Vacum retard - again

[Norton]

Hi. relatively new owner of a 1971 etype s3 auto.
I have read about this infamous vacum retard and is a little bevildered, but I am aware of its purpose. Late last autumn my ignition died, I was able to obtain an used original Opus system which I fitted and works fine. I adjusted ignition to 10’ BTDC, 750 rpm vacuum retard disconnected. But when driving a little longer trip now, the engine was running very hot. Timing rechecked and found ok. Checked the vacum line and found out it was sucking about same amount to very high rpm. Done witout any vacum gage, just by the feel of a finger. If it provides vacuum througout rev range it will not serve any purpose??

Traced it and found it connected to aft left intake manifold, NOT underneath aft LH carbutettor. Was able to connect a vacum hose to tube extending below LH carb. It was a very fiddly job! But testing sucktion from this connection I found that vacuum was decreasing and disappeared with increased rpm.
I never checked ign timing before I installed the replacement. I figure the previous ignition was operating constantly with vacuum retard and the present PO / mechanic compensated with a different timing setting. In other words compensated one fault with creating an other. Does this make sense??

Someone on this forum claims that the vacuum retard is via a thermostatic valve that cuts off when engine is hot. Does not make sense to me, then the idle rpm would be very high at idle??

Have not found any such thing in parts book or repair manual either. I think it is the positioning of vacuum source that makes it weaker with larger throttle opening??

Just did a longer test run with vacuum disconnected and seems to be working fine. Warm up time will be slighly longer, but who are idling for 10 minutes to warm the engine anyway. Considering to leave it disconnected. Will there be any other consequenses??

Rgds Harald

[PeterCrespin]

Hello Harald

I was probably the person who made what you consider a nonsense comment.

The vacuum retard is a system designed to make a very large engine warm up quicker and make fewer emissions. It does this by retarding the timing, which burns leftover fuel in the exhaust (less pollution out the exhaust), and that extra heat speeds warm-up and reduces time on choke (less pollution out the exhaust). If I can help you fix your car you will hopefully learn something (less pollution out of your exhaust)

Retarded timing is not needed once the engine is hot. At the back of the right water rail casting you should see a thermostatic valve to cut off the vacuum retard should the engine get too hot. It has four nipples on it for vacuum connections. Study your parts book and the drawings, Then look at your workshop manual and make sure your system conforms to the type designed for your transmission and the car's original market. If you don't have the thermostatic vacuum valve but wish to keep the vac retard, you'd better find one to avoid overheating your alloy engine. Also be aware that your automatic gearbox has a vac connection, which from memory is at the back on the left and would not be suitable for a timing connection.

When you've fixed the car I suggest you start saving for a non-OPUS ignition system for when yours fails. It is far less a question of 'if' and much more a question of 'when', since the older and more primitive electronic components are way past their expected lifespan and are not as durable as later parts.

[vee12eman]

Hi,
I recently changed to ported vacuum advance, however my efforts to decide which was better was thwarted by the discovery there were other issues with the ignition and tuning of the car prior to the change. However, I am happy with the results. I use a Re-opus system in place of the original, this is now unavailable new but should replicate the characteristics of the original closely.

Early UK spec 1971 cars don’t have the thermostatic element to the vacuum retard system, we established this some time last year, a search of the forum might find the information, if not it can be proved by the parts manual. I remember it being a little convoluted in there, but basically, if you don’t have a temperature sensor on the R/H rear water pipe casting, adjacent to the R/H rear carb, then you don’t have the thermostatic controlled vacuum.

This is the pre thermostatic design of casting:



The thermostatic sensor is fitted to a later version of this casting.

Regards,

Simon

[MarekH]

The 1974 Parts Manual shows you all of the possible connection schemes for the vacuum to control ignition timing.

I append a link to an annoted copy of the Parts Manual:- http://www.jag-lovers.org/snaps/snap_vi ... 1190800376 from several years ago. Where I wrote "vacuum advance", it should read "vacuum retard". The tan coloured US Owners Manual booklet describes how the other connections (electrical and mechanical) were designed to operate.

kind regards
Marek

[Norton]

Hi.
No Peter it was not you!
Thanks all for pointing me in the right direction. I was looking for a temperature device at the source of vacuum, LH rear carb. Was not aware that vacuum line is supposed to be routed via other devices. And found the pages Marek is refering to.

My RH aft water pipe rail is identical to the one in Simons picture, so I don’t have the thermostatic vacum valve. My car is a relatively early -71 us, auto. But the engine is from a 74-78 xj12 injection - as far as I can tell by eng no. Don’t know type of water rail on xj12, but could be a mixmatch of parts anyway.

Testing the thermostatic valve it should be heated to 104,4 ˚C to check for cutting off vacuum ( tan us book ). Leads me to believe that it normally is open, but shuts off vacuum in just case of overheat. ( which is sensible, but as long as it is shut you will have a high idle rpm )

Then there is the solenoid pressure switch, as far as I can tell only on auto cars. (also absent from my car)
This uses pressure from governor ( rises with road speed ) in autobox, its purpose is to direct vacuum from manifold below 35mph and above a proption of it. ( tan book ). Might explain why my vacuum was connected to LH rear manifold ( but job only partially done as this valve changes vacuum from manifold to carb ).

Tan book also describes a thermal over-ride switch. Which purpose also is to cut off vacuum retard when engine reaches normal temperature. Drawing in same book indicate a position below LH thermostat. (No traces of such a thing either on my engine). Which device this switch works in conjunction with I am not able to determine. Is this a developement that replaced something else at one point??

All this lead me to study pictures in sale ads for S3 etypes. Found that vast majority was without thermostatic valve and water rail blanked off. Some with valve in place but no vacuum lines connected except one or two. And a couple with railing as in Simons picture. So it seems quite common to disconnect this system.
Any comments on this? Consequenses? ( except warm up time and emissions )

For now I’more concerned about not ruining my engine than emissions, took a longer ride yesterday and car was running good. With no vacuum retard!

Rgds Harald

[Norton]

And Simon, I have read about the Opus reputation. And of course theese components are close to 50years old now and as you say, well beyound life expecteancy. But this ignition unit have the long wiring harness and have been attached to the picture frame all its life. So I decided to give it a chance and see if it maybee will last, living in the relatively cool breeze behind the radiator. The car it came from have aquired a replacement from sng barrat.

Rgds Harald.