20 hours of tweaking and I cannot get it right - knocking and after-fires. 1977 with and edelbrock carb. Major tune-up completed.

NOTE: I have read the timing posts and so much information can be confusing. I feel that there is a starting point that I am missing and thus I am playing patty-cake with this procedure. What I read here is somewhat different from the manual.

If you post, please be VERY SPECIFIC - as if you are talking to somebody who does not know how to do this.



1. My vacuum advance was originally ported, so I have it now set to full manifold vacuum per your suggestions. The vacuum on the manifold tested out at 18.

Sound normal?



With respect to the "set it at 36 and let initial fall where it may":

2. I can set timing to 36 at the 0 timing mark (with vacuum disconnected) by rotating the distributor. When I connect the vacuum hose, I advance 22 degrees and apparently am all in.

So my total is 58? This is bad? What does this tell me about my springs?




3. I am totally confused about the idle adjustments in the forum papers. Do you rotate the distributor to get the timing mark to 0 with the gun set at 36., then adjust the idle mixture screws to get your idle rpm to specs?

In other words, when I retard the distributor, my idle speed goes down to 600. So do I then adjust the idle mixture screws to get it back to 700... which in turn changes the timing mark... which means I then rotate the distributor which changes idles speed again and then.....

Please tell me what the "adjust idle" procedure is in respect to timing.




I really appreciate having a place to go for help.

 

1. Yep, sounds normal.

2. 36 total timing means :
- First : Your vacuum advance hose must be disconnected and plugged.
- You have to rev your engine until you get maximum centrifugal advance.
This should occur before 3000 RPM if you have small enough springs in your distributor.
Those are the conditions to get your 36*. You musn't get more centrifugal advance at any higher RPM.
Note that with stock springs, you sometimes have to rev the engine at 4500+ RPM to get full mechanical advance.
Once this is set, no carb adjustment will ever change this number ! It is purely mechanical !
Even if you set your idle at 2000 RPM, your total timing will still be 36* ( What will probably change is the initial timing ).
Did you try to adjust 36* at idle ????????

BTW, 58* with vacuum advance is too much. 52-54* is OK, no more. Which means your vacuum advance canister should only add about 16* ( 36 + 16 = 52 ).
Always choose a vacuum canister that will give full advance at idle, which means it should add full vacuum advance at idle vacuum + 2" Hg ( at least ).
But this shouldn't be a problem if you have 18" Hg vacuum at idle.

3. Idle adjustments and timing are two different things. Timing can affect your idle ( engine idles at a higher or lower RPM )
but changing your idle mixture or idle RPM won't affect your timing ( It will only change manifold vacuum ).
Don't mind about initial timing. As long as you are in the 12* - 18* range, you're in the ballpark.

 

[QUOTE=73StreetRace;1574669263]

36 total timing means :
- Your vacuum advance hose must be disconnected and plugged.

It is.

- You have to rev your engine until you get maximum centrifugal advance.


I can rev to 3000, but there is no change in advance with the hose disconnected and plugged. I will try rev'ing higher.



-BTW, 58* with vacuum advance plugged is too much.


I can set to 36* with the vacuum line plugged at idle. Once I unplug and re-connect the vacuum hose - I get an immediate advance of 22*, for a total of 58* with the hose connected and at idle. Am I understanding this correctly?

Is this too high with the hose connected?

Gosh only knows what it is if I get more mechanical advance rev'ing the motor to 4000+?

I am just not making the connections with the procedures.

With regards to idle - how do I know it is set correctly - and I assume I need to get this done before trying to set timing? The specs say to set it at 700 - and I can do that - but the idle definitely changes when the distributor is adjusted.

 

OK.... so this may be where I am failing.


I am going by the assumption that I want to be all in by 3000.

I set to 36* at idle (mistake), rev to 3000, but get no advance.

I am not getting any change in mechanical advance - because I am not rev'ing high enough to get the advance to kick in. So I must have stiff springs.


So, I need to change the springs in order to get all in before 3000?

 

36* with vacuum line disconnected and plugged at idle is way too much.
First check that your centrifugal mechanism isn't jammed. You should see the timing mark move when you rev the engine : the centrifugal springs should add about 16* - 24* to your initial timing. If the mark only moves a little or not at all, then smth is wrong with your distributor and you'll have to fix that first.

Yes, you can try weaker springs, assuming the mechanism itself isn't seized... If it is, changing springs won't have any effect on it.

