Petronix ignition issue

(Pete55Tbird) #21

Frankie fair enough. The distributor cap will only go on ONE WAY. Fine. Pull out all the plug wires from it
put the cap on. Your rotor is pointing to the #1 plug. Wire up, start up set timing. Pete

(Frank Andersen) #22

Neither do I, Frankie…:slight_smile:

The rotor position is decided, fixed, by the dog position - a ‘free’ preposition of the rotor is pointless. Positioning the housing aligns the rotor with the appropriate dist lid hole of choice for the selected cylinder, the cylinder ready to fire. Whether #1 or #6 cylinder is used as a reference depends on which ‘TDC’ is used. But #6, frontmost, is preferred for ease of access - and because it is the cylinder ready to fire whenever the head has been off, or the camshaft is aligned with the tool…

My question relates to your ‘clockwise’ turn - rather than the ‘counterclockwise’ space shown in the picture.

I’m sure you have explored all avenues, but as the engine is timed correctly and runs well - I’m not sure that further efforts is worth while at this stage.

I rather think that this dist configuration is not, for some peculiar reason, not suited to your layout. Even with a 20 deg repositioned dist drive/dog - it still looks awkward…:slight_smile:

xkj6 85 Sov Europe (UK/NZ)


Completely agree.

On to other improvements now - front brakes!!!


(Pete55Tbird) #24

Frank, good morning. I enjoy reading you advice and your comments and I appreciate that you take
the time and effort that you do. Sorry I am having some much trouble trying to explain this.
Please indulge me and imagine the distributor body and rotor shaft are separated. The engine is at TDC
and the cam lobes are up as required. Body in one hand and the shaft in the other.
Put the body of the distributor into the engine ( do not tighten yet so it can rotate ) THERE is NO SHAFT
Now put the shaft and rotor into the distributor. The tongue on the shaft MUST INDEX with the dog on the
gear for the shaft to fall into place INTO THE GEAR
Installing the shaft 180 out IS IMPOSSIBLE 180 degrees in not physically possible.
Wherever of 360% the rotor can point IS #1 firing position.
to adjust ignition timing BECAUSE of how the body positioned and location of vacuum can NOW
It is certainly a possibility
that I am completely misunderstanding the problem which I understood to be the vacuum can fouled some part of the engine before it can be rotated enough to advance or retard the timing.
If so can you please correct me. Thanks Pete


I wonder if there has been a misunderstanding here. It is not the physical action of distributor body rotation in situ that is the issue for me - I can lift the body clear of any obstruction, rotate it and then set it back down again - my issue is that I can’t align the rotor (at No 1 firing) with any HT terminal in the cap without some part of the body conflicting with other engine components nearby.


(Carl Hutchins, Jr. ) #26

I am with Frank.

It runs well. It ain’t broke, don 't fix it.

Different than others? No matter.

But kudos to Pete and frank for their detailed efforts.

Happy thanksgiving

I am going over to my son’s place for turkey and trimmings…

Carl …

(Frank Andersen) #27

Certainly, Pete the trouble ensues when the engine is set 360 deg to the ‘wrong’ TDC - while believing it is on the ‘correct’ TDC. There is of course no risk with the cams correctly set - the #6 (frontmost) cylinder is then ready to fire. And the rotor must point to this plug lead…

While the dist body can be rotated to position externals any which way; Frankie’s problem is that he can’t find a position of the dist body that aligns with any(!) of the dist lid plug lead connections with ample room for dist turning to time the ignition freely - without fouling something…

While this is indeed odd and unusual - but the rotor position is fixed, and needs to point to one of the dist lid plug lead connections…

xj6 85 Sov Europe (UK/NZ)

(Frank Andersen) #28

Just a coupe of more remarks, Frankie…

You have the Petronix; is there any documentation on that that requires 1700 rpms and 20 degrees? The blurring of the timing marks may be caused by ‘afterglow’ in the timing light - and setting can be more precise at lower revs. The more common is to set the timing in idle…

Question; what vacuum do you have on the dist in idle? If manifold vacuum is used; you have around 18" Hg - with ported vacuum you will read about 0. This requires different initial advance settings - and the Pertronix manual should reflect this. In fact, you get an idea of the idle setting by checking advance in idle…

