I have owned my 1986 Jaguar XJS for about 15 years. Incidentally, I recently restored my Rolls Royce Silver Shadow and I’m also an owner of a 1966 MGB. So needless to say I am a glutton for punishment and personally restored each one.

I recently rodded out the radiator and installed dual electric fans from a 2001 Nissan Maxima. The car currently runs at 82°C (82° thermostat installed). The car runs beautifully. However, I am concerned because after checking the timing at 800 RPMs it is 18° BTDC with the vacuum removed and plugged. It is 30° BTDC with the vacuum connected. Interestingly enough, I get an additional 18° advance when I reach 3000 RPM. I have driven the car vigorously, trained very steep hills with no preponderance of pinging or pre-detonation. How is this possible?

I was concerned that the harmonic balancer had slipped, so the timing marks on the balancer would no longer be relevant. However, I have chaulked the pulleys and there is absolutely no movement. I simply cannot imagine that the Woodruff key could’ve given way. Keep in mind, the timing has remained constant as reported above.

I should also mention that the current distributor adjustment screw is totally retarded. Also, the engine will accept more advancement and actually responds with increased vacuum to 18 inches.

 

Is the engine running as it should? I have heard someone with a Ferrari stressing over the same concerns, for him it was just the way the engine was made, and Ferrari even agreed to this saying "no two cars are the same".

Is there any difference in performance, idling or low revs- and at the risk of being offensive- is your test equipment fully functioning? I only ask as I got false readings from some second hand kit I bought at a garage liquidation auction (no wonder they went broke).

I too have restored my shadow, my knuckles have finally grown back 8 years later

 

Thanks for the response. I actually used two separate timing guns (although they were the same model with the variable timing knobs) and achieve the same results.

 

The official spec is 18° BTDC at 3000 rpm with the vacuum line disconnected. How close to that are you?

 

Kevin,

Being down here I look at things a tad differently.

I read this 4 times, and been thinking all day.

A couple of beers later I found the spare HE dist base in my stash, I really must sort this pile.
Anyway, the AUST HE has Mech Advance of 13deg, as stamped on the weights. thats it. The PreHE has 15.

What the vac does, dunno, never bothered, it works, so dont fix it.

Your 30deg is odd (I will stay polite), as there is only 30deg between cylinder posts in the cap.

I dont run a timing plate on any of them. I set the base at 10BTDC with a wire down the 1A plug hole, and lots of experience. They always start and run at that setting.
The micro is always preset in the full retard point, and then I drive time them, doing the adjustments as needed.
Vac attached as I would normally have the car.

Our engines are 12.5:1, and yours are 11.5:1, so there will be variations on any theme.

That vintage has basically zero emission junk to worry about down here.

If you are driving as you state, I doubt there is anything Jag wrong. I suspect the gauges, BUT, I have never used one. I was taught to LISTEN, and do it that way, and its stuck.

My HE can be nailed up hill, A/C on etc, and NO pinging. 240k kms, it cant be out too much I reckon.

 

FWIW, bad plug wires (or flaky plugs) can cause erroneous timing readings.

You get an inductance caused reading on the timing light from nearby wires if the plug wire (or plug) you are hooked to, is bad.

Been down that road twice in the last 48 years. Replaced wires and plugs allowed normal timing readings.

Doug

 

I looked at this for several days... I think it is all good!!!

Initial timing with vacuum line removed and plugged: 800 RPM @18* BTDC this is correct
Vacuum line reconnected: 800 RPM @ 18* BTDC ? and/or 3000 RPM @ 18* + 18* = 36* BTDC this is normal
18* initial + 0* Vacuum Advance + 18* Mechanical Advance = 36* Total Advance under power.

As you chop throttle from high RPM, the Vacuum Advance will kick in
18* Initial + 0-18* Vacuum Advance + 18* Mechanical Advance + 18* +18* + 18* = 54* Total Advance possible under light load and high RPM

Under light load and low RPM the Vacuum Advance will kick in, before the Mechanical Advance has reached 18*.

Light acceleration between 1000 - 2000 RPM will have a slow burning fuel air mixture, due to lower compression (restricted intake track), the Vacuum Advance kicks in to advance the spark, so you get maximum pressure during the power stroke. Once Mechanical Advance is fully reached @ 3000 RPM there should be no Vacuum Advance, or need for it.