 

Hi 81

Sounds like you're putting too many things together in the wrong order. Your Idle has nothing to do with the timing. That is set on the carb once everything else is in place. Your timing needs to be set first. I had the same issues when I started playing with all this. I started with everything at the base. So I set the engine at around 10* so that she could run to do the checks. I first found my Vac advance number (at idle vac connected) and wrote it down. I then found my centrifugal advance number (2500 +/- RPM for me and vac disconnected) and wrote it down. You ad those together, subtract from 51 as the papers say, and that number establishes what your base timing should be. Remove the vac if connected and set to that number (I believe mine was around 17* or 18*). Once that is set then you can now set your mixture and idle. That is all done on the carb, I find that the edelbrock makes it very easy. Idle screw on the throttle body and the two adjuster screws in the front of the carb. You should get a vac gauge to do the mixture, as it helps to fine tune and isn't that expensive. That will also be done with the vac unplugged, then fine adjust your idle and you're ready to go. Hope that helps.

Oh, like 73 said. If you're not getting a response from the centrifugal check then I would look into the dist. It could be gummed up or the springs too heavy. I would start there and work back.

 

OK, now we are getting somewhere.

So I will go ahead and get a rebuild kit (new weights and springs and bushings and plate).


Summary, to get to 36* total,

Timing light set to 36*

Hose removed and plugged,

I am actually setting the timing mark MUCH lower (retarded/before) than 0*, rev'ing the motor to get the advance to kick in and then getting that total lined up to the 0* timing mark with the gun set at 36*.

 

And you should get that 36* when the engine reaches 2800-3000 RPM, and get no more at higher RPMs.
Which means 36* is the maximum the engine can see at any RPM ( without vacuum advance ). With vacuum advance, it can be 52-54* MAXIMUM.
At idle, it should fall between 12 and 18* ( again with the vacuum hose removed and plugged ).

 

Check for harmonic damper outer ring slippage; Verify piston is at true tdc when timing mark is at true tdc "0".

If ring slips ... and they sure do with age, oil, heat & abuse ... the timing mark also moves with it and will fool anyone.

 

Yes, that cannot hurt...


http://www.summitracing.com/parts/SUM-900189/

 

I replaced the springs with one black and one gold.


I am getting 20* vacuum from the carb.

(When measured with a vacuum gauge on a full vacuum port, and when connecting the vacuum hose to the distributor, both measurements are 20*)

@ 2800 rpm:

I am having to set my total timing at 30-32* all in.

Then with the vaccum hose connected, I get 52-54* all in.


Anything wrong with these numbers (too much vacuum advance)?


Again - I want to say thanks for all the help. It really makes a difference .

 

Yes, 22 degrees ( 52-30 or 54-32 ) vacuum advance is too much, so you have to find a way to limit this amount. 36* total is what you want first. This is for performance.
Vacuum advance is just for economy ( part throttle / cruising ). Vacuum advance won't change your engine maximum power. Total timing will.
IMO, you're still very conservative with 30-32* total timing and you're loosing some serious horsepower.
Assuming you've got a GM HEI distributor ( large cap & integrated coil ), like this one :



Maybe you could buy a vacuum advance kit :

http://www.summitracing.com/parts/ACC-31035/

It's at an affordable price and quite easy to install. As you see, this canister is adjustable, so you can set the amount of vacuum advance you want, in your case, 16* instead of 22*.
This will allow you to set your total timing at 36* without exceeding 52* with full vacuum advance. Just check that this part will fit your distributor before you order.

 

Alrighty then - got one ordered and will get it in tomorrow.

Slowly getting this procedure within range. Whew....

 

OK - here is where I stand:

1. Ended up using the two black springs in the Mr. Gasket kit - that got me all in at 2600 rpm. They looked identical to the ones I replaced.


2. Installed an adjustable vacuum advance kit and got it dialed in to @ 16*.

So, rev'ing the engine, I am all in at 36* with the vacuum hose disconnected.

After re-connecting the vacuum hose, I am all in at 52*.
( I can set the timing light to 52*, rev to 2600 and be all in, and the timing mark lines up with the 0*)


S0, sounds like we are in the ball park? Please let me know if these numbers are incorrect.

Now - the next step is to take it out for a test drive. (I have started another thread about the procedure for adjusting the idle mixture)

Again - let me thank everyone for the tips and the papers. I could not have done it without the assistance.

 

Those numbers sounds great. 2600 all in may be a little soon, it all depends on your engine comp ratio and fuel octane, but you have to test drive the car to know.
Try to cruise at 2600 RPM. If the car jerks a little, try to remove and plug the advance vacuum hose.
Then try it at WOT from 2600 RPM, 4th gear. Now listen carefully : If the engine knocks, stop immediately and try the next stiffer springs. Test drive again.
If the car still jerks at cruise, remove some vacuum advance.
Fine adjustments can be made at the track ( best ET ) or with a G-Tech :
http://www.gtechpro.com/

 

Great Thread!

Thanks to everyone

Finally understand how to time my engine and the sequence.

 

Wanted to finish off this thread by saying thanks!!


The car is running better than ever - absolutely no afterfires at all!!

No more detonation, no more strong fuel smell, smooth idle.



Thanks to all who helped me through this process.

 

You're welcome. Nice to know your car runs great now

Enjoy and drive safe.