Question 2; the vernier adjustment has been mentioned but not elaborated on. What is the actual range of this device? Nominally, the ‘crude’ advance is set with the vernier in mid-positioned - then the advance can be fine tuned with the vernier. Which is very useful for fine tuning engine performance in driving conditions…

Neither of the above implies that the engine is mistimed - or indeed will solve the positioning problem. I’m just inquisitive…:slight_smile:

xj6 85 Sov Europe (UK/NZ)


Good point. I am taking the timing value from the workshop manual which is obviously based on the CB points system. I don’t have any documentation for the Petronix (previous owner) and couldn’t find anything on line. From memory the timing at idle (about 700 rpm) is 8 - 10 degrees. I might time the engine at idle by listening to the rpm fall off rather than by watching the strobe, and then use the light to see what advance that gives at 1,700. Would be an interesting check. Another interesting check would be to re-install the points system, time the engine to 20 degrees at 1,700 and see where that brings the orientation of the distributor body.

All my adjustments have been done with the vacuum advance disconnected as per the manual. My initial settings were with the Vernier at mid position adjustment but to minimise fouling I had to set it to max advance.


(Pete55Tbird) #30

Frank may I respond.
Can I remind you that the opening argument that began this thread was

A PO replaced the points system with a Petronix ignitor system on my 1971 4.2 litre and I have never been entirely happy with it. The distributor seems to be in the wrong orientation - the vacuum capsule pointing obliquely towards the block where it fouls the heater pipework when turned to the correct ignition timing position. I suspect fitting the ignitor pick up and magnetic cam sleeve has somehow altered the point at which the spark occurs necessitating rotation of the distributor to its present orientation to compensate

I took this at face value and I did not assume any fact that was not in the statement and explained how to orient the DISTRIBUTOR BODY in a more correct position to solve the stated problem.
Suggestions such as " move the shaft ONE TOOTH forward or back were CLEARLY WRONG
as far as redrilling the gear on the crank again beyond fantasy.
This information I hope will be of use to future Jag owners who run into their own problems in their future ownership of the " quirkey" British cars. To dismiss mis- information because a band-aide has solved the
problem. Pete

(Frank Andersen) #31

There are two separate issues then, Frankie…

One is the distributors advance curves, which varies with dis type - and the vacuum source, which influences the basic ign timing…

The engines’ advance requirements vary slightly with engine bore - but basically; all 4,2 xk engines have the same advance requirements. However, altering advance curves and initial timing gives different engine characteristics on the road - relating to performance, economy and/or emission concerns…

A fundamental dist input is vacuum source - and while ‘all’ initial timings are set with vacuum disconnected; the initial setting is fundamentally different depending on whether ‘manifold’ or ‘ported’ vacuum is used. (omitting ‘retard’, as Pertronix is unlikely to be so configured).

Measured at the dist; ‘manifold’ vacuum shows 18" (or thereabouts) Hg in idle, while ‘ported’ shows around 0 - which identifies your actual vacuum source, and dictates initial advance setting when idle is used. Crudely; ‘manifold’ vacuum. and advance, varies with engine load - while ported vacuum varies directly with throttle position.

In principle; any ‘advance’ distributor can run on either - but the initial advance setting must reflect the vacuum source…

As an aside; 'European’s use manifold vacuum - and the initial setting is 4 - 6 deg at 800 rpms. With ‘ported’ vacuum the advance is some 10 deg more. Basically reflecting that when vacuum is connected, both versions will have the same advance in idle…

As a further aside; varying advance (from some starting point) with the engine idling to achieve maximum manifold vacuum is sometimes used when advance specs are unknown. Very easy with the vernier fitted…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(Carl Hutchins, Jr. ) #32

Engine timing is basic yet intricate.

The factory spec’s are a compromise. And relate to the “fuel of the day”. In almost all cases, adherence to that criteria results in a car that is quite drivable.

But, it can be customized to the clime, driving habits and fuel of today. as well as engine health.

In olden days, the tech used his trusty Sun machine. A cabinet with a drive for the distributor nd tachometer and meters to read dwell and degrees. Spin it, observe and adjust My son rescued one a time ago, Reports that it works just fine on some of his old critters.

