DISTRIBUTOR VACUUM ADVANCE SHIMS AND SPRINGS

[PT81PlymouthPickup]

After rebuilding my distributor and installing Pertronix ignition. I set the timing at spec. using a timing light. The manual I have says it's supposed to be T.D.C. @ 350 rpm on my truck. I tested the mechanical advance and it's 11 degrees @ 1850 rpm or higher, exactly what my manual specifies. With the vacuum advance hooked up it's a different story. My manual says 11 degrees with 17 inches of vacuum. I do not have a vacuum gauge to check it that way, but when I open up the throttle the timing advances to about 22 degrees. I'm thinking this sounds excessive? How much vacuum do our flatheads produce? I was thinking 20 inches? Perhaps I'm misunderstanding how it works? The truck runs well. I'm not noticing any pre-detonation. I opened the vacuum advance and it has 3 washer / shims behind the spring. I doubt there should be more than that in there.  Maybe they made different spring tensions? Anyone have any experience with this?

[kencombs]

Sometimes the advance specs in manuals are distributor degrees.  Different from crankshaft degrees.  Crank full rotation of 360 deg only moves the distributor 180.  So your 11 spec matches your measured 22, IF the spec was intended to be used on an out of car test machine.

[PT81PlymouthPickup]

24 minutes ago, kencombs said:

Sometimes the advance specs in manuals are distributor degrees.  Different from crankshaft degrees.  Crank full rotation of 360 deg only moves the distributor 180.  So your 11 spec matches your measured 22, IF the spec was intended to be used on an out of car test machine.

Thanks Ken! I re-read my OEM Plymouth service manual more carefully and now I'm more confused.

 

It states:

 

"Automatic mechanical advance curve in degrees of distributor advance at distributor R.P.M."

 0 degree at 350rpm, 3 degree at 400, 6 degree at 950, 9 degree at 1500, and 11 degree at 1850.

 

"Vacuum advance curve in degrees of distributor advance with inches of manifold vacuum"

2 degrees with 6-3/4 inches of vacuum and 11 degrees with 17 inches of vacuum.

 

So, If I take this literally, and set my mechanical advance ("at distributor rpm") That would mean I should set it to 0 degree while running the engine at 700 rpm?

 

Not having a tachometer or a vacuum gauge, I set my timing at a very low rpm (with vacuum advance disconnected) at 0 degrees. It runs well there and if I open the throttle it will not go any further than 11 degrees, so I'm assuming the governor is likely working correctly? When I reconnect the vacuum advance line, it stays at 0 degree until I open the throttle and will advance to maximum 22 degree on the crank pulley marks. You're thinking this is likely correct? 

 

 

 

[DJ194950]

According to My Plymouth 46-54 repair manual that is exactly as it should be, not accounting for your particular dist.#. but what was in those years Plymouths.

 

Sounds perfect to me from what I just read!  ?

Those #,s where from back in the days where most gas was still lower octanes. No ethanol.

I do not know what you can buy but my Calif. car with a 230 flattie uses regular with ethanol added no problems. E-10 I think it is?  It is set close ( if I remember correctly)  to those #,s.

 

DJ

[Merle Coggins]

14 hours ago, PT81PlymouthPickup said:

Thanks Ken! I re-read my OEM Plymouth service manual more carefully and now I'm more confused.

 

It states:

 

"Automatic mechanical advance curve in degrees of distributor advance at distributor R.P.M."

 0 degree at 350rpm, 3 degree at 400, 6 degree at 950, 9 degree at 1500, and 11 degree at 1850.

 

"Vacuum advance curve in degrees of distributor advance with inches of manifold vacuum"

2 degrees with 6-3/4 inches of vacuum and 11 degrees with 17 inches of vacuum.

 

So, If I take this literally, and set my mechanical advance ("at distributor rpm") That would mean I should set it to 0 degree while running the engine at 700 rpm?

 

Not having a tachometer or a vacuum gauge, I set my timing at a very low rpm (with vacuum advance disconnected) at 0 degrees. It runs well there and if I open the throttle it will not go any further than 11 degrees, so I'm assuming the governor is likely working correctly? When I reconnect the vacuum advance line, it stays at 0 degree until I open the throttle and will advance to maximum 22 degree on the crank pulley marks. You're thinking this is likely correct? 

 

 

 

 

11 degrees of mechanical advance + 11 degrees of vacuum advance = 22 degrees of advance. Sounds like everything is working properly. 

 

Set the RPM as low as you can when you set the timing. As your research shows, 0 degrees @ 350 RPM. This is where you'd want to set it. By 400 RPM you're already getting a little bit of advance. If you are setting it at 700 RPM your mechanical advance will be around 5ish degrees, so you'd have to set to that. 

[PT81PlymouthPickup]

49 minutes ago, Merle Coggins said:

11 degrees of mechanical advance + 11 degrees of vacuum advance = 22 degrees of advance. Sounds like everything is working properly. 

 

Set the RPM as low as you can when you set the timing. As your research shows, 0 degrees @ 350 RPM. This is where you'd want to set it. By 400 RPM you're already getting a little bit of advance. If you are setting it at 700 RPM your mechanical advance will be around 5ish degrees, so you'd have to set to that. 

This is what I thought too. The thing that is confusing me is the manual explicitly states "at distributor rpm" which would be 1/2 of crankshaft rpm. I'm trying to understand why they would state it that way?

