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Old 12-15-2018, 12:59 AM   #1
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Default Vacuum brake on crab dizzy

Couple of questions regarding the vacuum brake. First, what is it's purpose? I see how it "works" in that vacuum pulls the plunger away from the plate, the spring pushes it towards the plate under engine load (low vacuum) but what affect does this have on timing?. Also, how is it determined when it is set properly?. Now, getting to my dizzy, where the vacuum port goes from the front cover to the dizzy body, there is a rubber plug in the dizzy blocking the vacuum from getting to the brake???? Also, when researching this I've noticed that the dizzy bodies that I've seen APPEAR to be aluminum, mine is cast iron. Is this a year/model thing??.
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Old 12-15-2018, 01:11 AM   #2
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Default Re: Vacuum brake on crab dizzy

...

Last edited by Tinker; 12-15-2018 at 08:41 AM.
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Old 12-15-2018, 06:25 AM   #3
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Default Re: Vacuum brake on crab dizzy

The cast iron crab was for commercial heavy duty use...and is rare/wanted compared to the aluminum one.
The "plug" is a rubber seal and there should be a brass tube in the middle letting vacuum pass through it.
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Old 12-15-2018, 08:03 AM   #4
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Default Re: Vacuum brake on crab dizzy

the vacuum brake is adjusted on a dist machine. it is set at a certain rpm with a certain vacuum signal to set the advance, Mark
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Old 12-15-2018, 08:12 AM   #5
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Default Re: Vacuum brake on crab dizzy

I back the vacuum break off and drive on. Works just fine.
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Old 12-15-2018, 08:21 AM   #6
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Default Re: Vacuum brake on crab dizzy

I think of the brake as a way to slow the advance curve. It is properly set by turning the screw out all the way by hand and then turning in back in 1 1/2 to 2 turns from there if your engine pings under acceleration screw it inward slightly until the pinging is gone. Using todays gas there is usually no need to adjust after initial setting. The rubber should be around the vacuum port as a seal, it should not block vacuum, check it out.
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Old 12-15-2018, 08:29 AM   #7
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Default Re: Vacuum brake on crab dizzy

In my opinion the vacuum brake is not needed with today’s fuels. It does not but slow the advance which hurts performance. However..... unless the shaft has been polished, you are confident the weights and spring move freely, and the springs are in good shape you may want to run a small amount of tension to insure the weights so back to static at idle
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Old 12-15-2018, 08:36 AM   #8
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Default Re: Vacuum brake on crab dizzy

If you look at the way the disc on the advance unit interacts with the flyweights, you can see what will happen when brake pressure is applied to the disc. It does the same basic thing that a modern distributor does accept it works in a different way. For most engines, it takes very little extra pressure on the spring to prevent spark knock. If it pinks under load, just tighten down on the spring a bit more until it stops. ALL engines benefit from an automatic retard system in the form of better mileage. As long as there is a low pressure acting on the piston (which is most of the time) it will not affect the timing. Its only when the throttle is opened considerably that it retards the spark. If you ever drive a model A or T where you have to operate the spark manually, you can then feel the difference when you retard the spark on a long hill.

Last edited by rotorwrench; 12-15-2018 at 08:48 AM.
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Old 12-15-2018, 01:53 PM   #9
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Default Re: Vacuum brake on crab dizzy

The basic operational details are this:
Vacuum will pull the piston off the advance disc and allow the advance flyweights to pull the disc into an advanced state. This occurs during low load cruising when vacuum is high. When put under load the drop in vacuum will allow the piston to restrict the discs response to the flyweights and the result is a retard in timing.
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Old 12-15-2018, 02:27 PM   #10
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Default Re: Vacuum brake on crab dizzy

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Originally Posted by Russ/40 View Post
The basic operational details are this:
Vacuum will pull the piston off the advance disc and allow the advance flyweights to pull the disc into an advanced state. This occurs during low load cruising when vacuum is high. When put under load the drop in vacuum will allow the piston to restrict the discs response to the flyweights and the result is a retard in timing.
Thanks. This is why I'm having trouble wrapping my head around it. Obviously it works so my thought process is wrong, however, its just seems to me that, the advance weights are operated by centrifugal force so, as the dizzy revs up, the timing advances. The you come to a hill, (for instance), the vacuum will drop before the RPM drops causing the brake to STOP the centrifugal weight from backing off and retarding the timing as the RPM drops and the engine demands require less timing to prevent detonation. . Where am I going wrong here
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Old 12-15-2018, 02:28 PM   #11
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Default Re: Vacuum brake on crab dizzy

