[Sun Tuned]

And here’s the money shot!

[Sun Tuned]

A few more…

[Sun Tuned]

Well maybe I can talk someone into fixing that mess…

Anyhow , let’s talk for a bit. You can weld em. You can epoxy them, hell I’ve seen them bronze brazed. Honestly, there’s only one other way to limit these advances that I’ve seen and I use it occasionally with what I’ve shown here together. You can go by a hobby shop or a hobby lobby type place or even the pet store that sells fish, buy some aquarium tube.
What I’ve got here is a roll that I got about 25-26 years ago and I swear I’ve barely knocked a dent in it over the years.
At roughly .250 of an inch at a time it’s gonna take you awhile.

That good for 3 crank degrees off whatever you started with without doing anything. The only other “easy” way is to take a small punch and a hammer and gently dimple the backside of the advance on each side. When you do this the pull arm will gently move to one side of the slot. Simply dimple the other side until the pull arm is centered in the rectangle and both side will be dead equal.

This works good if you don’t either have access to a welder, or just need it limited without all the erector set theatrics I’ve seen on the internet over the years.

It’s blindingly gosh damn fast too! If you got the hammer and the punch and have the vise opened up right, you can do this operation in about 15-20 seconds.

Now let’s get to the gist of it. There are two types of vac cans out there, both aftermarket…
The adjustable ones and the pro firm ones that say either “54” or “57” stamped on the arm. These pics are an example of each.

There is a limit to this procedure. You can shorten or close up the gap in that pull arm slot easily around .110 of travel without doing any damage to the advance unit at all. Now obviously you won’t need to shorten them all up this much. Some near all of that .100-.110 and a bunch will need less, some just need a very little help here.

I’ll give you a hint. .157 is the length of the slot on a 16 crank degree advance unit. So you may use this as a guide. You may wish to use the punch AND the tug on aquarium tubing together if you have an advance unit that’s got a giant slot as the adjustable units have.

As a side note on those I’ve run across quite a few that were adjustable but that still wouldn’t “adjust” correctly. Me personally I don’t use the adjustable ones here, and haven’t got about 25 years or so. Here , for me ,I’ve no need.

But I’ll tell you this. A guy can take the factory units that were say 24-26 crank degrees units and shorten these things up right nicely in this manner without any fanfare, blistering fast, with no more than roughly a 20 penny nail and for lack of a good hammer, you could use a pair of channel locks if that’s all you got handy.

[Sun Tuned]

Here’s a “57” number old proform unit…front and back after shortening travel…

[Sun Tuned]

One last one of the Accel setups that was shortened….

[Sun Tuned]

Lots and lots of ways to get this done but only a couple I’ve shown here that don’t require hardly any tools and no welding. Surely easier than the old Crane limit plate thing. Although I will say that once you go farther than you want it’s not gonna work well to try and undo it. Don’t!

Sneak up on it, or if you missed, just get another and start over. Or in the case of that, just chainsaw file a little extra room in the stop end of the slot.

This should take care of 99% of all the limiting needs rather easily.

Hope this helps.ST

[Greg Reid]

Just deform inner edges maybe?

[Kenth]

"A lot of screaming for a little wool said the farmer as he sheared the pig"

For most street driven cars just use the original or the B1 vacuum advance if the original is gone or broke, connected to whichever vacuum port suitable, then forget it.

Put the effort adjusting the carburetor idle/lowspeed and WOT circuits for todays fuels instead and you'll be rewarded.

JMHO

[Formulajones]

Now that you mention the crane limit plate that comes with some of these kits. Don't follow those instructions. They are incorrect. The way they want you to install the limit plate in front of the arm is not how you want to limit these things.

The travel has to be limited "behind" the arm. The instructions (last time I read them) are backwards.

[Formulajones]

Originally Posted by Kenth

"A lot of screaming for a little wool said the farmer as he sheared the pig"

For most street driven cars just use the original or the B1 vacuum advance if the original is gone or broke, connected to whichever vacuum port suitable, then forget it.

Put the effort adjusting the carburetor idle/lowspeed and WOT circuits for todays fuels instead and you'll be rewarded.

JMHO

I've found that with a good custom ignition curve I don't need to get as aggressive with carb adjustments, even on today's fuels. In fact most cars I deal with complaining about carb problems are fixed with a good ignition curve. I always start there first and won't even begin carb tuning until ignition is dialed first. A good ignition curve especially helps tip in and off idle transitions so I don't have to go crazy on pump circuit changes.

Then I'll drive into enlarging idle feed restrictors and what ever else might be needed.

I see people all the time approach this the other direction and really get the carb so far out of whack it's a whole bunch of effort to fix. Most of those instances the carb gets blamed.

[P@blo]

My method is the same as SunTuned's fish tank air line but instead use a hard plastic/teflon/nylon bushing that I profile the sides to fit in the slot. I can limit what advance I want and have different bushings with more or less meat on the limiting section.

Pretty nifty if you ask me. Takes a few minutes to make up and can reuse forever.

[68 Firebird]

All good info!

[Cliff R]

When it comes to this topic by far and above the biggest problem I see with ignition related issues and installing aftermarket spring/weight kits. They are JUNK and always have been.

Not only are the components used poor quality and inaccurate but for some reason I'll never understand is that folk are still following very poor advice on this topic and trying to get ALL the timing in right off idle.

