given. For the record (if static spec is given); the process is again to turn the engine to the ‘static’ spec. Then connect a spare spark plug to the #6 plug lead, loosen the dist - and ‘wriggle’ it around the trigger point. Each time the dist passes the trigger sensor (on CE; the pickup coil) - the plug fires…

If the graph you show is from your engine; you are running at 12 deg advance - practical advice; advance ign to spec…:slight_smile:

As an aside; the ‘European’ always used ‘manifold’ vacuum - the ‘ported’ vacuum was an ‘US’ thing. With light load the engine can stand much more advance without knocking - and as the manifold then is high, so is the advance. Opening the throttle; the manifold vacuum drop, cylinder pressure increases - but drop in vacuum cause the dist to ‘back off’, preventing knocking…

The influence of timing on emissions is somewhat complicated. This is not directly related to ‘manifold’ versus ‘ported’ vacuum - which is more about how close you can approach the knocking point - without overstepping the mark. Pure centrifugal advance have larger safety margin - and compromises engine power potential. Bearing also in in that the xk was designed with 98 octane in mind - which have better knocking resistance than lower octanes…

xj6 85 Sov Europe (UK/NZ)

(Carl Hutchins, Jr. ) #52


You bring back memories.

My first car, a “T” had the spark lever on the left side of the column in similar fashion. the linkage went to the “timer” on the front of the engine, turned by the cam shaft. But, I had to reach under the hood and pull out the linkage slop and get full retard. O’wise cranking by hand was risky !!

Decades later, a battered “little Olds” stock car came to spend some time here. Vacum can gone. timing advanced to the point that it would kick the starter. The drill. Two switches. One for ignition and one for crank. Crank with ignition off. when turning, on with the ignition. Solved. Quirky to drive. One front wheel had brake, the other side removed. So, a stab on the brake would toss the car. Good for dirt!! Really bad on hard pavement.


(Jochen Glöckner) #53


the man who bought my Jag new in 1975 was born in 1901 in England when Edward VII was reigning and cars weren’t even present in the vibrant London city life. In the 1980s he gave evidence to discover a historical scam - the fake Hitler diaries that even had fooled a major German news magazine - in fact that’s the point on the time line where my attention and his activity crossed, even though we never met each other. In a way the car has brought together a full century of individual experiences and history.

Chances are that it has only been roughly one century that is wearing the stamp of the automobile - at least as we know and appreciate it. It would be great if you collected your wealth of memories along the string of car-inspired moments. If you take your time with your PC and let your ideas flow we can make a great story book out of it I bet. Keep them coming!



75 XJ6L 4.2 auto (UK spec)

(Frank Andersen) #54

My father’s Buick had 3 levers on the steering wheel, Carl; idle, mixture and ign timing…

Though too young to do the hand cranking myself - the procedure was hammered into me, with dire warnings. Idle middle, mixture rich(ish) - ignition in fully ‘retard’.

But more importantly in this context; watching my father adjusting the timing while driving - and explaining why. And the mixture - again explaining why…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(Robin O'Connor) #55

And with hand cranking NEVER grip the handle like a hammer, always keep the thumb on the same side as your fingers.

(Frank Andersen) #56

Indeed, Robert - my father hammered that into me as well…:slight_smile:

Actually, I miss the hand crank - it made positioning the engine a lark…

xj6 85 Sov Europe (UK/NZ)

(Carl Hutchins, Jr. ) #57


My first "T’ was almost always hand cranked. although it was equipped with an electric starter, I don’t recall using it. Why, has escaped me over the decades.

I learned the lesson of “fingers and thumb on the same side of the handle” early on. Dad, and Billy, a mentor pal in the “life with a T”. Billy was pernuious (sp) and relied on batteries with one dead cell. Free at most gas stations!! Ok for ignition and almost for lights. Not for the starter.

Recently, watched a “You tube” piece with really nice T speedster. It’s owner pulled the handle from the 12 o’clock position. My teaching was from the 6 !!

As to turning the engine. Yes. When "adjusting the connecting rod bearings for wear. Over done with the first one.

The A’s had a neat feature. A bot with a nubbin on the cam cover. Remove it,. Insert the nubbin into the hole, turn the engine by hand crank til the nubbin falls into a slight depression on the cam gear. That is the firing TDC!!! Point eh rotor to #1 and the engine is timed…

The speedster “T” I built from cast off junk in 58, had an electric starter that fired the T almost each and every… Impressed some old time Fords mechanics…


(Paul Wigton) #58

My dad’s R-R PIII had an advance lever, and a ride lever (“Hard—Firm”), plus a bi-directional horn button: “Loud—Soft!”

(Frank Andersen) #59

Trust RR to cater for fastidious tastes, Paul…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(Carl Hutchins, Jr. ) #60

Some older USA critters had “trumpets” under the hood/bonnet. The driver had the option of choosing the toots Softly for city, and a blare for country.

And, after market musical horns. A simple key board to play a simple tune on the horns.

A"s had a motor driven horn different from others using a diaphram. The “ahooogah” was famous…

I may have one, somewhere ???