Timing

[Gary]

A friend brought over his 70 GTX so that we could help with some issues he was having.  440 six pac auto, not original, more of a conglomeration of parts.
Anyway we found he had 50 degrees total advance.  40 mechanical and the last 10 vacunm.  Idle was about 8. It's cammed up pretty good.  It doesn't seem to ping
under load, but it's hard to hear over the loud exhaust and rattle of a poorly installed exhaust system. I think that's too much advance.  I don't know what year or where his distributor
came from.  The orange box is dated 76, do later model distributors have more advance built into them. Is there a way to take some of the advance out.  Weights and springs just change the curve don't they.  What about not running any vacumn, would that be an option?

[Chryco Psycho]

I never use a timing light , I let the engine tell me what it likes best .
from experience I would have the Vacuum disconnected & more like 16* at idle , with the engine up around 3000 rpm what happens ? it will start popping if it is overadvanced you can hear the engine increase RPM as you advance to a point after that you hit a flat spot where rpm stays the same then the engine will run poorly if you continue to advance , I like to have the timing set right at the peak of the rpm rise . Often you have to weld in the slots in the mechanical advance plate so you can get enough timing at idle without overadvancing at higher RPM

[kawahonda]

Some of the math doesn't add up.

50 degrees total.
40 mechanical
10 vacuum
8 initial.

That means 58 degrees total with vacuum (which is too high) or 48 degrees without vacuum?

With that much advance, I'd disconnect the vacuum until the distributor gets curved. I think you want to be around 50 with vacuum with a curved distributor.

40 mechanical seems like a metric crapload. Personally, I'd want more than 8 initial...like Crycho said I'd target around 14-16 at idle, but that will probably cause pre-detonation with as much mechanical as it's adding. Some people with a curved distributor can even run more initial and still be within spec "all in".

If I was the mechanic and being that I didn't do dizzy curving, I would plug the vacuum advance and send him home, especially being that his exhaust is overpowering what pinging may/may not be there. It's not worth your time to advance the initial and send him away with an exhaust that's possibly masking any pinging which would destroy his engine. I'd recommend sending his distributor in to someone over the winter that can dial it in exactly and just plug the vacuum for now.

[RUNCHARGER]

That sounds messed up. If it has a fairly radical cam it would like something more along the lines of 18 initial and 36 total with the vacuum advance disconnected for checking it. Does it drop rpm's severely when put into gear?

[Chryco Psycho]

If the cam is radical it will bleed off enough cyl pressure so it will not ping @ 15-18 * initial timing

[chargerdon]

First...they sell a plate that will drop into Mopar distributors that will limit the amount of maximum mechanical advance.   see: https://www.ebay.com/itm/Mopar-Distributor-Mechanical-Advance-Timing-Limiter/323099910935?hash=item4b3a416317:g:6yAAAOSwQJhUdOgX&vxp=mtr

Second...i could be wrong, but, i believe that the vacuum advance merely advances the mechanical ahead of rpm..and that it cannot cause the timing to advance beyond the mechanical ???   So, then 8 static plus 40 equals 48 total...  its not mechanical plus vacuum.  I think!! 

Total advance for most mopar engines should not exceed between 36 to 38 without pinging...   On my 360 i have it set for about 20 degrees static (i would give it even more...but beyond 20 it will not crank when warm) and then i have the limiter plate in it for 18 for a total of 38.   My vacuum is on the manifold side to get some extra advance for idle ....  I backed it off to 10 for the vacuum...so at 900 rpm with vacuum connected i show about 30 with timing light.   

[kawahonda]

I'm only speaking for ported vacuum advance (stock):

Vacuum advance from the canister absolutely adds on top of the mechanical timing for everything that isn't WOT.

When you do timing, you should dial it in without your vacuum connected. You want about 34-36 "all in." 38 seems a little over the agreed number (mine would ping) to me. In-fact, mine pings if I go any higher than 33 "total" (without vacuum)...but I'm also at 2700 ft elevation and I have a carb that needs to probably be cleaned out better.

With vacuum connected and with a digital timing light, you will see that your "total" timing w/ vacuum connected will gain another 10-20 (depending on what the vacuum canister is rated for) and will absolutely blow by the 34-36 degree "all in" w/out vacuum number. You typically want to keep the "max" under 50-52 degrees (it's in the Chrysler manual, cannot recall the specific number but I do remember it's low 50s).

If your "all in" including the vacuum is mid or high 50s, or even higher, you run the risk of "mid-range" pinging. That means your vacuum advance is adding too much.

The idea is that WOT, the crankcase pressure drops, and your vacuum advance shuts off. So in those cases, vacuum advance doesn't add anything. This is why many racers don't care to run them because they offer no benefit...they also offer no performance loss either, it's just something else to "dial in" with no benefit for straight line WOT performance.

A LOT of street driving is not at WOT, so your vacuum absolutely adds advance over the initial + mechanical for everything that isn't WOT. That shouldn't confuse things, you still set the timing like I said above. But if you get mid-range pinging, you now know what's wrong. You should also keep your vacuum advance plugged for all "test drive" sessions. Vacuum advance should be looked at very last. Your first step is making sure it doesn't ping at WOT and to maximize your initial + mechanical. Once that's set, plug up your vacuum canister, and check the "everything" timing. If you are high 50s, you risk mid-range pinging. Test drive it, but if you get mid-range pinging...you know what's causing it...you can try to "tighten" the screw on the canister itself, but in my experience it doesn't get rid of the pinging if your canister is stamped with "18"...

Right now I have my vacuum canister plugged. It adds like, 18 advance and causes mid-range pinging. My initial is at 14 degrees. If I plugged in my vacuum advance, I'd get pinging, and I'd have to lower my initial to around ~9 degrees to get rid of it. During a re-curve, it's likely that dizzy person will replace the vacuum canister with one that only adds 10 degrees. I absolutely am a believer of vacuum advance for efficiency for street vehicles and think it should always be connected...but only after it's dialed in. Vacuum advance is an accessory, not a "vital" component. So test it last!

Also another thing to clarify, the amount of advance that the vacuum canister adds is NOT adjustable. What is adjustable is the "pressure" that it needs to advance. An 18 degrees vacuum canister is always going to add 18 degrees advance, no matter how much you turn the screw. Some people are really confused about this also. 

A digital timing light is the reason I am able to say all of the above. Seeing is believing! There should be NO guess work when timing a vehicle.

[Gary]

Sorry I took so long to reply, busy with harvest trying to fight the weather. If I understand right, 50* total is not out of line.  When I said 40 on the mechanical that included initial.  Sorry for the misunderstanding.  It's been running with that timing for a while so I don't think it's gonna detonate itself.  Anyway timing was not the main issue, it's just something we questioned.  The problem was loading up and hard starting, which we traced to a leaking power valve.  Replacing that and the car runs way better.  But fixing that led to the next problem.  Because of the leaking power valve someone had taken the choke way out of it.  So after getting that adjusted right, because it has headers [I am told] it takes forever to get enough heat in the  manifold to get the choke to open up.  It's stays on fast idle forever, which I know He's not gonna be happy with, especially since it's an auto. So I was told most people with headers run electric choke.  I called Holley, they said they didn't make one for this setup, 440 six pac center carb is a 4144.  Anyone have any ideas, short of hooking up a manual choke.  I think if I can get a choke to work properly put in the mechanical advance stop that was talked about and then bump up initial timing, I can make this car a joy to drive.  One more thing, give me some suggestions on a good timing light.

[kawahonda]

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