Hi everyone.

I have a 1974 series 3 V12. It is a US import, but has had the US emission system removed. However when I’ve had a look at it it looks like they have got a bit over enthusiastic and have taken almost everything off the vacuum system! They have connected the rear left carb directly to the distributor advance and that’s about it!

Now, having checked a local friends car that we know is correct to UK spec (he’s owned it from new!) I think I am happy to restore mine to the same as his. However, I would like to know just what a couple of the bits in the system actually do so that I can check they are working.

There are two units whose function I would like to understand. First there is a 4 port unit on the back of the inlet manifold (attached to a pipe on the cooling system, part number C37430) which seems to be known as the Vacuum Thermo Switch. In the image the 2 rear (left) ports labelled “C” and “D” connect together (when cold, not checked when hot) and are the connectors from the rear left carb to the distributor advance. But the other 2 ports seem to be closed (when cold). Does anyone know what is supposed to happen with this unit when it is cold or hot.


Then there is a small unit in each air intake (part number C38357) which seems to be known as the Air intake Air Temperature Sensor). If anyone knows if it is should open or close at a certain temperature perhaps you could let me know. Here’s what it looks like.


All info gratefully accepted.

 

All those parts are basically to help the engine better manage its emissions as it warms up.

Vacuum Thermo Switch - It changes the vacuum distribution as the engine warms up (you'll note it's screwed into the water jacket). As the engine warms it adjusts the vacuum to the distributor retard and the flaps to the air cleaners (see below).

Intake Air Temp. Sensor - This adjusts the mix of warmed vs. fresh air being directed to the carbs. The US cars have a jacket around the exhaust manifolds (yours may have been removed) and a round, ribbed paper/metal duct that is attached from the jacket to the side/front of the air intake horns on each side. When the engine is cold warmed air is diverted from this jacket to the intake but as it warms up the valve at the front of the intake opens shutting off the warmed air and allowing cold air in. The idea is that the warmed air allows the fuel to vaporize faster when the engine is cold so lowers the startup emissions.

The only one that really matters to the basic functions is the vacuum line to the distributor. I'm not sure if yours is connected the same as on the non-federal cars so check that out to make sure it's set up correctly, otherwise you can probably ignore the rest.

 

Thanks Irgridley. Useful info, although it sounds like it'll be difficult to check if they are operating properly! I guess if I connect it up and get it running ill see if it is any improvement. I could probably check if the various valves open by seeing if I can suck through the pipes once the engine is hot...

I'll try and let you know.

Thanks again

Rick

 

If you don't have one you'll find one of these very useful in testing the vacuum lines and what they operate...

 

Thanks. I'll take a look at it.

Rick

 

The previous post is the most correct in my opinion. However I think the vacuum to the bypass valves on the carbs have to be addressed. This and the distributor vacuum are the most important on the thermostatic switch

 

I have learned it is not a vacuum advance. It appears the vacuum on the distributor is totally unnecessary and retards the timing until the car warms up and then it is turned off. I would consider just blanking it off and plugging the port on the left rear carb. It is a notorious source of leaks.

 

That's interesting. As I have started reinstating everything to European (non emission control) layout I have found several anomalies. I have seen a few comments elsewhere about not needing the distributor vacuum, and in fact when I tried sucking on mine it turns out it isn't working and just leaks continuously, so that's air leaking into the LH rear carb! Rather than block it off I am hoping to get it all back to factory standard. I guess Jaguar put it on for a reason! Hopefully with all new pipe it won't give me any problems for a few years and at least I'm getting to understand the system.

Having taken off the air filter housing to sort out the Air Intake Thermo switch I noticed that the pipework inside the air intake case doesn't connect to anything at the moment. These are the pipes I an talking about.


Firstly, does anyone know what they are for?

The centre pipe comes out on the back of the box (so carb side) and looks like this on mine..

The parts catalogue shows this connected to pipes through a hole in the front part of the case, before the filter element. I have the hole, but no pipes. But it says 'Not Canada USA' so I'm unsure what the UK non emission control setup should be. Any ideas?

I'm missing the rubber pipes C37524/C32910 and the metal pipes C38736/C38737 near the top of the diagram.

All help gratefully received..

Rick

 

Tarapoocchy


There are four balance pipes going across the engine. One is the main vacuum port going to the gulp valve. The second goes from the breather on the drivers side cam cover to all four carbs and to the carbon canister. (very Important to get this one hooked up without leaks or the crankcase pressure will blow out oil through the rear main seal), The third is for fuel to all the carbs. and the fourth and the one you are asking about connects to a balance pipe that pulls from both sets of carbs. This balance pipe goes to the carbon canister. I believe it creates more vacuum to power the other emissions ports such as the breather vacuum, the gulp valve vacuum etc. I am leaning more and more to removing everything going to the carbon canister. It is almost impossible to keep all of the lines from leaking.

