Vacuum Advance source dual SU carbs on A15?

[rbastedo]

I've had my vacuum advance line connected to the intake manifold and just this weekend while checking my timing had the idea that might not be correct.

It seems my Vacuum Advanced is always advanced as far as it can be, and that's not right.

With it disconnected I've got 5 degrees BTDC. Connected it jumps to 25 degrees BTDC at idle (about 800 rpm).

Should there be a vacuum port somewhere on one of the carbs?

Where is this supposed to connect on cars that had the twin SU type carbs?

 

[metalmonkey47]

Vacuum advance doesn't go to manifold vacuum, it goes to the carb. I think the front carb should have a port.

What are the SU's off of? 

[ggzilla]

Vacuum advance traditionally goes to manifold vacuum, but with twin SUs is not used at all.

[rbastedo]

They are from an MGB, motor from a Datsun 210 sitting in an MG Midget.

It is a hodge podge but it's all mine...

The carbs are SU HIF4.

[ggzilla]

A-series engine runs good with no advance. But will get better fuel economy with it. So connect it to manifold and set timing with it disconnected.

[rbastedo]

Will there be any trouble with it running effectively at 20 - 25 degrees BTDC all the time?

[ggzilla]

No because vacuum advance traditionally goes to manifold vacuum. Only with advent of emission controls did they start retarding it at idle (by connecting to carburetor throttle port).

[MikeRL411]

 

Vacuum advance traditionally goes to manifold vacuum, but with twin SUs is not used at all.

 

Not so. The rear Hitachi SU has a vacuum port for the advance. Check your MGBs, they probably have the same connection. It's on the rear [engine facing] side so is hard to see. There are 2 types of connections. It can be a simple slip on rubber tube connecting to the hard line to the advance mechanism or a pressed on toroid with a screw-on attachment to secure the advance connection. It all depends on the vintage of the SUs.

[datzenmike]

Will there be any trouble with it running effectively at 20 - 25 degrees BTDC all the time?

 

It won't be at 20-25 all the time. It follows manifold vacuum so when you step on the gas it drops. At full throttle, presumably you have almost zero vacuum advance, the static 5 degrees??? (i don't know what the exact advance is) plus the 20-25 degrees mechanical for a total of 30-34 degrees?.

[ggzilla]

Is so. Traditionally goes to manifold vacuum, maybe you are too young to remember.

 

With twin Hitachi specifically it goes to carb ported vacuum. Maybe HIF carbs have a port on them too. But generally most A-series engine swappers leave the advance can disconnected.

[metalmonkey47]

Just throwing this out there, my 210 ran better with just the centripetal advance in the dizzy. I found about 5 BTDC -9 ran best. 

[rbastedo]

Gawd now I'm totally confused!

[ggzilla]

Yes it will run better plugged if the can is leaking, which 90% are now after 35+ years.

 

What is "5 BTDC -9"?

[ggzilla]

Gawd now I'm totally confused!

Don be confused. Try it both ways. Or all three ways and see for yourself (1. unconnected, 2. port vacuum, 3. manifold vacuum).

[ggzilla]

To ensure it is working:

 

1. See if it holds vacuum. If not, new vacuum advance cans are $30 to $130.

 

2. With cap removed, visually check that the plate rotates when vacuum is applied. Many 210 distributors are stuck (those that are not leaking). New bearing plates are $80.

 

[wayno]

It really depends on if the distributor is made for the manifold or carb type connection, it sounds like his is made for the later type of connection at the base of the carb, it should not advance to 35 degrees when one connects the advance, it should only do that when one gives it the gas.

I have 2 complete setups for the A series engine, one uses a very early datsun roadster set of SUs that have no vacuum port for the diatribtor, that works off the manifold and has the port for it, the other is a more modern set and has the vacuum port at the base of the front carb.

It sounds like your going to need to find a distributor with a mechanical advance, or maybe you have the port on the base of the front carb, and you just have not found it yet.

You have any photos of your setup?

When you put a timing light on the running engine and you give it the gas, does the timing start to retard, like going from 25 degrees btdc to 15 or 10 degrees btdc?

[ggzilla]

All Datsun distributors have mechanical advance, except the 280ZX Turbo distributor as far as I know.

 

When you put a timing light on the running engine and you give it the gas, does the timing start to retard, like going from 25 degrees btdc to 15 or 10 degrees btdc?

Are you thinking of vacuum retard? Some 210s had that.

[smoke]

Gawd now I'm totally confused!

I'll keep this very general to give you a grasp of it, then you can go from there with the small detail. Your average distributor has two sources of advance, the weights and springs inside advance your timing to X amount at X amount of rpm, (this is really the most important set) the second is your vacuum. Vacuum advances at light load, light cruise (in most). This is because many engine like a whole lot of timing advance when there is not a ton of load on the engine, they will get better mpg and get more of a complete burn, some even run up into the 50s like this and run good. Sounds like a lot I know, but it does not matter, because when you hit the throttle heavy, as in making the engine pull, the engine loses vacuum (well, there really is no such thing as engine vacuum, but that a WHOLE other topic) when the vacuum drops off from engine load, so does the amount of advance your vacuum canister gave you. This brings you back to your the mechanical advance part of the distributor. Try not to think about the vacuum advance part too much. As far as hooking it to the manifold, yes, it will idle with a lot of timing, but it will not harm anything, as a matter of fact, contrary to popular belief, many engines idle at a much cooler temp this way. So, don't worry about that. When you set your timing, unhook the vac can from any vacuum source and just pretend it does not exist, get your timing set, then hook it up. 

 

If you do not have a ported advance, that's fine, nothing "bad" is going to happen. If you do, hook it up and see if you like the way it runs/drives better. Which ever way you personally think your engine runs better, this is where you should hook the vacuum advance to. On most of my engines, I prefer full manifold vacuum, but I know of many who prefer it ported. So, that part is up to you, it's not going to hurt anything.

[rbastedo]

Thank you all, I'm starting to grasp this.

Yes I have a picture of my setup, trying to include it here. This is an old picture, some hoses have been moved / removed.

So the rubber covered brass tubes on the sides of each carb may be vacuum ports is what I'm thinking now.

If so would it be better to connect the two with a hose and a three way connector then a hose to the distributor ot merely connect up the back carb to the vacuum advance?

 

[smoke]

yeah, give it a try and see if u like the way it runs better.

[ggzilla]

would it be better to connect the two with a hose and a three way connector then a hose to the distributor ot merely connect up the back carb to the vacuum advance?

Just connect it straight to the distributor. Carburetors that have a spark port only one needs to be connected.

 

Let us know how it runs.

 

To test if that is the spark port, put a vacuum gauge on it.

 

Spark port

* 0 vacuum at 800 rpm

* full vacuum ~20 inches at 1100 rpm

* vacuum becomes less as throttle is opened

 

EGR port

* 0 vacuum at 800 rpm

* slight vacuum at 1100 rpm

* vacuum get stronger as throttle is opened

 

Manifold vacuum port

* full vacuum ~20 inches at 800 rpm

* vacuum becomes less as throttle is opened