Ignition Timing from an old Service Bulletin

Came across an old Jaguar bulletin that references dynamic timing. Has anyone else seen this? This bulletin has timing at TDC @ 750 rpm. I have a 70 SII with Strombergs. Trying to keep it all original. Would this be the correct timing for my car?

There was a distributor change for US and Cdn cars in Oct/69 7R7974, 7R38580. I believe this was the vacuum retard dist, which is timed at 10 degrees BTDC. There was also a change to the so called parabolic cams in Nov/69 so this bulletin likely refers to these two items.

How would I know if I have one of these distributors? Or do I assume it does since I have a build date after that?

Is your engine number before or after the numbers I gave you?

Sorry didn’t fully read your email. There were two distributors used on early and later Ser II cars for N.A. and Canada. The parts manual for these cars refers to the engine numbers as:

For the first (older number cars), their dists had no vacuum cannister of any kind on them, only a plug where they entered the dist body. The second list had vacuum retard cannisters pictured below. I’m not entirely sure how to read the reference If you are after 7974 it would be the vacuum retard dist.

Juast for greater certainty I belive the 10 degree timing is at a static setting, when the car is running at idle and the vacuum is engaged the setting is at TDC ie the vacuum while running retards it to that.

You have to be very careful here. IIRC, you had said your vacuum capsule was disconnected. If that’s still the case, the dynamic setting doesn’t apply. The reason is that you have high vacuum at idle, which is why dynamic is ten degrees retarded from base static timing per the bulletin. If you set dynamic timing to zero WITHOUT an operating retard capsule, your timing would be retarded all through the operating range, so that at high loads where vacuum was low, your timing would still be retarded.


I would set it at 3000rpm. No one drives idling.

My motor is 7R9913-9, and the distributor does have a vacuum pickup from the carb to the unit. So does this bulletin supersede the standard 10 BTC? Or is current view better?

David I assume your distributor is the same as in the photograph - that is the hose to it attaches to the vacuum cannister in the same place. This is a vacuum retard distributor. The timing for it is 10 degrees before TDC if set statically - engine not running. If the engine is running (idling at +/- 750 rpm) and creating a vacuum the timing is set with your timing lite at TDC. I suspect the 10 degree difference is created by the vacuum at idle retarding the distributor 10 degrees. Having said all this the general view is that the vacuum units were very unreliable and failed early. I don’t believe new ones are available. You can test yours by sucking on the hose, and seeing if the plate in the distributor rotates as you do this. Unless you are in a state that requires smog/pollution testing it doesn’t make any real difference. If the vacuum cannister is not working just set the timing at idle at 10 degree BTDC, preferably using a strobe light.

Terry, thanks for your advice. That sounds like a good plan. I just wanted to bounce that off someone with more experience with an e-type. Cheers!

Moss supposedly has them:

Appears to be the retard unit. Not that I am advocating getting one. Terry’s advice seems spot on.

I am also not going to advocate someone use or not use it, but I will ask the question-
What is the advantage of not using it? From what I read, the vacuum retard will make the engine run slightly hot at idle which reduces pollution. That certainly will not hurt anything. How much does the engine “suffer” at speed? I thought, but I am not sure, that it is mostly doing nothing at speed. So if there is indeed little downside, but it helps reduce pollution at little cost, why would someone not want to use it?

And I have a new one in storage. Happy to sell once I get moved and everything out of storage later this year.

The cooling margin on these cars is not large. My Series 2 is most vulnerable to overheating after a long run on the freeway followed by stop and go traffic when I enter a town - ie when doing a lot of idling. So, a vacuum retard would be very unwelcome on my (pre-vacuum retard) car.

I would presume that the vacuum unit has no “knowledge” of the speed of the car, but just retards the ignition when vacuum is high. So, when cruising under light load, the ignition is going to be retarded - exactly the opposite of what the vacuum advance unit did on the earlier cars to reduce fuel consumption. So, another black mark against the vacuum retard function in my mind.

All this having been said, simply removing the vacuum retard capsule from a 41322 distributor which has been designed to work with one, is not going to be ideal either. If I planned on removing it, I would either replace the distributor with one better suited, or change the springs to more closely match the earlier 41207 that was designed to work with no vacuum capsule. YMMV.

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The ONLY smart thing to do with a vacuum retard distributor is to disconnect the vacuum. If you want it to look stock, leave the hose connected, but plug it so no vacuum reaches the advance capsule. Vacuum retard is 100% an emissions thing, with NO functional or performance advantage whatsoever. Exactly the opposite, in fact. It increases engine temperature at idle, and, if anything, reduces off-the-line performance.

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Yep: broken-off golf tees do nicely!

Aruuuuugh! The vacuum retard is fine if fuel mileage deterioration, performance degradation and overheating doesn’t bother you.
Best ever update/improvement was Rays ignition system!

I’m suspicious of people who don’t like dogs.
But I trust dogs who don’t like people.


