Recurve Duraspark

[pbrown]

3) Vacuum advance. This only operates at idle and cruise, but highest vacuum is at idle. This is added to initial advance as soon as you set that and reinstall the vacuum line. So, in reality, this is what your "idle advance" is: initial plus vacuum.

The vacuum advance is typically connected to a port that is just above the throttle blades. This is different than the old Load-o-Matic stuff. In this manner, there is little to no vacuum advance at idle. It's when the throttle just opens that the vacuum advance will be greatest and it will vary based on engine load. It is still best to set base timing with the vacuum capped off. Then reconnect the vacuum and go. The vacuum advance can is adjustable on Kenny's new dizzy. You can use a small allen wrench to adjust the amount of advance you get at a preset vacuum setting. The wrench adjusts the preload on a spring in the vacuum can.

[Luva65wagon]

This is contrary to the thread on Classic Inlines thread. They indicated this to be called "ported vacuum" which was used in the early smog emission days, but was not the way to go today. But they did indicate it was a "debated subject" and then proceeded to explain what each was. The Load-o-matic carbs only offered a load-sensing vacuum using the little diaphragm doodad on the carbs.

So, anyway, I guess it is still debatable.

I've spent all of 20 minutes so far looking into it. I only know that when doing a DSII swap when I have a 1100 carb - I only have manifold vacuum as a real source and should not be using the original feed from the 1100. I will continue to study the pros and cons, but I'm hooking it up to manifold vacuum tonight, for now.

[Luva65wagon]

This is the thread on ported versus manifold vacuum sources:

http://www.classicinlines.com/VacAdvance.asp

[Luva65wagon]

This is a pretty good article on ported versus manifold. Might also explain why, now, with manifold vacuum connected it seems to be running rich. If only it was easy to gain ported to try it.

http://www.lbfun.com/warehouse/tech_..._explained.pdf

[redfalken]

The vacuum advance can is adjustable on Kenny's new dizzy. You can use a small allen wrench to adjust the amount of advance you get at a preset vacuum setting. The wrench adjusts the preload on a spring in the vacuum can.

I wish! I tried several sized allen wrenches and it is what it is. I think Crane and Standard make one for about $35-40.

When I get mine hooked up I'm going to compare ported vs. manifold. I borrowed Jeff's vacuum gauge so I can see what it's doing too. I've read plenty of forums where just as many people use one or the other and the consensus always seems to be "whatever works for you".

[Luva65wagon]

Well, first of all, you have to have the option. Perhaps your Weber offers the option, but the 1100 doesn't, really.

I've not done a lot of research yet, so for me the jury is still out, but tonight I added a manifold vacuum source and plugged off the Load-o-matic feed source on the carb. With initial timing sitting at 7 BTDC (I didn't try to adjust it to 6) I plugged in the vacuum advance and it jumped to 27 deg, or 20 deg added - at idle. With rev to about 2500-3000 it max'd at 41 degrees total advance. I do [now] have a spare vacuum source, so maybe I'll check vacuum reading too.

I will say that if I had ported source and the thing was running at 6 BTDC, it runs pretty crappy that way.

I think I still have some carb issues though. It is running pretty rich and part throttle run is not real smooth, but WOT acceleration is very responsive under no load.

[Jeff W]

I was having trouble with that carb as well (I had a filter screwed right on the carb so I hopefully can't be blamed for the dirt)... but I was testing it when I had that bad resistor wire.

Have you swapped the one off your wagon for testing?

[Luva65wagon]

I was having trouble with that carb as well (I had a filter screwed right on the carb so I hopefully can't be blamed for the dirt)... but I was testing it when I had that bad resistor wire.

Have you swapped the one off your wagon for testing?

Gene and I were staring at it tonight and this was floated. The answer is no, though. But it wouldn't be really hard to do. It's a slightly different carb though, so I'll hold off just until I give up trying other things here.

FWIW, with what was in it when I opened it, I can see why you would have had troubles. So no, the once rusty tank in the flarechero would be to blame.

[Luva65wagon]

A little more insight from my readings....

Basically manifold and ported vacuum are the same except at idle, where ported is OFF at idle and manifold is ON.

Initial advance is determined by whether you use or don't use one of these.

Still looking when I can, but nothing so far has helped me learn whether the recurve I did on mine, based upon the CI instructions, is doing what it was supposed to do.

[Luva65wagon]

Not trying to beat this horse anymore than needed (OK, maybe I am), but I can really begin to see why there is no consensus out there on the subject of "which port to use." Even MSDtechsupport gets a basic idea of engine vacuum wrong, which I don't think is a debated subject. I was very surprised by this, but doesn't help clear the confusion out there:

We recommend that the vacuum advance cannister be connected to a ported vacuum source. This will provide vacuum at idle, part throttle, and cruzing speeds.

This is not correct. It is very well established - reading 99.9% of the sites I've read (this would be the .1% that didn't agree) - that there is NO vacuum at idle from ported source because this port is above the throttle plates.

He then continues with:

Do not connect it to manifold vacuum because this will increase with RPM. With that, you would run the risk of having too much timing which can cause detonation.

This too is wrong, partly. Manifold vacuum and ported vacuum have the exact same characteristics and provide the same readings off-idle. And do not increase with RPM at all. At cruise, say 3000 RPM +/- you will have all mechanical advance and a good portion (if not all, depending on the vacuum canister) of your vacuum advance, which will hopefully put you in the upper 40's, low 50's for advance. Total mechanical is hoped to me about 36-38 (initial plus mechanical) and adding vacuum to this, which is ~14 deg added, puts you at 52 degs at cruise. Too much advance, when you don't want it, or need it, or under the wrong conditions, can cause detonation. That part is true.

I think I can echo Kenny now when he said most people just say:

do what works best for you...

...because there is nobody out there that is saying anything that is the same.

Most of those who are in agreement pretty much say it depends on the cam you use as to whether you might benefit from one source or the other. I will continue to play with it.