With only one spring the timing is going to be all in RIGHT NOW off idle is my guess.

nah. As long as its primary spring, which is looks like. The black spring is a reasonable starting point.
The total advance is set almost maxed out. That will also increase the intial tension on the primary spring.
So idle to maybe 1500 will be OK. Above that it will advance way too quick and too much.

The engine started life as a 68 2bbl. However, it was rebuilt along the way, and before I got it. The important things, 150 cranking PSI within 5% of each other in every cylinder. I had to put a rear sump pan on it to clear the AlterKtion (also adding a windage tray, yeah somewhat pointless at this RPM level, but didn't cost me much), cross hatch in the bores and bottom end was clean. Top side the valley and under valve covers super clean. It pulls really strong and steady vacuum at idle (with vacuum can on manifold vacuum) as well. I'm not sure what cam is in it, but with that kind of vacuum at idle it is pretty tame.

If you look up towards the top of the thread (from a few years ago), that is what I measured. That was with my dialback light. Now that I have confirmed TDC (and have added timing tape), I might just put it back together and make sure everything is where I think it is. But a few years ago, initial was 18 degrees and adding 20 degrees of mechanical advance all in by around 2500. Seems like a lot of initial timing, but with this and the vacuum can attached it pulls 18-20 inches of vacuum, and has a clean idle. I keep reading that the engines like more initial with the newer/ethanol fuel.

I'm going to get a jet kit for the carb and richen the primaries (after I verify ignition timing). Maybe 74 or 76 (has stock 70s in it right now) and see if that changes anything. Setting idle mixture works fine, transfer slots are not over-exposed, and blades are closed. Put new 30cc pump diaphrams in it a few years ago, with the red cam (forget which hole it is mounted in), but it pumps with even the slightest movement of the throttle.

What is the Holley List # and dash cast into the choke horn ? I would look up the original stock specs. for that carb dash number, IE 4779-6 and jet it back to stock unless your more than 1500 Ft above sea level, if so then try one Holley # size smaller. for every 1000 Ft above sea level to get you in the ball park.
If your 4000 Ft above sea level and the stock primary jet was suppose to be a # 76 I would try a 72,73 or maybe even a 71.
Leave the secondary jetting alone until you get the primary side jetting and air bleeds and spark plug heat range and ignition timing set up exactly the way you love it!

Sounds like good advice.

What I was looking forwith respect to timing was which cam is in it? I guess from what you know, we don't know. That's OK. The curves aren't that far apart.


It is alot. That amount of initial is needed on relatively radical cams, maybe around 240* at 0.050 in 383.
18-20 " of vacuum is a good indicator that the cam is quite strong at idle and should be fine with stock initial timing (pre-CAP). It probably could be made to work with the emmissions specs but that's just a headache for what your doing and hurts economy.
You can't really use vacuum to guide setting initial timing with a manual transmission. Out of gear, all engines love lots of timing and lean mixtures. Life's easy. Too easy. Start to put any load on and there's no power.

My suggestion is to go for timing curve based off the 1968 curves for a 2 bbl automatic and a 4 bbl 4 speed, but use the '67 initial.
Initial timing for all 383s in 1968 was reduced to reduce certain emmissions. Just use 12 - 15* BTC and try to get the idle speed down. Factory was 650 rpm with emmissions. You should be able to get in that ballpark.
Try the black or orange spring on the one side, and the secondary spring should have an long loop so it doesn't engage until the rpms are up around 1500 -2000.
If you don't do that, my experience (small block) is that on longer trips the engine will ping at part throttle load.
With the YH advance it is very difficult to slow down upper end of the advance the way the factory distributors did. Just do the best you can. If you can keep it from hitting 38* until after 3000 rpm you'll have done well. You can then use vacuum advance, and tweak it to provide around 56* when crusiing on the highway and it shouldn't ping at part throttle. 56 is what Direct Connection recommended as a target for b/RB engines and guess what? Shop manual shows a 383 4 bbl vac advance can add 24* at 15"Hg. Add that to factory curve which shows 32* at 3000 rpm and its 56*. There seems to be a pattern here.

