8 replies to this topic

[Grayedout]

Most of the advance is done by the weights in the dizzy and the vacuum does not actually add much !

What you need to do is make sure you plug the manifold end or the engine will be running lean !

Pinking is usually due to too much advance whereas you now actually have slightly less !

[fikus01]

the vacuum compensated for part throttle running!! it advances the timing because there is effectively less air in the engine so less chance of pre-ignition/pinking but a greater chance of poor runnign and poor economy!!

at full thruttle the engine produces almost no vacuum so therefore uses just the weights!!

although it pulls ok i'll be tickover will be a bit bumpy!! depends where the vac takeoff is on the carb!! some were positioned badly!

[dklawson]

There are two possible connection points for the vacuum line depending on the vintage of the car. The vacuum canister on the dizzy is designed to work specifically with one or the other... not both. When you fit a replacement dizzy, it's often best to move the vacuum canister from the old dizzy to the new one (assuming its diaphragm isn't damaged). The first connection type is ported vacuum which comes off the carb body and is tapped into the carb's venturi. The faster you go, the more vacuum and the more advance. This supplements the bob weights. The second vacuum type is manifold vacuum. As its name implies, the connection is to the intake manifold. The faster you go, the more you open the throttle, the lower the amount of vacuum (higher pressure) in the manifold. This is the type of connection mentioned above. It gives you MORE advance at part throttle to improve fuel economy.

Whenever you tune the car and set the mixture you should disconnect the vacuum line and plug its connection at the carb or manifold as appropriate. Once you've set the mixture and are happy with that you reconnect the vacuum line and adjust the idle speed as necessary. If the vacuum advance is effective at idle, it will affect your idle RPM when reconnected. Also as mentioned above. If you aren't going to run the vacuum advance (perfectly acceptable) plug the open port on the carb/manifold so your mixture won't go lean on you.

[fikus01]

i never realised there were 2 types or vac advance!! i've used loads of diff types of carb/dizzy and always conncted the vac gauge to the same place as the vac advance of the dizzy and always got the same effect!! less vacuum at higher throttle openings till the revs came up!!

was the type i did not know about fitted to any certain year cars??

learn something every day here:)

[dklawson]

I wish I knew which type was fitted when. My car originally had the 23D4 dizzy which is strictly mechanical. Those Mini owners I've worked with have had transplanted dizzys so I never knew what was correct.

[njw26]

I've just fitted my new dizzy (thanks Jammy), and found my vacuum advance pipe is bust. Ive set the timing as best I can and there is no pinking under hard acceleration. Why?

Without the pipe I thought acceleration would be pants - its not. Why?

Now when I fit a new pipe, I'll need to adjust the timing - right, but turned in what direction?

A few things there so any help would be great.

Cheers

Matt

I have just had a MG leccy dizzy fitted to my 12G295 998cc stage 1 Mini and had it RR to have the mixture and timing set. The RR operator was having difficulty with the vac advance pipe setting it up, so he removed it and blocked the advance off, timing and mixture set ok and on the power run got 58BHP, well pleased.

Not sure if this helps.

Cheers
njw26

[fikus01]

he was having trouble because the advance curve for a mg is totally inapropriate for the requirments of a 998!! you'd probably get a better drive and possibly more power with the original non electronic standard 998 dizzy!!!!

distributors are designed for the specific needs of the engine, the cc, cam and compression ratio all count and rarely will they jsut be instantly swappable!!