[Merle Coggins]

That would make sense. I missed the "distributor RPM" part. The specs are written this way so that It can be set up on a distributor machine. 

So you were correct. At anything below 700 RPM on the engine you should be at 0 advance. 700 is a fairly high idle speed for these engines. 

[kencombs]

51 minutes ago, PT81PlymouthPickup said:

This is what I thought too. The thing that is confusing me is the manual explicitly states "at distributor rpm" which would be 1/2 of crankshaft rpm. I'm trying to understand why they would state it that way?

 

 

They state the spec because it was assumed that distributor repair and test would be performed on a dedicated test machine.  Back in the day most high end shops, especially dealerships, had a Sun distributor tester.  In my area in the 60s and 70s only dealerships and guys that specialized in performance tunes had them.

 

Here is an interesting read, if you're an avid DIY guy  .http://www.ourdigitalmags.com/publication/?i=567450&article_id=3305126&view=articleBrowser

 

 

Edited August 24, 2020 by kencombs

[PT81PlymouthPickup]

1 hour ago, kencombs said:

 

 

They state the spec because it was assumed that distributor repair and test would be performed on a dedicated test machine.  Back in the day most high end shops, especially dealerships, had a Sun distributor tester.  In my area in the 60s and 70s only dealerships and guys that specialized in performance tunes had them.

 

Here is an interesting read, if you're an avid DIY guy  .http://www.ourdigitalmags.com/publication/?i=567450&article_id=3305126&view=articleBrowser

 

 

Interesting article Ken! I'm not quite that "avid" however. Lols!  I'll stick to adjusting it in the vehicle. Funny, how although this stuff is not rocket science, it sure can get confusing. Well, I do confuse easily.

I remember my father who was a professional mechanic his entire career (1940s-1990s) telling me before computers, and less traffic, they often set timing on the road. After making sure ignition components were set and working correctly, they would take the vehicle for a test drive and stop occasionally to rotate the distributor slightly to advance the timing until they could hear pre-detonation. Then just back it off a bit until that stopped. Perhaps I'll try that and then check it with a timing light to see where it ends up?  So far, it seems like everyone reading my post agrees I should be OK at setting it at 0 degrees below 700 rpm? Although I won't be building a distributor test machine, I probably should purchase a good tachometer and a hand vacuum pump with gauge.

[Merle Coggins]

There are guys in here that talk about setting timing with a vacuum gauge. Essentially set your idle as low as possible, then adjust the timing to achieve the highest possible manifold vacuum. The RPM will probably come up a bit during this. Readjust the idle as needed, then take it out for a road test and listen for detonation/pre-ignition under load. Back it off, if needed, until the detonation goes away. I've never tried that, but they claim it works well.  

[kencombs]

39 minutes ago, Merle Coggins said:

There are guys in here that talk about setting timing with a vacuum gauge. Essentially set your idle as low as possible, then adjust the timing to achieve the highest possible manifold vacuum. The RPM will probably come up a bit during this. Readjust the idle as needed, then take it out for a road test and listen for detonation/pre-ignition under load. Back it off, if needed, until the detonation goes away. I've never tried that, but they claim it works well.  

 

Still my preferred method .  Remove distributor to do points/condenser,, check play and wiring.  Drop it back in with the crank timing pointer at 0.  Put test light on coil wire/point side.  Rotate distributor until points open and light goes on.  Start it up and set with vacuum gauge, at max less 1/2" or so.  Road test as described.  If any further adjustment needed to get rid of a ping, remember the vacuum level afterward and you can return to it if needed later

[JBNeal]

I've done this a few times, as there are 3 steep hills nearby that really tax the engine if it isn't adjusted properly...on initial settings, the engine runs ok until going upgrade then the unpleasant noises become pronounced...once everything is dialed in, the flathead can pull them hills and sound as sweet as flying across the floodplain

[PT81PlymouthPickup]

3 hours ago, Merle Coggins said:

There are guys in here that talk about setting timing with a vacuum gauge. Essentially set your idle as low as possible, then adjust the timing to achieve the highest possible manifold vacuum. The RPM will probably come up a bit during this. Readjust the idle as needed, then take it out for a road test and listen for detonation/pre-ignition under load. Back it off, if needed, until the detonation goes away. I've never tried that, but they claim it works well.  

Yes, I've read about several guys setting timing with vacuum gauges. Not having a vacuum pump or gauge it prevented me from trying.  I've been searching for a decent hand vacuum pump with gauge. There's been several times I could have used one for checking vacuum pots and for brake work as well. I know I can get them cheap, but you guys will probably laugh at me, because I have been trying to not purchase anything made in China.  It's almost impossible to find anything that isn't. Just a pet peeve of mine due to job losses over the decades. Even Snap-On Blue point is made in China. Like JBNeal, I want my "flathead pulling them hills and sound as sweet as flying across the floodplain" without supporting China to do it. Lols!

[wagoneer]

Couple of things please:

 

1) Where can one get the vacuum advance shims (also called calibrating spacer washer) in various thicknesses. 

  I cannot find them anywhere?!

2) Testing this on the bench or even on the car is described in the B2 service manual, but I'm keen to test it on the bench.

 

   Service manual specifies specific advance based on inches of vacuum, though I would say the method @PT81PlymouthPickup is a great refinement that adjusts based on engine condition.