David, my S2 is also pre vacuum retard, so I cannot say just how it will affect things. I can only go by what I read. Each car is different. I have never noticed any overheating issues with my S2, so to me, I do not see some additional heating as an issue. To me, I do not care if the car runs at 185F or 205F as long as it does not overheat. To me, the reduction of air pollution would be worth it.

From a Jaguar Technical Bulleting:

Note it is ported opposite of ported vacuum advance. It has manifold vacuum at idle and then drops off as the throttle is open and then, from what the bulletin says, mechanical advance does the advancing. So as I read it, the vacuum retard is really only in play when the throttle is closed and should cause no ill effects while driving. If so, again, virtually no loss for less pollution.
Yes, it does sound like one loses the fuel economy of vacuum advance. But I also thought I heard this distributor has enough mechanical advance to equal the previous distributors with vacuum advance??? So is there actually a loss?

Yes, I would agree it is 100% an emission thing and no performance advantage. But it does have a very important function- to reduce emissions. And as stated above, no real performance loss. Win-win.

It seems to me in the world we live in today emissions is a very important function. I would readily give up some power to have a cleaner burning engine.
I have a 1967 S1 I got in 1980. In 2015, I got an S2 for several reasons. I wanted and Etype that I hoped to drive more so I wanted one an Etype that never had to have the body rebuilt. I wanted some safety features such as head rests, better lighting, collapsible steering column. And I wanted one that polluted less. IMO, the stock engine with stock distributor (yes no vac retard in 1969) ZS setup runs great.

We are currently in a time where gas powered cars may very well be on their way out. That is fine. There are climate issues, etc. Some of us still like and enjoy the vintage cars as they were built and do not really wish to convert to EV. I like the old cars as built. I believe if we hope to continue to drive these cars in any kind of even slightly regular use, we should be doing all we can to present ourselves and our hobby in the best light as we can. We may reach the point where we need public support to keep our hobby alive. We should be asking for the public’s support by showing them that as much as we do enjoy these cars, we recognize they emit extreme amounts of pollution compared to what the public wants, and we should show the public we want their trust by trying to make these cars pollute the least amount we can and not just snub our noses at a public by disregarding any pollution concerns to a public that could care less about our hobby. Sure we can try to justify these cars do not get driven that much and so do not contribute that much. One of our cars probably pollutes more than several hundred new cars. We may justify it, but will the public?
As I state previously, I am not telling anyone what to do. I am not advocating we should be making these cars cleaner than stock, just hoping to keep them as they were. Just mentioning some thoughts.

As I understand it, the port used for the vacuum retard module monitors manifold vacuum, rather than ported vacuum. Ported vacuum (which is used by the vacuum advance modules) is pretty much the same as manifold vacuum with the one exception that ported vacuum drops to zero when the throttle plate is closed (as it physically blocks the port), whereas manifold vacuum is at its maximum when the throttle is closed. As soon as the throttle is cracked open, even a little, the two vacuum ports will read pretty much the same level of vacuum.

I don’t think I agree. The table from the document you quoted clearly shows that the vacuum retard capsule continues to retard the ignition (though not as much as at idle) when there is less manifold vacuum. When the engine is lightly loaded (eg medium speed cruising) manifold vacuum is still significant. So, for example, if manifold vacuum is 8" of Hg, the retard module retard the ignition by 4-6 distributor degrees - which translates into 8-12 crankshaft degrees. In contrast (see the distributor tables for the S1 vacuum advance module on page P.X.s.1 of Bentley), in the same circumstances (8" of Hg depression), the S1 vacuum advance module advances the ignition by up to 4 distributor degrees, or 8 crankshaft degrees. In doing so, the fuel consumption is significantly reduced. I would hazard a guess that the reduction in fuel consumption results in a corresponding reduction in air polution. So, I see the vacuum retard being at best a win-lose, where the win is reduced NOX polution when a cold engine is idling, and the lose is increased CO/CO2 polution when a fully warmed engine is lightly loaded whilst cruising. I am not qualified to judge whether the balance of the win/lose is positive or negative, but suspect that it could be argued both ways.

Yes there is. The problem lies in the fact that the vacuum retard distributor mechanical advance curve is limited by the need to prevent pinging at wide-open-throttle settings when manifold vacuum is very low. When the engine is lightly loaded (at the same rpm), the vaccum retard unit actually reduces the advance, whereas the vacuum advance unit would increase the advance to take advantage of the light engine load and providing better fuel economy.

I agree with your comment that it is important to avoid the perception that drivers of classic cars don’t care about the level of polution that they create. In this particular case, however, it seems to me that what Jaguar did in 1970 was a compromise that allowed the car to meet one federal requirement, but arguably didn’t result in any less polution, and indeed may have resulted in more. That said, California requires all vehicles to be fitted with the emissions control equipment that they had at the time they were originally built, so anyone who converts a twin Stromberg car to triple SUs is non-compliant. And who would think of doing that?..