Vacuum retard test

[marcus]

On my V12, when I take the rubber pipe off the vacuum unit and suck/blow there doesn't appear to be any resistance in the unit itself.

I guess there should be, to operate the linkage inside the distributor?

If this is the case and I need to replace the vacuum unit, is this just a case of swapping it for a new one?

The engine is uneven during acceleration underload, in all gears and I'm just working my way through the usual suspects at the moment and I came across this query with the vaccum.

Any advice appreciated, thanks.

[christopher storey]

Assuming this is a UK distributor which DID have vac advance, rather than possibly a North American spec one, then the lack of resistance sounds like a failure of the capsule. However, before condemning it, try the following

1. put your finger over the end of the capsule take off nozzle, turn the baseplate to the fully advanced position, and then take your finger off the nozzle. If you feel suction on your finger in doing so, then the capsule is still intact.

2. Examine the baseplate and capsule linkage carefully to see that it spigot of the capsule is still engaged with the baseplate

If neither of these bear fruit, then the diaphragm has probably failed . You can check the functionality with a xenon type timing light . With the capsule connected to vacuum, run the engine up to say 2500 rpm and observe the reading. Then blip the throttle suddenly, when the amount of advance should reduce suddenly and markedly ( shown by the flash of the light moving in the same direction as crankshaft rotation). If this does not happen, then clearly the vac adv needs attention . You can find the capsule code on the face of the diaphragm capsule, but you may well need to clean it ruthlessly with a wire brush to see both the part number e.g 4409117 or similar, and the vacuum spec such as 5-13-8. The Distributor Doctor is probably a good place for spares and he does give good advice over the phone

[PeterE]

Marcus, this is part of my S3 V12 with which I'm very familiar! Christopher, the unit is a vacuum retard unit (and not as per the six cylinder engines for advance) - it was implemented I believe for emission control and faster warm-up purposes.

When I bought my car my unit did not work either, the rubber diaphragm had perished inside, a common problem on the V12 I understand because of the level of heat in the 'valley of death'. Sucking on a hose attached to the capsule pipe just gave me a lungfull of petrolly-smelling air.

I replaced it with a new one from SNGB and all was well for a while but this also developed a leak after a few months, this time along the shaft that goes into the distributor; because the unit provides retard and moves the distributor pick-up plate in the direction of distributor rotation the vacuum is applied to this actuator shaft side of the capsule diaphragm and not the other side which is what you see with advance capsules. I replaced mine again with another one under warranty from SNGB.

The vacuum retard applied is not proportional to carburettor vacuum, it is either full on or off. Thus what you end up with is a jump from 7-9 degrees of distributor retard (14-18 degrees of crankshaft retard) at 600rpm idle to none at all when you open the throttles slightly. If you have the timing set at +12 degrees advance at idle then the crankshaft timing jumps from -6 degrees to +12 degrees very quickly - I've seen this using my venerable 1970s Gunsons timing light! This is why the ROM tells you to set the idle timing with the retard unit disconnected.

However, I have now blocked off the vacuum retard altogether so the pick-up plate remains fixed. There are a number of forum threads (most on Jag Lovers') about the advantages/disadvantages of doing this, my reasons are perhaps a little specific; my right ankle is fused because of arthritis and thus lacks the level of 'bendyness' that a normal ankle has. With the vacuum retard connected there is a pronounced jump in the engine speed as the timing advances 18 degrees at the crankshaft and I found that getting smooth take off from standing (mine's an automatic) was difficult, with the retard disabled this problem has gone.

I've covered about 1,500 miles with this arrangement with no ill effects although I sometimes get exhaust popping on the overrun. US spec cars had some extra vacuum plumbing for the retard unit which was routed via a temperature-controlled valve which shut off the vacuum when the engine was up to temp; mine which is a late V12 has this unit but it's connections are sealed off. I've not mentioned the problems I had with the centrifugal advance getting stuck - partially advanced - but that's another story!

