Rover V8 Distributor Vacuum Advance Connections

[Raggylad]

I'm about to replace the knackered Stromberg carbs on my 110 V8 with a new pair of SU HIF44s (acquired on ebay for considerably less than Burlen sell them for).

A couple of questions:

- There are currently 2 vacuum pipes running from the distributor vacuum advance unit to the near side Stromberg:





The pipe on the left side in the photos runs to the top of the carb on the inlet manifold side (there is an equivalent connection on the SU carb):



The other runs from the right to a connection under the carb on the air inlet side:



On the SU carbs there is no apparent connection for this second pipe (what is its function, anyway, if the first pipe is the advance - being after the throttle ?).
The vacuum pipes run from either side of the advance/retard diaphragm. The left hand (in photos) one definitely advances the distributor when you suck on it - which makes sense as it goes to the inlet manifold side of the carb and hence the vacuum it delivers will vary with the size of throttle opening. That's the easy one.

The right hand one runs from the other (retard ?) side of the diaphragm, through a trap of some sort (you can see it in my first pic) to the underside of the carb on the air inlet side. If I understand my sidedraft carb theory correctly, that should mean that is delivers a pretty constant vacuum unaffected by throttle opening or closing ? If you suck on the tube, there's no visible effect on the distributor - which sort of makes sense if I am right about the constant vacuum ? So what happens with SU carbs which have no connection for this second tube ?

I've chased this question around through google and come up with the 3 conflicting pieces of advice/options - so, do I:

- Leave the right hand (in the photos) connection on the vacuum advance open to atmosphere ?
- Block it off ?
- Tap a connection into the air inlet manifold ?

[Jono FD3]

On the engine I'm running (SD1 electronic ignition) the conection you are talking about is blocked off with a rubber bung but then this engine was injection, but can't see that making any difference to the ignition side of things?!

[RoverP6B]

Hello Raggy,

If your vacuum advance module is more than a few years old, then it is very likely the diaphragm has perished in which case it isn't working anyway.

However if it is working and or you replace it with a new one, then as you correctly surmised only the advance (furthest from the distributor) is used by the SU carburettors. The rear takeoff is vacuum retard and is not required. Just put a rubber cap over it.

Ron.

[mgbv8]

Thats interesting???

One vac pipe to the carb from the normal side of the diaphragm on the dissy which is where I would expect it to go to for normal use. ie max vac at idle / over run which then decreases as the butterflies open so the mech advance can take over.
Then another vac line from the other side of the dissy vac module diaphragm to the inlet manifold as well.

How odd??

That would appear to make those connections cancel each others vac signal out ??

Or is the connection on the carb only generating vac as the throttle opens.
This would create an advance signal at part throttle which would then die off at wide open throttle wouldnt it??

I have to admit that I have never seen a dissy vac module with those connections ??

Could it be some sort of an emmisions thing??

[RoverP6B]

Hi Perry,

I don't know if it is the same with Stromberg carburettors, but with the SU the vacuum point is on the air filter side of the butterfly, so the system runs ported vacuum as opposed to manifold vacuum.

In this case there is no vacuum advance at idle as the butterfly is closed. Once off idle and with the butterfly open there will be vacuum advance of varying degrees dependent upon throttle position. The more the butterfly is open, the less the vacuum advance.

Ron.

[mgbv8]

Hi Perry,

I don't know if it is the same with Stromberg carburettors, but with the SU the vacuum point is on the air filter side of the butterfly, so the system runs ported vacuum as opposed to manifold vacuum.

In this case there is no vacuum advance at idle as the butterfly is closed. Once off idle and with the butterfly open there will be vacuum advance of varying degrees dependent upon throttle position. The more the butterfly is open, the less the vacuum advance.

Ron.

Thats the same as old B series engines in the MGB. No vac at idle, then a bit as you pedal it and then none at wide open throttle.

But I'm confused as to what the other pipe does??