In DIY practice, not hard, even if the points and condenser still is present. If gone, even better!! Three elements to consider:

  1. Static setting. Usually somewhere between 10 and 17 degrees BTDC . The base from where to begin.

  2. Add in the degrees of advance by the action of the vacuum can. Say 15 degrees. Example only.

  3. Then add another 15 or so, The actionof the centrifugal weighted arms in the interior of the distributor.

  4. Full on advance in the vicinity of 35 or more…

The smart timing lights can read these stages.

And finaly, ears and seat of pants. Run up the advance a couple of degrees, drive the car. No “ping” on pedal down? run in some more degrees. repeat. Aha a ping. Back it down.

Tuned to a “T”…

Carl .

(Pete55Tbird) #33

Frankie, I will leave the timing of your engine in the hands of these experts.
After you get your car back on its wheels and running you might want to take a look at
the architecture of your engine compartment. Something or some item has been added
after your car rolled out of the factory. An item that was not there then is preventing
you from rotating the distributor. My 67 420 compact with 4.2 is laid out with P/S, Alt twin SU HD8s and there isnt any interference ( vacuum points straight back) so you might check. Pete


Frank and Carl:-

Initial timing for this engine is 8 degrees; dynamic timing is 20 degrees at 1,700 rpm. Therefore the advance is 12 degrees - yes? The attached advance curve plot shows my distributor as tested by the specialist following rebuild and recalibration earlier this year. This would seem to match those figures pretty well. On the car, advance is 20 at 1700 and 8 at idle. Am I correct to conclude then that the distributor is advancing as it should and my timing is correct? If so, the issue of orientation must have another cause.


(Carl Hutchins, Jr. ) #35

I’ll avoid further comment so as not to obfuscate rather than help.

n my opinion, and I do not claim to be an expert, merely a guy that has messed with many of these IC’s over my fourscore and nine,

Timing is to spec. leave it.

Distributor orientation is another matter. And with respect to Pete for his efforts, , I’d leave it there.


(Frank Andersen) #36

This is plain centrifugal advance, Frankie - but from the initial idle of 8 degrees…

Both settings are dynamic; 8 deg in idle and 12 degrees ‘more’ at 1700 rpms. The engine does not differentiate between the two - the advance is 8 deg in idle and 20 deg at 1700; there is no ‘addition’ or ‘subtraction’…:slight_smile:

The confusion may stem from specs for ‘static’ initial setting. ‘Static’ means with the engine not(!) running - and this spec is sometimes given and set accordingly. In both cases; set to specs - the dist then varies the centrifugal advance with rpms. When ‘static’ specs are set, commonly at 8 deg; this is indeed ‘added’ to the advance caused by the centrifugal advance - which admittedly is not necessarily a hill of beans from 0 rpms up to normal idle…:slight_smile:

Note also that the diagram shows that your centrifugal advance peaks at some 25 degrees at some 3000 rpms. No further advance is provided beyond this…

Which brings up vacuum ‘advance’; the engine needs, and uses, both centrifugal and vacuum to get the ignition timing right. With one of them wrong; it impairs engine performance - and while your centrifugal advance seems (is!) to spec for that dist - vacuum must be addressed for correct engine timing on the road…

I do not know the vacuum specs for the Petronix - but generally, vacuum advance is around 10 degrees at some 18" of vacuum. So the point is; what advance does an engine need at various rpms (centrifugal) and load (vacuum). The you have 20 deg (centrifugal) at 1700 rpms does not mean that this is the total advance that the engine needs…

The is why the initial ‘ported’ and ‘manifold’ vacuum is different. As said (often enough); ‘ported’ vacuum is 0 with throttle closed (idle) - while manifold vacuum is 18" Hg. So the normal idle advance for ‘manifold’ vacuum is 4 - 6 deg - while ‘ported’ vacuum setting is usually 14 - 17 deg in idle…

Hence the necessity to check, at the dist, which vacuum you are using. The Petronix can likely use either - but the initial setting ‘should’ be some 10 deg higher than if ‘manifold’ vacuum is used.

