Recurve Duraspark

[redfalken]

Picked up the DS II today. Hopefully I can install it tomorrow and go for a spin...weather permitting. I peeked in through the holes on the base plate and noticed he had flipped it to the 13R slot and replaced both springs with the Mr. Gasket kit springs which are lighter. I've heard some say to only swap one of the springs We shall see!

Here are the specs he emailed:

Advance begins at 800 rpm
22 deg of mechanical @ 2300
14 deg of vacuum advance
36 deg of total all in at 2560 rpm

[MacDee]

I did the "replace-only-one-spring" thing to mine, and I have to zing it pretty high, way more than 2500 rpm, I think, to get full advance. (Need to tee off the tach wire from the Street Fire box so I can use my test tach under the hood!) I really think I need install the other Mr. Gasket spring. My throttle response still sucks....

[Luva65wagon]

One thing I'm still a little vague on is how the vacuum advance works - or doesn't - when I converted to the DSII dizzy. I still have the stock carb on this thing and last night when I was debugging my run-time issues, I did watch the timing marks as I accelerated and it's not advancing 20+ degrees much less 30+ degrees. I didn't have RPM to watch, so I'm not sure what I got for that.

I shut it all down last night after only confirming I got the run issue resolved, so I have more investigations to go on this, but you both have vacuum options I don't have, I think. And I know the original distributor was for using the power valve port on the carb.

Any wisdom out there on this issue?

[redfalken]

Is this the Load-o-Matic setup? If so, here's a link to a pretty good write up and I'll cut/paste the text on using this carb with a DS II. Basically, you can plug the spark control valve and run it off manifold vacuum, wherever you can find it.

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

Distributor Swaps
The "Load-O-Matic" distributor uses the "Spark Control Valve" (a spring and diaphragm mechanism), to determine the proper amount of vacuum advance, which is commonly a mixture of manifold vacuum (sensing load) and venturi vacuum (sensing rpm). On the other hand, a conventional distributor uses mechanical weights (speed) and manifold vacuum (load), to provide the proper amount of ignition advance for any given situation.

While you can use a Duraspark or DUI distributor with a stock Autolite 1100 carb, both of these distributors utilize a mechanical advance mechanism that was designed to operate with ported or manifold vacuum. As such they will not operate properly using the vacuum port on the stock Autolite 1100 carburetor. To correct this you need to plug the port on the carb, so you don't have a vacuum leak, and run the vacuum line from the Duraspark or DUI distributor to a manifold vacuum source. This can be accomplished using an existing vacuum source on the intake manifold, or the bottom of the carb adaptor. Or you can create a new source by drilling a hole in the intake manifold and installing a vacuum barb. However, you'd need to remove the cylinder head before drilling, to make sure the manifold is free of any debris that could work down into the cylinders and severely damage the motor.

So...what happens if you do try to use the vacuum port on the Autolite 1100 carb? At idle, the "Spark Control Valve" is open, sending manifold vacuum to the distributor. As such, the engine will idle just fine. However the "Spark Control Valve" will not provide the proper vacuum signal under load and/or speed. All engines need more advance when cruising, and less to none at wide open throttle. However since you are now using the carb (venturi vacuum advance) and the distributor (mechanical advance) to compensate for speed, the ignition system will receive to much advance at cruise and wide open throttle. As a result, the engine may chug and jerk from the over-advanced condition at cruise.

[Luva65wagon]

Very interesting. The part that always cornfused me is the thought that vacuum advance actually advanced with acceleration. I'm sure it was a long-ago forgotten thing I learned in auto shop. I always did well in those classes, but I can't remember much from that time to know why. But that's another story for another time.

I have always thought that as engine speed increases, so does the advance, which of course is true with mechanical advance. But since manifold vacuum actual goes down with throttle plate opening, I always assumed (until today) that the reason you hooked the distributor to the carb was that they had a means of increasing vacuum with throttle opening. This is actually true, to a degree, with the old Load-o-matic distributor and carbs. This article, and a related link on vacuum advance, helped cure me of that assumption. Though this is true with this style carb, all distributors with mechanical advance, which the Load-o-matic distributors lacked, required manifold vaccum.