Rgds
David

 

Thank you for everyone’s input, I really appreciate the amount of time that’s been invested in each of the responses. I will of course respond to each of the questions raised by those who submitted comments.

Integrity of Testing Equipment
i’m currently using a timing light with variable advance. I suspected that the equipment may be faulty so I purchased another timing light and received the exact same timing results.

Quality of Spark Plg Wires
very intrigued by this comment. The spark plug wires have barely been used. However in an effort to be thorough, I have substituted one a with a couple of different spark plug wires from my other vintage vehicles.

Varied Timing Results
December 3 - 750 RPMs 18° BTDC without vacuum 30° BTDC with vacuum advance
December 4 - 750 RPMs 12° BTC without vacuum, 22° BTDC with vacuum advance

Accuracy of Timing Marks on Harmonic Balancer
Applied white paint across all of the pulleys including the harmonic balancer and observed absolutely no movement. Hopefully, the Woodruff keys are intact.

Consensus Desired Timing Results Target
750RPM - 0 to 3° BDTC without vacuum advance and 18° of additional advance at 3000 RPMs. Keep in mind that this figure should be 18° of additional advance on top of the initial static advance.

 

A comment on plug wires,

Depending on the type of plug wire, if you have fiberglass/carbon cored wires, as used for radio static suppression, you can break the inner core with sharp bends, or sometimes the metal ends get separated from the core. metal core wires of course, have much lower resistance.

IIRC, graphite core wires should have something like 600-800 ohms/inch resistance, if you get one a lot higher, something is wrong.

My theory, on induction induced bad timing readings may also include dirty distributor caps such that some amount of voltage crosses over to other plug wires even if all plug wires are good, giving an improperly timed flash on the light.

Just about a year ago, I had bad timing readings on a V8 motor, the light said the timing was 20* retarded, attempts to rotate the dizzy to make the reading right, resulted in an engine that didn't want to run as it was way too advanced, so i reset it using a vacuum gauge (vac advance connected and operational, tee to vac gauge in the vac line to dizzy, adj dizzy to highest vac reading at idle and back off 2" of vacuum.)

The wires were new looking Packard wires, and I didn't want to remove or damage those, so i replaced the plugs and the dizzy cap. Timing reading went back to normal, the vac gauge timing method had me with in 2* of the 10* static advance (vac adv disconnected) I was seeking. It was either the plugs, which were very dirty, or the cap causing the problem. The problem back in 1975 was bad plug wires.

Doug

 

FYI, all of the spark plug wires that I’ve been using our carbon core and actually did not even consider a dirty distributor with rogue carbon trails. Interesting phenomenon with the Packard. For what it’s worth, I too used a vacuum guage and I teed into the back of the passenger side manifold. Oddly enough, the 18° BDTC without vacuum advance would need to be increased to 19° in order to achieve the highest vacuum of 17 inches.

 

Varied Timing Results
December 3 - 750 RPMs 18° BTDC without vacuum 30° BTDC with vacuum advance
December 4 - 750 RPMs 12° BTC without vacuum, 22° BTDC with vacuum advance

This is not normal. The drop of 5* BTDC within 24 hours, makes me think mechanical advance hung up ( 2.5* on distributor) on-day-one and was freed on-day-two. The vacuum advance seems to move about 10 - 12* when re-attached each time, which seems normal.
Good cleaning and light lube of advance mechanisms inside the distributor seems to be in order.
Rgds
David

 

David, thank you very much for taking the time to comment. A hung up centrifugal advance was my initial suspicion as well. However, please keep in mind as I’ve indicated, I still receive 18 inches of advance at 3000 RPM on top of what I observe at 750 RPMs (most recently reported at 12° BTDC without vacuum).

 

But just to be sure, since its a Lucas car, have you actually serviced the distributor? Does it spring back firmly or is it a bit mushy when you turn the rotor by hand?

 

I still receive 18 inches of advance at 3000 RPM on top of what I observe at 750 RPMs (most recently reported at 12° BTDC without vacuum).