There is no doubt that the issue with yours may be causing your uneven running as I had this too - a leaking retard unit will let air into the rear LH carburettor and when I replaced mine the first time it stopped this and the engine ran much better; idle/static timing needs to be set up carefully on these engines too.

[marcus]

Thank you very much Christopher and Peter. Excellent advice.

I'm sure we'll get to the bottom of it soon.

It's tempting to check her in to a garage, but that'll be admitting defeat, besides which, I bought this car to stay with me long term so I intend to learn along the way. More fun this way. I don't have the cash to splash about anyway at the moment.

[PeterE]

Marcus, a no-cost option would be to try blanking off the vacuum hose where it goes into the the bottom of the rear LH carb, this way you should seal off any air leak and run the car without any retard. Basically what you have now with a non-working retard unit but without any possible air leak.

I was surprised how much impact a leak on the carb vacuum feed made to the running of the engine.

[marcus]

OK Peter, I see what you mean. Perhaps a suitable bulldog type clip. Small one on the rubber hose I guess.

Marcus

[PeterE]

That would do it but I just 'borrowed' one of my eldest son's golf tees :)

[marcus]

I now know that the timing (which I thought I'd set correctly) will be incorrect, simply because of the air leak through the vacuum diaphragm.

So I need to disconnect the rubber pipe where it attaches to the vacuum unit and plug the hole with some blue tack to seal the leaky hole.

Then set the timing at idle to 12 BTDC.

Would this be correct?

[PeterE]

Marcus, yes, timing on the V12 is always set with the retard unit disconnected and the hose from it and to the carburettor plugged, if not plugged the air bleeding in will increase the idle speed which really needs to be set as low as possible at around 650 rpm.

12 degrees BTDC is the setting I use with Shell V Power fuel, 10 degrees BTDC is another setting I've seen but for US spec cars.

[vee12eman]

Hi Marcus/Peter,

Read this with interest because I too have had issues with a new SNGB vacuum retard for a UK spec V12, which failed after just a couple of months. I chose not to replace it but to repair it by carefully grinding it open, replacing the diaphragm with rubber from nitrile gloves (used in double layers with some cloth sandwiched between and bonded with silicone). It was tricky even Heath Robinson and when I put it all back together I had serious doubts it would work, but it has. Along with carefully tracing and sealing possible air leaks my car went from a hesitant engine beat at constant rpm to a smooth and steady beat. I used self tappers to close the unit with carefully applied sealant and so far so good.

Regards,

Simon

[Woolfi]

- For low gas consumption a motor needs a vacuum-advance capsule.
- The EV12 has a retard capsule, because of emission reasons.
- If you have problems with the retard unit, close the rubber hose with a 5 or 6 mm screw. The retards unis is only working at idle. At driving it is not managing the timing.
- If disconnected there will be no difference in driving. If the capsule makes problems, disconnect ist und close the hose.
Since ten years I have a advance unit in my EV12. It reduces gas consumption with roundabout 3 - 5%. There will be NO difference in max power. For correct working and timing, a advance capsule MUST be connected to a special port, which is ending roundabout 2 - 4 mm in front of the "round brass metal turning door" inside the carb.
- At idle the vacuum is near zero. No changing of timing.
- At low part throttle maximum vacuum is at the port and therefore maximum timimg advance of roundabout 16 degree. This is depending of the type of capsule.
- At higher part throttle the vacuum is becomming lower at this port and therefore the timing advance.
- At full throttle the vacuum is lower than the vacuum, the capsule needs for small timing advance. Therefore no change of timing.
The timing-management resulting from the special vacuum curve at this port is "exactly" the motor needs for proper working.
Best solution for your EV12: no retard unit or correct connected advance unit. To renew a redtard unis is a waste of time and money.
I have seen Stromberg carbs with this port. My Stromberg didn't have this. Therfore I have drilled a whole, ending roundabout 2 - 4 mm in front of the "brass-door" towards the carb mouth. To this whole I conneted the rubber hose to the retard unit.
My car is driving like this since years and works without any problems.
Regards Wolfgang Gatza