Note that the specs given, 20 deg at 1700, does not reflect which vacuum it is spec’ed for. The ‘European’ specs invariably refers to ‘manifold’ vacuum - and the centrifugal advance is set accordingly. The complication here is the uncertainty about the ‘ported’ vacuum output at say 1700, as compared to ‘manifold’ vacuum…

In my view; the less power generally quoted for ‘US’ xk engine is partially due to difference in timing. The original ‘European’ specs also refers to 98 octane fuel which allows for more ign advance - and indeed that the ‘European’ aim was maximising engine power. With no regard to other issues…:slight_smile:

xj6 85 Sov Europe (UK/NZ)


The line to the vacuum capsule comes from a nipple and drilling in the rear carb above the throttle butterfly. When the throttle is closed (idle) the edge of the butterfly covers the drilling, so it seems no vacuum can reach the dist. Setting engine timing at idle must therefore be purely on mechanical advance. When setting engine timing at 20 degrees and 1,700 rpm the drilling is “open” to the dist but the manual requires the line to be disconnected. Therefore 20 at 1,700 is also set with only mechanical advance and the orientation of the distributor is thereby fixed.

The advance graph horizontal axis is noted “Distributor rpm x 100” and my interpretation of the curve assumed that crankshaft rpm would be twice these values and with damper advance degrees different accordingly.

The petronix unit fitted is simply a magnetic pickup and collar in place of the points - I am still not clear how this may interact with the vacuum and why a different advance may be needed?


(Carl Hutchins, Jr. ) #38

Great description.

I can only add two things.

  1. At idle with no exposure to vacuum, the mechanical advance is not in, yet. That is why timing starts there. Add vacuum and the cylinders fire earlier. More RPM’s and the mechanical begins to make the cylinders fire even earlier. And at some RPM, the advance is all ion… The graph depicts the curve.

Pertronix set one that would suit most. As did Jaguar in the beginning.

Now, the curve is customized by some. It takes skill and equipment and some experiments. Some vacuum cans can be adjusted. A teeny Allen screw in the pipe!!

Now alter sots, weights and spring tension and the mechanical can be “adjusted”.

For the experienced, yet not “expert” DIY guy. Just bump the static til the engine pings under load. Back t down a couple of degrees and drive it and enjoy.

Done here, thread exhausted…


(Frank Andersen) #39

That’s must be the wrong interpretation, Frankie - that would meant that advance was tested (red cross) to 17 deg at 9000 rpms…:slight_smile:

Timing is set using the crankshaft scale - the test should show degrees advance on this scale at the relevant rpms. In this case the graph shows an engine timed to some 12 deg at 1700 rpms. Considering that a ‘standard’ setting for ported vacuum is 14 - 17 deg in idle - explain…

Basically, the type of ‘advance’ distributor fitted will have roughly the same centrifugal advance, used for timing - there is no reason for the Petronix to deviate much.

You description of ‘ported’ vacuum is to the point - however, the pick-up port may be placed at different points in the carb for ‘special’ effects. But standard is usually as you describe - giving 0, or very low, vacuum with the throttle closed. Crudely; manifold vacuum reflects engine load, the ‘ported’ vacuum reflects how much air is going through the carb - the faster the airflow past this port the lower the pressure…

As an aside, the porting for ignition timing is of course irrelevant for fuelling. Petrol added to get the mixture correct is regulated by a different port, controlling the carb piston/needle, and the air speed past the jet. Where the same applies; the faster the air flow the lower the pressure at the relevant points…

In short; the graph shows a mistimed engine - but how this pertains to your dist ‘interference’ problem is somewhat unclear…if at all…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(Robert King) #40

From my experience with engines other than the XK, that looks to be reasonable non-racing curve; remember that the advance is stated in “distributor degrees”- you need to multiply both the rpm and advance degrees to read at the crankshaft.
Most hemi engines like 36-38 degrees at wide open throttle at their torque peak; advance increasing above that rpm usually gains little; this curve shows advance all in at 5000 rpm, about 33 degrees.
Set your base timing at 6 degrees BTDC at low idle and you will be real close. Vacuum advance can be connected to either manifold or port, whichever gives the most consistent idle. You may see 50 degrees advance under light load, but the engine needs this advance under light load for maximum efficiency. Vacuum disappears as the throttle opens and load increases, and the vacuum advance is not in play at wide open throttle.