BTW, what holds the leather in there?. Mine should be replaced but i don't want to start digging at it until I understand how it's assembled.
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Old 12-15-2018, 02:31 PM   #12
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Default Re: Vacuum brake on crab dizzy

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Originally Posted by Terry,OH View Post
The rubber should be around the vacuum port as a seal, it should not block vacuum, check it out.
Yes, that is what I see on all of them when I was researching it. Mine has a rubber plug with no brass vacuum port but it did have what appeared to be a hole poked in the middle of the plug with an awl or something. This hole was not really open as you can imagine when you poke a hole in rubber it just closes up when you pull the poker out.
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Old 12-15-2018, 02:53 PM   #13
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Default Re: Vacuum brake on crab dizzy

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Old 12-15-2018, 05:32 PM   #14
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Default Re: Vacuum brake on crab dizzy

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Originally Posted by petehoovie View Post
Thanks. I had come across that in my research but seeing it again brings up another question, Are those coils any good??? I see they are available but are rather pricey. I have a normal cylindrical coil adapted to the original bracket and would like to use the proper one unless they are junk. Thoughts?.
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Old 12-15-2018, 06:42 PM   #15
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Default Re: Vacuum brake on crab dizzy

Most of the repro coils are crap. There are several guys that rebuild the old original coils so most folks go that route. Original coils can sometimes be found on flea pay but they would likely need rebuilding.

Contact cement would likely secure the leather pad on the piston as good as anything.

The vacuum port should not have a solid plug in it since the auto retard feature would be inoperative if it were. It should have a packing like an o-ring there.

The flyweights go to full advance at a moderate rpm setting then just stay there until the rpm drops back below that point. Bad springs can cause them to go to full advance too early. The type of springs they use are generally pretty reliable but everything wears out in time. The only way to check them is on a distributor machine. There are several guys that work on distributors who frequent this board.

Last edited by rotorwrench; 12-15-2018 at 06:50 PM.
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Old 12-16-2018, 12:22 PM   #16
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Default Re: Vacuum brake on crab dizzy

Quote:
Originally Posted by Licensed to kill View Post
Thanks. This is why I'm having trouble wrapping my head around it. Obviously it works so my thought process is wrong, however, its just seems to me that, the advance weights are operated by centrifugal force so, as the dizzy revs up, the timing advances. The you come to a hill, (for instance), the vacuum will drop before the RPM drops causing the brake to STOP the centrifugal weight from backing off and retarding the timing as the RPM drops and the engine demands require less timing to prevent detonation. . Where am I going wrong here
This may help: The flyweight will extend with rpm, but not necessarily advance the timing if the plunger rubs the advance disc. This is possible, because the flat springs are overridden by the brake plunger. The weight assembly and the disc are not directly connected. Kinda like a clutch effect.
Hard to explain.

The leather pad is just glued in. The edge of the disc needs to be highly polished to prevent early wear of the brake.
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Old 12-17-2018, 12:58 AM   #17
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Default Re: Vacuum brake on crab dizzy

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Originally Posted by Bill OH View Post
I back the vacuum break off and drive on. Works just fine.
Quote:
Originally Posted by 35fordtn View Post
In my opinion the vacuum brake is not needed with today’s fuels. It does not but slow the advance which hurts performance. However..... unless the shaft has been polished, you are confident the weights and spring move freely, and the springs are in good shape you may want to run a small amount of tension to insure the weights so back to static at idle

see here...
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Old 12-17-2018, 01:05 PM   #18
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Default Re: Vacuum brake on crab dizzy

Vacuum Brake set to music??
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Old 12-17-2018, 02:35 PM   #19
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Default Re: Vacuum brake on crab dizzy

Remember the vacuum brake is a starting aid, as the engine has low vacuum when first cranked . The spring would push the piston inward causeing a slight retard. ( perfect when cranking) when engine starts the vac would increase and pull brake outward advancing the timing. If ya got it use it !!!
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Old 12-20-2018, 08:26 PM   #20
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Default Re: Vacuum brake on crab dizzy

Bubba I don't want to be a pain. But the vacuum brake never effects the low vacuum under load accelerating conditions at speed? I run mine out and it runs well.


That makes sense, but I was always taught otherwise. Probably wrong info. Some hit/miss engines have a start/run lever. Start retarded spark, of course.

Last edited by Tinker; 12-20-2018 at 08:38 PM.
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