Distributor vacuum brake question
 

When the engine is at idol and the carburetor butterfly's are mostly shut would the distributor vacuum brake lift off as the manifold vacuum is high?

And when you are driving and butterfly's open the manifold vacuum will be lower and cause the spring to force the piston to act on the cam and weights to advance the timing?

Re: Distributor vacuum brake question
 

It works like you say with the vacuum lifting the plunger but you can design for increase or decrease of timing so no issue there, the brake plate holds the centrifugal weights back when actuated by the plunger retarding the timing.

Re: Distributor vacuum brake question
 

The spring actuated plunger has a leather-like pad. At lower vacuum the pad rubs on the rotating plate and resists the mechanical advance. It does not retard timing, but rather, delays the application of advance.

Re: Distributor vacuum brake question
 

It is a vacuum controlled retard system to act as a brake on the advance disc under load conditions. It only has braking affect when manifold pressure is high (low vacuum) as was stated. A person can say that it doesn't physically retard the timing but in effect it does so by slowing the advance mechanism.

Re: Distributor vacuum brake question
 

My chart from a old Ford training guide says :

Governor weights spread as speed increases thereby advancing spark.
The same vacuum force that operates your windshield wipers also retards the ignition spark.

As i run one of these almost every day on a machine. I would explain it like this:

Engine off leather brake holds the advance ring by spring tension causing a slight retard in timing allowing a easier start.

Once the engine starts vacuum pulls brake off the advance ring allowing a proper advance as soon as the engine starts.

Driving down the road and hitting a hill (increase load) vacuum will drop off causing a retard to timing preventing spark knock.

There are some good pictures on our web site
http://www.bubbasignition.com/crab-42-48.html

I guess its how you word it?????

Re: Distributor vacuum brake question
 

Cool thank you for the replys, I was reading something that was wrong a lot of people insist it works the other way round with manifold vacuum.



I unscrewed the plug in the shaft like bubbas website recommended and oiled the wick.

Got a pertronics electronic conversion and am putting that into a 2nd distributor,

Re: Distributor vacuum brake question
 

I set my distributor up in the timing fixture and set that scale on the side witch the fixture, I am thinking I could advance it a lot more as its at half now

Re: Distributor vacuum brake question
 

The scale is marked with 6 divisions. each division is 2 degrees giving total of 12 degrees for adjustment. 6 degrees either side of the mid mark.
Normally set with 1 division advance ie 2 degrees static advance.
Unless your actual timing is not correct.
Road testing normally confirms you have this correct.
Phil NZ

Re: Distributor vacuum brake question
 

The flyweights move out at rpm to give most of the advance. It does have to be set for starting though. Once it's set, it should need no further adjustment.

Re: Distributor vacuum brake question
 

Are you saying, just back off the vacuum brake totally and let the breaker plate run without a brake?

Re: Distributor vacuum brake question
 

You want it to drag a bit - otherwise the timing can "flop around" a bit. I tighten it down until I just feel it start to drag.

Re: Distributor vacuum brake question
 

Manifold vacuum pulls the plunger away from the plate. So during normal everyday driving the plunger does not contact the plate, so I don't see how it can stabilize the timing. If you've got wobble, the distributor needs to be serviced.

Re: Distributor vacuum brake question
 

There a lot of disagreement as to the purpose of the vac brake . Most very untrue.

Heres what actually happens in slow motion : ( assuming vac brake is properly adjusted) You can see this action on a distributor machine .

Engine off = no vacuum , piston is spring loaded down to the spark ring . Leather is pushed against ring.
Engine cranking = very low vac piston is still forced inward , causing a slight timing retard. ( desired)
Engine starts, manifold vacuum is increased ( above spring tension) piston moves off the ring allowing some advance as engine is now running.
Then driving down the road hitting a large hill etc vac again drops allowing the leather to again drag making the spark retard slightly..

I actually have this somewhere in a old ford training manual, will look it up as time goes on.

Re: Distributor vacuum brake question
 

I understand the operation, it is just that I cannot see how the plunger is said to stabilize timing.

Re: Distributor vacuum brake question
 

watching the spark timing on a distributor machine without the piston the spark lines kinda jump from side to side at idle. These units dont ever show wear and even a new one shows the same looseness. ( they would need to be loose to work). When piston and leather with some spring tension on the ring the spark lines are very stable on the machine , would be the same using a timing light on the pulley. Just something we have noticed , i seems to learn something on every rebuild and test .......

Re: Distributor vacuum brake question
 

The only time I've seen that on my machine is when the distributor is worn. At idle, the vacuum is high so the piston is away from the ring.

Re: Distributor vacuum brake question
 

On my cars I have the vacuum line plugged and use only slight tension on the piston. Personally on today’s fuels I don’t think the vacuum brake function is needed on a stock flathead engine.