Problem

[greeners]

I've got a wire going into the the carburretor that has become disconnected due to the plastic connector having broken.

I have attached a photograph showing the problem.

What is the purpose of the wire and can I simply buy another connector?

Any help gratefully received.<br><br>

[bmcecosse]

It's the vacuum advance connection - it runs down to the dizzy. You need a stiff piece of plastic tubing to connect it - so it doesn't collapse with the vacuum!

[mmjosh]

i have a spare plastic tubing part

[greeners]

Thanks for the quick responses guys.

The mogster doen't seem to mind this too much - certainly was starting and going as well as ever until I noticed...!

What function does the vacuum advance connection actually perform?

[PSL184]

If you didn't notice any difference then its probable that the vac advance unit in the distributer has failed. You need to remove the cap and suck on the pipe to see if you get any movement in the baseplate of the distributer.....

[StaffsMoggie]

Although its only a couple of degrees of advance, a working vac advance unit does make a difference on the road.

[ndevans]

It'll affect fuel consumption, particularly on a run. Should start & idle ok, wil run fine, just use a bit more fuel.

[bmcecosse]

It's a lot more than a 'couple of degrees' - and yes - it makes a very worthwhile difference to the 'brightness' of the engine - and the fuel economy.

[rayofleamington]

It makes absolutely no difference at full throttle / when accellerating, so most people would not notice any difference.
It is ther only to improve economy when at low throttle, the ignition can be advanced further without pinking occurring. That gives more power for less fuel.

[bmcecosse]

But if it's failed - the engine is sluggish to 'pick-up' when flooring the throttle - hence the 'brightness' remark.

[MarkyB]

How often do you use full throttle?
It is definately a noticeable difference when accelerating.

[rayofleamington]

if you open the throttle even half way at low revs you loose the vaccuum. If the vac advance is very slow to come off you'd be having a few seconds of pinking.

[PSL184]

For normal automotive applications the vacuum advance will benefit the engine as follows.

1) Improved idle cooling
2) Improved idle quality
3) Improved fuel economy
4) Improved throttle response
5) Improved drivability
6) Enables improved spark knock control under full throttle accelerations
7) Enables leaner fuel jetting at light load to further improve fuel economy.


The basic reason for all these improvements is that the vacuum advance mechanism allows the distributor to supply a more optimum spark timing proportional to the load and speed output. Without the vacuum advance the distributor can only vary spark timing in proportion to speed and ignores its need for between 10-20 additional degrees of spark timing ("advance") at light loads: (idle and cruise conditions)


The basic reason for the change in optimum timing at light loads is that when operating at light loads, the mixture is leaner for fuel economy and less dense because of light load. These conditions cause the charge to burn slower, and thus, to reach peak pressure at optimum point in the cycle, the spark must be initiated earlier. Failure to do this will result in "retarded" spark timing and all the aforementioned losses.


All engines are different, and have different spark timing requirements, but they are all the same in that as load is decreased, additional spark timing is required for optimum combustion.

[greeners]

Many thanks to all respondents.

I fear I may well have lost the vacuum as I do recognise some of the symptoms.

I will check out further.

Thanks again!

[bmcecosse]

Well - not difficult to fit a new one - although due to fiddly nature of the little clip - it's best done on the kitchen table. ie Dizzy removed from engine !