Ignition: Distributor Advance Question

[Robert Crandall]

At what rpm should the vacuum and the centrifugal advance begin to function? At idle, 700 rpm, my timing mark shows here:



At 1500 rpm the pointer shows at the yellow painted mark by the 1-6 stamp. When I repeat this check with the vacuum advance disconnected, the pointer shows the same as before for both 700 and 1500 rpm. Does this tell me that the centrifugal advance works, but the vacuum does not? How far and and at what rpm should either advance move the pointer?

This question is for a 1949 2R5 truck, champion engine.

Thanks.

[RadioRoy]

A point of clarification, please.
1. With the vacuum advance disconnected, the readings are the same as they are with it connected?
2. Or... the timing stays at the same number of degrees regardless of RPM?

Going with the first scenario (With the vacuum advance disconnected, the readings are the same as they are with it connected) My memory may be rusty, but this tells me that:
-The vacuum advance is not working at all
-the mechanical advance is does not have the range it should

Maybe the mechanical advance weights are rusted, but they should advance farther than they are doing now. The vacuum advance is definitely bad.

[Mike Van Veghten]

I assume you are speaking of the original setup ?

There is no "should" when speaking of advance, either mechanical or vacuum. You can buy kits to change the mechanical to come in faster, you can buy adjustable vacuum advance modules to make the vacuum work how you want it to work.
It also depends on how Studebaker set "that" particular distributor up. What engine, what car/truck, even what state (Colorado vs. Arizona or California).

So, verify that the mechanical is working properly. Verify that the vacuum is not (or is) working. There are tools that you can use, or clean it well and suck on it yourself (removed from the engine of course). Watch the point plate move (or not), and does it stay in place when you block the port so air does not enter the line.

For the best mileage and performance, the mechanical should start coming on about 1100 to 1300 rpm and be full by 1300 to 1500 rpm.

The vacuum will depend on where the vacuum source is, either full or ported vacuum. The original Stude most early cars) the vacuum is pulled from a "full" vacuum source. That is full vacuum at idle and it drops as the throttle is depressed...it's always moving..!
Vacuum advances do not last for ever. They are just a rubber diaphragm, eventually rot/crack and have leaks.
Either check yours or just buy a new one, make sure the point plate rotates freely, is clean and dry, then install the new vac. advance mechanism. Probably not a bad idea to buy a new one anyway if it's more than just a couple of years old anyway.

For what it may be worth, I normally run my engines without the vacuum advance.

Mike

[Robert Crandall]

Thank you for your advice. This is the time of year for rain or snow for us in SE VA. I do not drive my relics in rain or snow, so I work on them. I bought the truck in 1980, and I have not yet changed the vacuum advance, so it is at least that old. I will pull the distributor and check the centrifugal advance, and I will check the vacuum advance. I am happy to buy a new one, but I will check first.

If you run your engines without vacuum advance, then you are running as I am now. Why I question is because my truck is, for now, just a novelty that I drive around my large suburban development. When I accelerate in 2nd gear I very soon come to a point where the engine will go no faster, and that is only in a 25mph area. If I ever venture out of the development I will want to accelerate to more than 25 mph.

I will check then update. Thanks.

Thank you for your advice.

[JWL4541]

An inoperative distributor vacuum chamber is most likely due to a cracked diaphragm which will cause a vacuum leak. This will result in a lean mixture, higher operating temperature, and poor performance.

(o[]o)

[RadioRoy]

An inoperative distributor vacuum chamber is most likely due to a cracked diaphragm which will cause a vacuum leak. This will result in a lean mixture, higher operating temperature, and poor performance.

(o[]o)

It will also burn the valves eventually.

[PackardV8]

Most here know this, but vacuum advance is of greatest benefit at steady highway cruise where vacuum is high. At cruise, the vacuum typically provides 10-20 degrees of additional ignition advance beyond what the centrifugal is providing and noticeably improves fuel economy.

Another general FWIW, flatheads don't use as much ignition advance as do OHV engines, so when checking the Shop Manual, be sure to use the numbers and graphs for the appropriate engine.

jack vines

[Bob5341]

I converted the distributor to Pertronix in my 289 and discovered the vacuum advance was not working. After searching all the local auto parts stores I gave up and ordered one from studebakerparts.com. A major tuneup and the lack of points solved my problems. I was worried about the condition of the engine since I just bought the 1955 Commander last fall and it had not been started in many years. Runs great.

[Robert Crandall]

I made some vacuum measurements today. Next work session I will take the distributor out and check the centrifugal advance workings. I will buy a new vacuum advance.

