vacuum advance

[kobra]

I as told by my engine builder that the vacuum advance connection from dizzy to the carb should nt be connected yet. He did explain why but i still did nt understand. Can any one explain in laymens language why. I have sbc 351 when he reset the dizzy and the carb i had done about 800 miles. when the tube was connected it did seem more responsive. So is there a certain mileage where the vacuum advance should be connected. An info would be helpfull. Regards Ray.

[craggle]

It should be disconnected while you are setting the timing but connected whilst you are driving the car.

Without the vacuum advance connected you are messing up the timing at higher revs.

At least thats what I did to my engine.

Craig.

[wilf]

For a performance-oriented engine, you don't need vac advance.

It is there to improve emissions/fuel consumption on light throttle openings.

Go back and ask your engine builder why they don't want it connected - I suspect it is because they have the distributor advance curve and your static timing set to not need it. I have known engines run very poorly when it was connected.

If your engine "picks up" better with it connected, then your timing is not set properly - as soon as you hit the throttle with meaning, inlet vacuum pretty much disappears, and so should any vacuum advance, so vac advance should make naff all difference to large-throttle-opening-performance.

[neil still building it]

think of it like this:
if you have a high(ish) compression engine,sensible(ie sporty cam) the fuel ingested 'll burn pretty quick anyway so won't need the help of a 'spark advance' timed to help the mixture burn in time
but if you run a low compresssion ,poorly cammed 70's emissions type 'smogger' it need all the help it can get.......(more advance helps oor quality fuel,low compression,'emissions cam-low lift/duration/overlap allowing it to burn quicker)
bear in mind this has all to happen before the power stroke gets all used up and the exhaust opens,more ingested ,compressed,and another power stroke.......add some revs and it gets mighty a short space of time . hence advancing the timing to make sure its all burnt in time..our poor fuel doesn't help of course..
neil

[craggle]

As the builder has fitted a distributor with vacuum advance and has gone to the trouble to run a length of tube from the carb to the distributor I would say connect it.

I know you can do away with the vacuum and use purely mechanical advance but doesn't this usually mean a different distributor? Different balance weights and springs at least.

I had a Range rover 4.2 that lost the vacuum advance and it became less powerful, used more fuel and was difficult to start. Once the canister was replaced it ran fine again. Obviously an engine designed to run with vacuum advance.

If the engine seems more responsive and goes better with it connected then use it. Better still talk to the guy who built the engine and clear up what he meant.

Craig.

[TonyD]

With the vacuum advance unattached do you block the hole into the manifold. If not you have an unmetered air leak into the manifold and it is ruining your A/F ratio.

A prolonged burst of full throttle and lean mixture may cause detonation, overheating and poorly engine........

Cheers,

Tony

[kobra]

Originally Posted by TonyD

With the vacuum advance unattached do you block the hole into the manifold. If not you have an unmetered air leak into the manifold and it is ruining your A/F ratio.

A prolonged burst of full throttle and lean mixture may cause detonation, overheating and poorly engine........

Cheers,

Tony

Hi Tony, yes the manifold is blocked. And thanks guys for all the info. Regards Ray.

[queeniee]

Originally Posted by wilf

For a performance-oriented engine, you don't need vac advance.

It is there to improve emissions/fuel consumption on light throttle openings.

.

I experienced that. It has been a good idea to disconnect my vacuum advance yesterday, when a new Mallory Hyfire VI was installed. I had a lot of problems with timing, an hesitating engine under load (high rpm) and these problems dissappeared with the vacuum advance disconnected.
(Two days searching why the engine was not running properly after changing all the ignition system: distributor, coil, ignition box, plugs...).

[dingocooke]

Aaahh Queenie, you discovered the inverse rule of fault finding:
The more expensive the parts you replace at first, the cheaper the final solution becomes!

[queeniee]

Originally Posted by dingocooke

Aaahh Queenie, you discovered the inverse rule of fault finding:
The more expensive the parts you replace at first, the cheaper the final solution becomes!

Not exactly: things had to be changed.
First: the ignition Hyfire IV died completely. So, I replaced it by the Hyfire VI since I could not find a new Hyfire IV.
Also, a spark plug change was required with the mileage I had on the old ones.
The distributor gear (bronze) was in a very poor condition and the breakerless distributor was 9 years old, I had one in stock so the change was made.
And it costs nothing to change the coil since I had a new one in stock.
The wires were also old, after some years, it is better to put new ones.
But after that, the car was really not running well in spite of many carburetors and timing tuning.
Only the removal of the vacuum tube made the engine happy. Maybe the Hyfire VI is related to this .
I keep my old parts as spares (the working one). I go to Poland at the end of June so I have to trust the electrical parts (as well as the other ones).