It is difficult, if near impossible to make that happen anyhow without some of the curve being in at idle speed.. Talk about inducing problems that act like a carb issue when it's simply irratic timing at idle speed, base timing not the same every time you let off the throttle or roll up to a stop, and/or timing falls out when a load is placed on the engine. Another issue I see is idle speed not returning to where it should be or idle RPM too high with the speed screw backed all the way out.

Don't get me wrong, not complaining here at all as those POS spring/weight kits have kept my wallet full over the years, and happy customers after removing them and installing the stock parts with maybe just a little "tweak" in a few places.......

[Shiny]

[Kenth]

Originally Posted by Formulajones

I've found that with a good custom ignition curve I don't need to get as aggressive with carb adjustments, even on today's fuels. In fact most cars I deal with complaining about carb problems are fixed with a good ignition curve. I always start there first and won't even begin carb tuning until ignition is dialed first. A good ignition curve especially helps tip in and off idle transitions so I don't have to go crazy on pump circuit changes.

Then I'll drive into enlarging idle feed restrictors and what ever else might be needed.

I see people all the time approach this the other direction and really get the carb so far out of whack it's a whole bunch of effort to fix. Most of those instances the carb gets blamed.

I have found igniting a too lean A/F mixture with timing out of the scale the engine wont reach it´s full potential.
I prefer to use close to factory ignition curves as they are a part of engine design parameters and add the required amounts of A/F mixture needed for best performance, driveability and economy.
The timing settings out of the scale or "all in" at 2000 rpm´s, usage of multispark systems are all remains of the past.

Over the years, it has always been more difficult, as well as the lack of knowledge among the general public, to modify the carburetor's low-speed circuits, many have not known how this worked or even that they existed. Instead, they have gone to the ignition, as it is easier to turn the distributor, and mount weaker springs at the weights.

FWIW

[Formulajones]

Originally Posted by Kenth

I have found igniting a too lean A/F mixture with timing out of the scale the engine wont reach it´s full potential.
I prefer to use close to factory ignition curves as they are a part of engine design parameters and add the required amounts of A/F mixture needed for best performance, driveability and economy.
The timing settings out of the scale or "all in" at 2000 rpm´s, usage of multispark systems are all remains of the past.

FWIW

I've never been a fan of running anything lean. Especially anything even remotely performance oriented. In fact with todays ethanol blend fuels the old 14.7 AFR is pretty much out the window and it was rare that I'd run anything that lean with real gas. I just haven't found any substantial benefit from doing so.

I've also never found any benefit, or even a need, to bring timing all in at 2,000 rpm. That's a bit excessive, lol, but I do find substantial benefits with almost any engine when I shy away from the lazy factory ignition curves. The factory is notorious for being super conservative in that respect, even on todays new vehicles.

Lean mixtures and conservative timing is just an attempt from OEM to keep the tree huggers happy and keep warranty claims in check.

[Shiny]

Originally Posted by Greg Reid

I hope this isn't a dumb question but as far as limiting the upper end of centrifugal advance and determining how distance of movement translates to degrees of movement... Can it be as simple as measuring the diameter of the distributor body and dividing by 360 then making a pointer from, say, a piece of wire properly placed and indexing it all?

Greg, I did what you suggested. Pumped with Mightyvac, marked at multiple vac levels, took a photo, then superimposed a degree wheel.


Is in line with my guesses from measuring displacement of the rod... the max definitely exceeds the 16 degrees expected for a B1. Hopefully I get to find out if it's too much!

Originally Posted by Formulajones

If you want to know the limit of the vacuum can currently, You can simply put your hand pump on the vacuum can while the engine is running while checking the timing at idle.

So if the initial idle timing is 12 degrees with no vacuum, pump up the can and subtract your initial and you'll have at least a rough idea.

I hope all this actually helps get mine running!

Originally Posted by Kenth

"A lot of screaming for a little wool said the farmer as he sheared the pig"

For most street driven cars just use the original or the B1 vacuum advance if the original is gone or broke, connected to whichever vacuum port suitable, then forget it.

Put the effort adjusting the carburetor idle/lowspeed and WOT circuits for todays fuels instead and you'll be rewarded.

JMHO

Amen... I hope the effort to understand and verify gets me to the next level.

Mike

[Kenth]

Originally Posted by Formulajones

Lean mixtures and conservative timing is just an attempt from OEM to keep the tree huggers happy and keep warranty claims in check.

Yup, and that´s why folks have always set the timing out of the scale to gain some "performance", until the bearings knocks out or/and the pistons melted down.
Now we know better.

[Formulajones]

Originally Posted by Kenth

Yup, and that´s why folks have always set the timing out of the scale to gain some "performance", until the bearings knocks out or/and the pistons melted down.
Now we know better.

I do enjoy threads like this, it shows at least some are curious enough and want to understand it.

I see cars frequently with vacuum advance unplugged for various reasons explained to me. Pinging is one I hear a lot, but it boils down to not knowing how to modify them to give the engine what it wants. Easier for owners to run without, then complain about fuel mileage. Lol

[Formulajones]

Originally Posted by Shiny

Is in line with my guesses from measuring displacement of the rod... the max definitely exceeds the 16 degrees expected for a B1. Hopefully I get to find out if it's too much!

It'll depend on the engine in question and whether or not you dial in the initial and total to a level the engine performs well at.

Generally with most performance oriented engines, once I have initial where I want, and total is dialed in where it makes best power from dyno tuning, I find another 16 degrees from a vacuum can a bit much for today's pump gas.