My gage reads in Hg. The distributor vacuum connection is at the base of the carburetor between the idle adjustment and the linkage. At that port at 700 rpm I read 2 in Hg. At 1100 rpm I read 3 in Hg. At 1500 rpm I read 11 in Hg. At the manifold port for the windshield wipers I read 20.5 in Hg at 700 rpm and 21 in Hg at 1500. Do my readings seem logical or normal? Thanks.

Thanks for the advice in post #7. The charts in the repair manual show readings as they should read using an 'electrical analyzer', which I don't have. I don't know how I will duplicate that with only my timing light. My current plan is to see if the marks as seen with a timing light move more after I fix things.

My setup is, as far as I know, the original one.

Thanks.

[TWChamp]

With a timing light you can check the spark advance on the damper with the vacuum line disconnected, then again with it connected at the various RPM.
This should let you know if the vacuum advance is working.
I suck on the vacuum line and block the end with my tongue to see if the diaphram holds a vacuum.

[bensherb]

It will also burn the valves eventually.

Only if the timing is incorrect. Vacuum advance is not nessisary for the engine to run properly. It just alters the timing to BETTER cope with changes in RPM and load. Mike Van Vaghten is absolutely correct.

I have a GM HEI in my 289. I set up the advance to function as the original Delco window distributor advance did. At idle the staic timing is at 4 degrees, as stock, with the vacuum dissconnected. At idle with vac connected timing is around 25 degrees @ 15"hg. I use an unported vacuum connection (direct manifold vacuum). Ported vacuum sources are a result of emissions laws and manufacturers doing whatever they could to pass smog. Using an unported, direct vacuum source will allow it to engage at idle, which is good for a number of reasons. Much like cruise conditions, engines run leaner at idle than they do under load. This means the mixture burns slower and needs an earlier spark to optomize the burn. Ensuring the mixture has a complete burn helps the engine run cooler at idle. Carbureted cars used unported vacuum to the distributor before emission requirements came into being.

My mechanical advance comes in off idle and is all in by 1500 rpm, bringing timing to about 34 degrees, at about a 2500 rpm cruise (little load) the vacuum advance brings timing up to around 40 degrees. These numbers are estimates based on measured marks I made on the damper, not absolute, I don't have a degree wheel or tape mounted.

It runs great!

[Robert Crandall]

Thanks for the further advice. I will update with what I find after checking the advances. I am curious about why the vacuum readings are so low at the port at the base of the carburetor where the vacuum advance is connected. There would be no smog setting on this 1949 2R5 champion setup.

[TWChamp]

Page 69 in my 2R5 Shop Manual has the distributor advance charts.
Maximum mechanical advance of 6 to 8 distributor degrees is at 1400 distributor RPM, which means 2800 crankshaft RPM.
Vacuum advance should be 18 crankshaft degrees with 14" vacuum.

Harbor Freight sells a hand vacuum pump with gauge for about $20, and would be a good tool to own.

[64studeavanti]

Your vacuum readings may be normal. I don't know much about sixes, but, on wcfb carbs I have observed that the throttle plates at least partially block the vacuum advance port at idle The AFBs on my shelf all have indentions in the throttle plates to allow the port to be uncovered at idle. All my Avantis have Edelbrocks, so i can't easily measure vacuum for stock AFB. The vacuum for the speedsters (WCFB) is similar to what you describe.

FWIW, my Avantis i have full manifold vacuum going to the advance. They run great and do not over heat.

[Bill A]

You actually don't need to remove the distributor to check the mechanical advance. it consists of an upper and lower distributor shaft one turning on the other. That allows the lobes to be moved in relation to the crankshaft hence advance. If you remove the distributor cap and grab the rotor can you turn it and does it return to it's original position when released. If it does that you have nothing else to check unless you want to know how much advance you get. If you have a modern timing light it will likely have an advance meter capability. Using that you can run the engine at idle and check the timing then adjust the meter until the reading is 0, rev it up and while holding the engine speed repeat the process.... do the math and you know the amount of advance. You can use the timing light and advance feature to check the function and amount of vacuum advance. With the light the entire process is usually under 5 minutes. I did that stuff for years. The leakage from a broken vacuum hose at idle could effect the mixture a small amount on a 6 cylinder but on a 250 inch engine at 2500 rpm's it would be nil. The real positive effect of vacuum advance is fuel economy, the earlier you start the fire the greater the energy released. The amount of ignition lead used in engines is usually determined by the combustion chamber shape, the more efficient the quicker the ionization/ flame front travel so less lead is needed. This is all from memory as I haven't done it for a number of years but it should be close.