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MG Midget and Sprite Technical - Vacuum advance

Not changed anything and car running OK but pipe and connectors look old so just wondered how I check the vacuum advance?

If I switch to electronic does the electronic ignition do the advance? And if it does what happens about the existing vacuum advance?
Dave Squire 1500

Dave

Pull of the pipe and see if it makes any difference to the engine speed and behaviour in general. Check for any leaks on pipework when connected back up using snoop or dilute washing up liquid, but hey, if it ain't broke, why fix it?

I have the 123 electronic and it still requires the vac advance. You set it with it disconnected though.
Mark O

Dave - the best way to check the vac advance is to disconnect it at te carb end and suck on the pipe (clean it first if you must ;) )

You should hear the bellows in the dissy operate, and the vac in your mouth/pipe/bellows should stay constant with time (it should be a clsoed system, and the bellows should return when you release the vac.

If the dissy cap is off, you will see the top-plate rotate as the vac is applied (and rteturn opn release).

Any 3D ignition system won't need to use vaccum... engine revs and throttle position (along with crank position sensor) are all that's needed.

HTH - A
Anthony Cutler

An electronic module fitted to your dizzy still needs the vacuum, an electronic dizzy still does, 321's still use vacuum (some of them). The vacuum just advances the timing on part or closed throttle, to give the sparse mixture more time to burn., which helps idle and slightly improves mpg. Highly tuned engines often dispense with it on 3 counts. 1. The engine doesn't spend much time on part throttle. 2 mpg is not an issue. 3. Hot cams, with lots of overlap, don't have much manifold vacuum to operate the module anyway.
Assuming the pipe and rubber ends are in good condition, flex the ends to see if they are sound,check by sucking and watch the top plate in the dizzy move. You should be able to put your tongue over the open end and the plate should hold it's position.
Allan Reeling

Dave,
you can get the vacuum pipe connectors and thin nylon pipe as new parts

I had problems with petrol pooling towards the dissy end of the vacuum pipe connector and crazing the rubber connector so I replaced the tiny plastic vacuum pipe and connectors with a bigger (advance, reteard tube) rubber hose and things seem fine now

(the 1/4" fuel hose joining the carbs cracked after 2 years use so was replaced with modern 6mm fuel line that fitted better and hasn't cracked)


Nigel Atkins

I can vaguely remember when the vacuum pipes were steel and there was a trap in the middle to catch fuel and act as an evaporator. Presumably there to stop fuel contamination of the vacuum diaphragm. The problem is worse now with the ethanol addition. Lifting the dizzy end of the pipe up should help. If you look at Paul Hunts site, he has made an evaporator/trap out of a 15mm copper stop end.
Allan Reeling

yes the earlier cars had a metal pipe and trap of some sort, I got one but didn't fit it for reasons I forget now (probably all in the Archives here)

I'd have thought lifting the dissy end of the pipe would have had the liquid running in easier(?)

the consensus view was that it didn't on the 123 anyway but I didn't like the thought of petrol being there

fitting the bigger vacuum (advance, retard as the seller calls it) hose seemed to do the trick so it remains

I also tried blocking off the vacuum as some suggested and even bought vacuum blocking kits that I didn't even knew existed - I must admit I didn't notice any difference in the running with the 123 vacuum blocked off or not but thought as I'd paid extra for the vacuum on the 123 I might as well have the use out of it
Nigel Atkins

ETA:
sorry word drop out again

>>the consensus view was that it didn't (matter about petrol getting in) on the 123 anyway but I didn't like the thought of petrol being there<<
Nigel Atkins

Nigel, Just thought lifting the dizzy end would cause any fuel to pool towards the manifold end where it would evaporate with help from the heat at that side.
Allan Reeling

Allan,
thing is you can't lift it where it joins the dissy and on mine it points upwards from the dissy so any pipe ends sloping down to the dissy and the dissy is lower than the manifold

I made a high point on the manifold side by cable-tying the vacuum hose to the breather pipe and having a looped climb of the vacuum pipe from the carb connection

just fitting the bigger I/D vacuum hose seemed to cure the problem but I must admit I've not checked for a good while
Nigel Atkins

Nigel,
It's me not thinking midget!!!
Allan Reeling

arh right, I often do the same with other models
Nigel Atkins

This thread was discussed between 11/12/2013 and 13/12/2013

MG Midget and Sprite Technical index

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