simple centrifugal advance question

[danno 128]

The disti was changed years ago, and I am not sure it is correct.  What is the correct centrifugal advance slot width for a 1969  standard 302 automatic?  It should have a C9AF-n disti, but the Ford shop manual does not specify the slot width.   The book says 12 to 16.5 degrees advance at 2000 rpm with vacuum disconnected. It also has NA for total centrifugal advance, but 14 seems reasonable. 

I am using an 18L slot, which looks to be way too much! I should be using 8L or less ?    

[danno 128]

somebody must know, or be able to tell me where to find it? 

[barnett468 418]

Ok, your question is FAR more complicated than you can imagine, so for the moment I will try to simplify the answer.

The safest thing to do for the moment is run it in the 8 degree slot with 8-10 degrees of initial timing and leave the vacuum can disconnected with the line to the carb plugged, HOWEVER, since you already have it in the 18 slot, you can do the following.

 

Disconnect and plug the vacuum hoses to the dist if you have any.

Start your timing at 8 degrees BTDC.

With the engine idling, advance the timing 4 degrees . Listen for an increase in rpm and irregular/rough running.

If the rpm increases and it still runs smoothly, reset the idle speed then increase the timing 2 more degrees and check for the same things.

Retard the timing to 8 degrees.

Reset the idle speed.

Increase the rpm to around 2000 and  advance the timing 4 degrees . Listen for an increase in rpm and irregular/rough running.

If the rpm increases and it still runs smoothly, reset the engine speed to 2000 rpm then increase the timing 2 more degrees and check for the same things.

POST RESULTS

 

Is your engine stock?

What is the static compression?

What is your elevation?

What is your rear gear ratio?

What cam do you have in it?

 

 

 

[danno 128]

Barnett, I knew you would probably reply.  Thank you.   Ok, it will take time to try your solutions.  The stop arm in the existing distri has a rubber sleeve over it to limit the advance.  I would guess you know what I am referring to. So I want to measure the exact amount of advance compared to a stock distri.  

I think the 13 opening for 26 degrees advance is correct for my car.  This is what I am trying to find out. 

My engine is stock, compression about 9.5 :1 in all cylinders. It has new pistons, but the bore and stroke are original and the pistons are stock

I am in Minneapolis, about 900 feet. 

Stock gear ratio, I think that is 2.78?

Stock cam.

Keep in touch, I will get back to you. 

[dcm0123 15]

If you want to limit the advance without taking the distributor apart, put on a thicker black sleeve (shrink sleeve or rubber tubing) which limits the clearance between the 2 parts and the timing advance.

Just replaced my points with a Mallory electronic module and timing is more consistent. Was jumping around at ~2000 RPM before on a 390.

Attached a file from 69 manual in case you do not have it. The way I read this is if it says max advance above 2000 rpm then you can expect further advance above 2000 RPM. If it says NA, then the max advance will occur at 2000 RPM

69mustang-dist0001.pdf

[danno 128]

11 hours ago, dcm0123 said:

If you want to limit the advance without taking the distributor apart, put on a thicker black sleeve (shrink sleeve or rubber tubing) which limits the clearance between the 2 parts and the timing advance.

Just replaced my points with a Mallory electronic module and timing is more consistent. Was jumping around at ~2000 RPM before on a 390.

Attached a file from 69 manual in case you do not have it. The way I read this is if it says max advance above 2000 rpm then you can expect further advance above 2000 RPM. If it says NA, then the max advance will occur at 2000 RPM

69mustang-dist0001.pdf 1.02 MB · 4 downloads

Thanks for the suggestion. I have the same book, and it says I have a C9AF-N disti.  I was not sure about the NA for max advance, your explanation makes sense. I might try a thicker sleeve, or even a metal ring around the tab.  

What I am surprised about in the table you have is that it does not specify what the mechanical advance is for a C9AF-N.  I guess as long as the advance is 8.25 to 10.75 degrees at 2000 rpm, it will work.  I have also read that ( because of today's gas mixtures) if it is a bit further, it is better.  So maybe 11 or 12 degrees is better?   But I also want to do what Barnett suggested. 

[barnett468 418]

2 hours ago, danno said:

Thanks for the suggestion. I have the same book, and it says I have a C9AF-N disti.  I was not sure about the NA for max advance, your explanation makes sense. I might try a thicker sleeve, or even a metal ring around the tab.  

What I am surprised about in the table you have is that it does not specify what the mechanical advance is for a C9AF-N.  I guess as long as the advance is 8.25 to 10.75 degrees at 2000 rpm, it will work.  I have also read that ( because of today's gas mixtures) if it is a bit further, it is better.  So maybe 11 or 12 degrees is better?   But I also want to do what Barnett suggested. 

You are way over thinking this. You do not need to mess with the sleeve to limit the timing. In your case, if you simply use the 13 slot and set the initial at 8 degrees and plug the vacuum advance, you will be fine. If you use the method I posted, you can fine tune it which is helpful but not really necessary on a stock or near stock engine.

To change the actual timing curve, you can change the springs. They are Mr Gasket part number 925D and summit racing sells them, but I typically only need to install one of these springs and installing both of them trpically causes the distributor to have too much advance too soon, but the springs have zero affect on the amount of total timing an engine has.

