7 replies to this topic

[Spannermanal]

I'm looking for a dizzy for my A series 1098 which has 266 cam and 295 head with twin 1 1/4" HS2 carbs and have been told that the dizzy from my 850 will not have the right advance curve.

I believe that a 43D dizzy does not have a vacuum advance and was wondering how this affects the performance .

I don't quite understand how the advance works, or why it needs it, and what effect not having it has. I would like to know what distributor I should look for. I have seen some on ' an internet auction site' without points, which I assume is a good upgrade.

Any help would be most appreciated.

[sammidonut]

the vacuum advance is vital for getting the timing of the spark right all the way through the rev range. as the speed of the engine increases the earlier the spark needs to happen. this is achieved through air being forced from the carb to the dizzy by way of the vacuum hose. this inflates a small diafram in the dizzy pushing the timing forward. without this the engine would idle perfectly but as you open up the throttle it would seriously affect the performance and it may not even run at high revs.

45d is your best bet from what i can see, electronic ingnition is good for a reliable spark and not to much more expensive. hope this helps

[yellowmonster]

The 43D doesn't have a vacuumcontrolled advance, but a mechanical controlled one.
There are two little bobweights and springs inside, which move due to centrifugal force: the higher the revs, the more centrifugal force, the more advance...
It should work fine as long as you keep the insides lubed...

Edited by yellowmonster, 25 August 2011 - 02:27 PM.

[lrostoke]

I'm a little confused by Sammidonuts explanation.
I always thought the vac unit worked by air being removed not blown in ??
Hence when you test a unit you suck on the vac pipe to see if the ignition plate advances
The vac unit does most of its work when the engine is under light to no load, At higher revs the weights inside the dizzy are doing most of the work in advancing the timing,

[Dan]

It does indeed work under vacuum rather than pressure, hence the name. Although vacuum is merely pressure in a different place and technically you can't actually suck anything because a vacuum is nothing and so can't exert a force.... never mind that though.

The 43D is not unusual in having mechanical advance, all dizzies (except the SPi one obviously!) do. A proper ignition system uses both.

I'll compare it to a digital system so you might understand. Digital systems use a 3d ignition value map, the dimensions are engine speed, MAP or throttle position, and ignition timing. The MAP or throttle position are giving an indication of the load on the engine because the load determines how hard the engine is working and can be derived by comparing the engine speed to the throttle position or manifold depression. The timing of the spark has to be controlled to a specific point that works best for every combination of the other 2 values. In a dizzy the mechanical advance (bob weights) are monitoring the engine speed. The vacuum advance is monitoring manifold depression, giving the load compensation to the ignition system. You need both to be able to set it up well. By using different combinations of bob weights (the two weights inside the dizzy are not the same and operate in different speed ranges from each other) and using a dizzy that has both vaccum advance and vaccum retard you can build a very detailed ingition setup.

You don't have to worry about lubricating the bob weghts inside the dizzy at a service, you'll do more harm than good constantly opening up the dizzy.

[Spannermanal]

OK thanks for the information, but which dizzy should I go for - the car will be a normal road car - but with a little more ooomph. Would I be OK putting a standard 45D4 with electronic ignition in - will this give me smooth running but also take account of the mild improvements - or are they so mild that it won't make much odds?

Thanks

[dklawson]

OK thanks for the information, but which dizzy should I go for - the car will be a normal road car - but with a little more ooomph.

There is no simple "right" answer to that question. Your engine will no longer be "stock" so choosing the right advance curve is not as simple as looking in a table and saying "that's the one".

One option for you if you have the money is to consider buying the 123 distributor. It has several (16?) switch selectable advance curves. I believe the instructions come with suggestions for where to start based on the build of your engine. From there, you either test drive or dyno the car to determine whether to select a curve "up" or "down" on the list. See the link below.

http://tdcperformance.ca/
The web site above is owned by Canadian, Marcel Chichak. He helped select the A-series curves for the 123. He is not the manufacturer. If you click on the "sales" link, you will find U.K. suppliers for the 123, including Mini Spares.

I can almost guarantee that others posting soon will suggest you install MegaJolt, which is an ignition system of the type Dan discussed above.

[Spannermanal]

Thanks everyone. I think I have a starting point!