Bottom line is that vacuum advance is NIL off idle and during acceleration. It provides advance in lean-burn conditions, most common at idle and cruise. Lean mixtures need more burn-time (more advance) and rich mixtures require less advance. Mechanical advance is all RPM based and is the advance you have access to when you are accelerating, AKA "getting it on."

So, that being the case, the specs they provided you, Kenny, of:
Advance begins at 800 rpm (I assume all mechanical movement starts here)
22 deg of mechanical @ 2300
14 deg of vacuum advance (I assume they can check this below the 800 RPM)
36 deg of total all in at 2560 rpm
... would seem to imply that you add mechanical advance to the vacuum advance to get this total. Since they can apply vacuum at any point while on the test bench, this doesn't mean you will have 36 degrees of advance at 2560 RPM at wide open throttle (WOT) under load. So under WOT, I think your max advance may be closer to 22 deg. I can't see you getting another 14 deg out of another 260 RPM. So... how did they come up with this total?

On a side note, I stumbled onto a thread yesterday on the TFFN as I was looking around the Internet about what spark plugs people used when doing a DSII swap since I needed a set (this was on my phone - so it wasn't a serious search) and a guy did a bunch of elaborate mods to his 1100 carb to create "ported vacuum," which these guys on CI seriously denounced the use of. The guy on TFFN really thought this was a requirement to do a DSII swap.

[redfalken]

Well...it made sense at one point...

I think maybe he meant 14 deg of "initial" advance and I should have corrected that but just did a cut/paste. So when your engine is at idle with the vacuum disconnected from the distributor and plugged, this is where you set the initial timing. As you begin to accelerate, the vacuum advances the timing until the mechanical part takes over and the vacuum from the manifold or carb port dies off.

Somebody way smarter than me can explain it better but here's a pretty good web page if you need some light reading:

http://www.angelfire.com/on/geebjen/timing.txt

I didn't get to install it yet but I have Friday off so I'll do it then for sure and report my findings.

[Luva65wagon]

Yeah, that link pretty much echo'd the CI document.

Initial advance is set by rotating the distributor in the hole, so they can't set it for you. They can recommend it and use that as a reference point to say what all the other values can be, but it still seems odd that they include vacuum advance. The best they could do is to indicate the maximum vacuum advance you can attain at idle (when the vacuum is greatest).

Essentially it is like this:

1) You set initial advance without any vacuum assist and at an idle speed below any introduction of mechanical advance. This setting, as seen in "the books" was all based upon compression ratios, fuels, octanes, etc... at that time. Though still a good place to start, the reason you may want to vary from this is because we are in a different day and age of fuels and you may not be running a completely stock setup anymore.

2) Mechanical advance is 100% rpm based. The old Load-o-matic distributors didn't have mechanical advance, so Ford actually did use vacuum (a mixture of manifold and venturi) to make a fake "mechanical" advance - which was needed (is always needed) during acceleration.

With the DSII distributor and any post-1967 distrubutor, which has mechanical advance, this is the thing you are modifying with new springs. There are two weights inside that "fling out" with centrifugal force and these drive against a cam, of sorts, to rotate the breaker plate on Fords. Changing the weights effect how quickly the weights can move and how quickly the breaker plate rotates and how quickly you see advance.

Since there are two weights, you can, depending on how the springs are setup, have one weight move to allow for fast advance action and then the other comes into play later. At this point - both are in play together. If both spring are in play, their tension is summed. So, if you have one weak and one strong spring (as is the case with the suggested mod) and both are "under tension" all the time, you could actually have two equally sized springs as well - as long as the sum of two weaker springs are the same and one strong and one weak. If you have one spring slightly un-sprung (possibly the cause of them wanting you to bend a tab), then it might allow a quick advance - at first - and then when both are under tension it allows a slow and steady advance over the rest of the RPM range.

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.

That's the way I see it so far. Sorry for typing this all out and hijacking your thread Kenny, but hopefully it will come in handy. I'd still like to know whether they added vacuum advance to mechanical advance to give you total. Seems awfully coincidental that they just happen to add up.