After I fine tune the advance curve, I typically use an adjustable vacuum advance unit and st it so it adds around just 6 degrees of advance.

 

 

 

 

 

[danno 128]

Maybe I did not say things correctly. I do not have a 13L slot.  I have an 18L slot in my Pertronix distri.  I used a piece of metal to limit the advance to the same as a 13L slot.  i also used a piece of shrink tube over the stop.  Next I need to see how car it advances at 2000 rpm and start trying your tests. The car seems to be running fine, but I want to insure it is correct. I need an engine analyzer now so I can measure the rpm and  see how far the timing is advancing at 2000 rpm.  I have a timing light, but no tach. 

[danno 128]

Ok,  I am getting closer on this.  I have it set at 8 degrees at 500 rpm ( with vacuum adv removed).   It advances to the maximum of 20 degrees at 1500 rpm, so I have the wrong spring in. I need the stiffer spring rather than the easier spring.   

From what I can see, the advance from 8 degrees at idle to 20 degrees is correct. I just need to get the springs so it reaches the 20 degrees at 2000 rpm. 

 

[barnett468 418]

15 hours ago, danno said:

Ok,  I am getting closer on this.  I have it set at 8 degrees at 500 rpm ( with vacuum adv removed).   It advances to the maximum of 20 degrees at 1500 rpm, so I have the wrong spring in. I need the stiffer spring rather than the easier spring.   

From what I can see, the advance from 8 degrees at idle to 20 degrees is correct. I just need to get the springs so it reaches the 20 degrees at 2000 rpm. 

 

Not necessarily, you can simply bend the tabs the springs attach to with some needle nose pliers to alter the timing curve. It should have a low tension spring with a little preload on it and a high tension (larger wire) spring with around 1/16 of an inch of free play on it.

Also, why is your car idling so low?

The distributor should not advance until just over idle speed which is around 650 for a manual car and around 750 for an auto.

There are no aftermarket springs that are stiffer than stock.

If it is advancing just above 500 rpm, bend the tab outward slightly that the small spring is attached to then recheck the timing.

You also don't need it to be exactly 20 degrees at exactly 2000 rpm.

You should also test drive it first before you do anything to see if you can hear any pinging or feel any hesitation or surging in the engine etc. This way you have something to compare your changes to.

 

 

 

 

 

 

[danno 128]

Great, I will increase the idle to 750 and play with bending the tabs.  I did not realize the stiffer spring should have play in it at idle, that is why it was not advancing at 1000.  

What is the best timing at initial idle (750) with vacuum removed?  I have read that it really needs to be as far as possible without pinging. I will start with 10 degrees and see how the whole curve is. 

Should I get the curve correct at 10 degrees idle and leave it there even if it is advanced to 14 degrees at idle ?  Or should I get the timing at idle be correct first and then set the curve based on timing at idle? 

[barnett468 418]

Do the test posted above.

[danno 128]

Is the purpose of this test to determine if an advance of timing ( measured at idle) can be tolerated?  

I will see if I can do it tonight, but it might be tomorrow night.

Thanks, I will post results.

[barnett468 418]

2 hours ago, danno said:

Is the purpose of this test to determine if an advance of timing ( measured at idle) can be tolerated?  

I will see if I can do it tonight, but it might be tomorrow night.

Thanks, I will post results.

The test is for determining best idle timing and timing at around 2000 rpm and above, but it is helpful to do the test at around 2800 rpm as well.

Wear eye protection.

 

 

[danno 128]

Timing is now 10 at 750 ( idle) ,  10 at 1000 ,  12 at 1200,  22 at 1500, and 26 (max) at 2000.  So I have 16 degrees total centrifugal advance.  I probably need to insure the stiff spring is not holding it at 1000 rpm. It should be advancing more by then.  I will look into that. 

 I completed your test.  Increased timing 4 degrees at idle and it makes no difference.  Returned timing to 10 degrees.  Took it up to 2000 rpm and increased it from 26 to 30, and it seemed to not make a difference.  What does that tell you?  

 

[barnett468 418]

11 minutes ago, danno said:

Timing is now 10 at 750 ( idle) ,  10 at 1000 ,  12 at 1200,  22 at 1500, and 26 (max) at 2000.  So I have 16 degrees total centrifugal advance.  I probably need to insure the stiff spring is not holding it at 1000 rpm. It should be advancing more by then.  I will look into that. 

 I completed your test.  Increased timing 4 degrees at idle and it makes no difference.  Returned timing to 10 degrees.  Took it up to 2000 rpm and increased it from 26 to 30, and it seemed to not make a difference.  What does that tell you?  

 

The test results tell me that in your case, you do not need  more advance than you currently have at idle or at 2000 rpm, but I would do the test at around 2800 rpm as well because unless you have fast burn heads or something, your engine should need around 32 degrees of timing at higher rpms.

Yes, it should start advancing a little sooner than 1000 rpm, however, as I mentioned, you need to drive the car before and after every change.

[danno 128]

OK, I will test at 2800 tonight after I find the safety glasses again!