Did you get 18 inches of advance at 3000 RPM (without vacuum) on top of the original 18* BTDC initial timing, for a total of 36*? or did you get 18*+ 12* for a total of 30*?
Rgds
David

 

18° +12° for a total of 30° vacuum Advanced disconnected. However, I neglected to mention that I’m missing the timing plate and have been estimating TDC to be directly between the two mounting bolts on the pan. Upon further review of timing plate images available on the Internet, that does not appear to be the case.

 

Varied Timing Results
December 3 - 750 RPMs 18° BTDC without vacuum 30° BTDC with vac advance > 750 RPM 18* BTDC (no Vac) +12* @3000 RPM = 30*
December 4 - 750 RPMs 12° BTC without vacuum, 22° BTDC with vac advance > 750 RPM 18*- 6*=12*(no Vac) +18* @3000 RPM = 30*

December 3 - (18* @ idle) Mech advance stuck at 6* with 12* left until 3000 RPM reached 18*+12*=30*
December 4 - (12* @ idle) Mech advance at 0* with 18* left until 3000 RPM reached 12*+18*=30*

Would be interesting to know how hot was the engine bay was on the Dec 3rd and 4th when you took the readings..
Advance Mechanism binds when hot or loosens up when hot????
Rgds
David

 

AND

Those timing plates DO NOT have mounting holes, they have SLOTS, so the plate can be positioned correctly once TDC 1A Comp Stroke is established, and that is where my wire down the hole comes out to play.

REMEMBER

NEVER EVER rotate that engine backwards, as the Plastic Chain Boomerang can/will snap, and your words will be choice and the wallet will groan quite loudly.

If you miss the mark when setting the TDC, go around 2 more times and PAY ATTENTION, and I am NOT being nasty, or trying to upset you, but I have replaced too many of those Boomerangs due to people "sneaking" the crank backwards a couple on mm's, coz they blinked at the wrong time, oops. Its about a 30 hour task IF you are familiar with that engine, and the Boomerang is $600 last time I looked.

 

Grant – Thank you very much for your post. Duly noted with respect to turning/sneaking the crank backwards. I’m absolutely paranoid about the health of that chain tensioner. Also, with regards to completing two revolutions in the event that I miss TDC, would that be necessary because even TDC on the exhaust stroke would be suitable. I’m basically interested in the point wedge the piston reaches it’s Zenith at the compression or exhaust stroke. Again, as a relates only to the correct positioning of the timing plate. Which incidentally, I will be making with some sheet metal. I’m only interested in TDC as I use a variable rate timing gun.

Basically, I have created a piston stop tool using an old spark that I tapped to accept a 3 inch bolt. I will obviously be incrementally loosening the bolt in an effort to vary the length to enable the crank to rotate through.

 

Kevin,

Thank you.

The wire is from the old days, and my learning curve without many tools, manuals or money.

Lots of discussion on my method, as used by many my age/era.
I, and you, know there is a variable to the wire method as the piston "rocks over" TDC, which equates to degrees of crank rotation.

Since I only use it to set the base timing for the distributor, then "drive time" after that, the few degrees that will be passed on to the jackshaft are negligible at best, and then the wear factor of the drive and driven gears at the distributor, make it zero to me.

That Boomerang is fine, except for the backward nonsense. My PreHE is just about to nudge 800K kms, and the base engine is untouched since Coventry in 1976.

The biggest killer, apart from the above, of that Boomerang, is OVER HEATING that engine, and sadly that is getting more common than I like.

 

800,000 km! That is absolutely remarkable. As a relates to accurately identifying top dead center when attaching the timing plate, my method does not require the crank to be rotated counterclockwise. However, it is an iterative process to say the least. I have removed the porcelain and electrode portions of the spark plug, drilled and tapped the remaining portion to accept a bolt and nut. Position the crank in the neighborhood of top dead center, remove the spark plug in 1A and thread in this “homemade” piston stop. Next, I hand tighten the bolt in order to make contact with the piston and then back off a couple of turns. And then rotate the crank clockwise using the 7/8 wrench until I make contact with the bolt and the piston stop. Repeat the process until the crank makes a successful pass. After which, I tighten the bolt “slightly“ and repeat the process. At this point, the additional incremental adjustments, if any, will be very minute. This is a more time-consuming process, however one that does not require a counter clockwise revolution or calculation.