 

Hi egold56,

Thanks for all your advice. I think I am starting to understand most of the system. You're right that trying to stop leaks in the vac system is a nightmare.

Sounds like you still have all the US/Canada emission control bits on your car. Mine has all been removed already so I have no gulp valve, no air pump, no metal air distribution pipes and no carbon canister. All the holes in the manifold have been properly plugged. Unfortunately I think whoever did all this got a bit carried away and left me with a few leaks and only the distributor vacuum unit directly connected (which was leaking anyway!) While the car did run it wasn't smooth and felt like something really needed sorting out. Lots of backfires and needing choke even when hot. I have discovered this has been exacerbated by the wrong springs in the carbs and leaking seals on the needle adjuster, so oil disappeared from the dampers within a mile or two of driving.

I'm still not sure if I should have the pipes on the back of the filter housing that I mentioned in my last post and I'm not sure if the EX/EM version is connected up in the same way. I'm sure something should be connected to the pipe otherwise anything could get drawn in without going through the filter.

I do have a friend who has a very original UK car so I hope to get a look at his sometime so I can get mine set up like his (which runs like a dream!)

I'll let you know what I find.

Rick

 

The gulp valve is actually useful. When the engine is under load and you suddenly lift off the accelerator the carb slides close up but the vacuum continues to pull gas into the carbs creating a momentary rich mixture condition. The gulp valve allows a "gulp" of air to be added to the fuel/air mix at that moment to lean it out. If the gulp valve is not functioning you'll get backfiring when you lift off the throttle under load.

 

No you don’t need those pipes.
Having constant vacuum to distributor is very bad.. It constantly retards your timing by 12 degrees and when hot will cause loss of power and most of your other symptoms. I don’t think you need the gulp valve without the air pump.
It is imperative to make sure your bypass valves in your carbs are not leaking and are properly. They will do the same thing as the gulp valve. If your carbs were not rebuilt they are bad and will cause all of the problems you say you have.

 

I'll have to look again when I'm next at my car but I don't believe the gulp valve is connected to the air pump, it relies on the manifold vacuum to draw the air in (which by the way increases when you lift off the throttle). The air pump is for the air injection system that dumps air into the exhaust just downstream of the valves. That's designed to help complete combustion of any unburned gasses and the system needs a pump to offset the obvious pressure in the exhaust.

Rick - the metal parts in the air intake I believe are part of the EGR (Exhaust Gas Recirculation) system, yet another emissions system, this one to try to reduce NOx emissions. It sounds a bit nuts but it actually bleeds a small amount of exhaust (which has cooled a bit) into the intake system in order to dilute the air/fuel mix with a non combustable gas that will cool the combustion slightly (without messing up the A/F ratio), which in turn reduces the NOx emissions. If you look closely you'll likely see the remnants of exhaust soot in the pipes. Those pieces would have been fed by a tap into the exhaust manifold, IIRC one on each side, likely blanked off on your car. These are frequently removed as it can slightly improve performance. This and the parasitic power drain from the air pump are two of the bigger contributors to the HP differential between US Federal and ROW cars.

If all this sounds pretty crazy and primitive it is. Remember this is the era where catalytic convertors were just beginning to appear and smaller production car companies like Jaguar were struggling to make their engines smog compliant without spending a lot of money on redesigns so they tacked on whatever they could cobble up to get by.

 

A gulp valve sense a change of vacuum in the intake manifold. The gulp valve is intended to stop the air flow over the exhaust valves when a rich mixture is added to the system, i.e. whenever you let up on the gas pedal. If the gulp valve does not operate correctly, then this rich mixture would hit the exhaust manifold with the forced air and explode in the exhaust system.

 

gulp valve was part of the early emission control systems of that time. This system included an air pump that forced air through a manifold of small tubes to the area of the exhaust valves to oxidize (burn) unburned fuel before it exited the exhaust system into the atmosphere. The problem could occur when you let up on the gas pedal and a rich fuel mixture passes through the engine. When oxygen is added by the air pump and this mixture hits the hot exhaust manifold it ignites, causing the backfire.
The backfire suppressor. The Gulp Valve.

To prevent backfiring, there is a second path from the air pump to the intake manifold through the vacuum controlled gulp valve. The gulp valve remains closed most of the time directing air from the air pump to the exhaust system. When the throttle is released intake manifold pressure falls, and through a small hose connection to the gulp valve, this vacuum causes the gulp valve to open. This allows air from the air pump to enter the intake manifold and lean out the fuel/air mixture, preventing backfiring.

 

Strangely though it must only be a problem with the US/Canada models (also a few EEC countried like Sweden, and Japan) with emission controls (EX/EM models). The Gulp Valve doesn't seem to be fitted on the non EX/EM models!

 

What does Ex/‘Em mean?

 

EX/EM seems to be how Jaguar refer to the US spec emission control system.