Cab, the carb is a 4779-5. Its got stock jets 70s/80s. I am in NE Ohio, think we are around 800-1000ft above sea level here. I cannot remember which plugs I have in the car. I'll look today.

Mattax, this is great info. Thank you. I'm not sure how much mechanical advance this distributor will give me? But, it sounds like I should open the slots the whole way (to give me full mechanical advance) and I'll bring my initial back to not overshoot all of the mechanical being added. At the moment I closed the slots down some to stop the mechanical advance at 20 degrees since my current initial is 18, for 38 total.

I'll have to see what springs I have. I don't recall having more than came with the distributor, but I'll look. If not, I'll pick up a spring kit.

650 RPM idle speed with a manual, or was that auto with auto trans?

Thanks again.

The spring that I removed from the mechanical advance...

editLOL. I see you were ahead of me while typing!
I found your other thread. You wrote it was the secondary spring was purple. That's certainly OK to start with. I wouldn't rush out to buy the kit. They've gotten pricey last I checked.

There is a silver secondary spring that may have a slightly higher rate. I say may because I have two different measurements for purple springs.

Assuming the YH advance your Accel is the same as all the others, the total advance possible is at probably 28*, maybe 30*.
In the kits with extra springs are keys to assist in setting the advance. They're plastic spacers of different thickness. You might be able to make your own and they'd be just as accurate. I've seen the thickness measurements posted on the interweb, but if we can't find, I'll measure the set I have.

My guess from eyeballing the photo above is there is 20 - 24* degrees advance where you have it set. I know you measured 20* advance from 950 rpm to around 2400 rpm. But could there be a few degrees more below 950 rpm? In other words if you slowed the engine, does the timing stay steady or go down?

The purple spring you have is 8 coils. The one I have that has 8 coils was made with .035" diameter wire. The other one I have was made with .032" diameter wire and has 9 coils so you probably have the stronger one.

Instructions with the kit confirmed the advance adjustment available is 28* max.

I think if you start with it adjusted around 24 to 25* that would be good.

Light dotted lines show where it might be at now.

With the advance set to 24*, if the purple spring engages around 14* out, it will slow the timing and the curve should look something like shown with dark dashes if initial is 14* around 650 rpm.

See what you get and then we can figure out what is needed to adjust it as needed.

One of these: https://www.summitracing.com/oh/parts/sum-850535-1/overview/

Good point on the lower side of the RPM/advance. It is idling more than 650 at the moment and you are probably right that the mechanical is likely adding few degrees.

I'll start with 12 initial and with 24-26 that will put me at 36-38 total.

I'll put the purple spring back in and see what happens. I need to run to summit today anyway, so I'll probably pick up the curve kit above just so I have it.

Typing at same time again! Thanks, I'll start tweaking.

OK. The nice thing about the curve kit is the extra springs.

If not. here's what crackedback posted about the keys.

Thickness of some keys from the mallory kit.
14*: .140"
16*: .155"
18*: .170"

And from FABO post https://www.forabodiesonly.com/mopar/threads/mallory-29014-advance-curve-kit.351641/#post-1971314327

14° = .130” or 3.3mm or #30 drill
16° = .149” or #25 drill
18° = .167” or #19 drill
20° = .184” or #13 drill
22° = .202” or #7 drill
24° = .225” or 5.7mm or #1 drill
26° = .240” or 6.1mm or B drill
28° = .260” or 6.6 mm or G drill

This is weird, looks like my first post this morning disappeared.

650 rpm was the idle speed for 4 bbl manual and automatic, and 2 bbl manual 383. 2 bbl automatic was 600 rpm. All '68 383s were CAP equiped
Non-CAP 1967 383s all had idle speeds spec of 550 rpm. (that's with A/C on if so equiped).

The reason for the higher idle speed was that CAP traded some power and efficiency at idle for big reductions in CO and HC (but worse NOx).
But the loss of even a little power at idle has a noticble impact so higher rpm was needed to keep them runnin smooth and handle the loads when A/C was on or the car put in gear.

Thanks. Yeah I didn't see that original post either. Hmm. I'll shoot for 650. Shouldn't be a problem.

A *master reference* on the advance springs would be nice. I'm sure one exists somewhere. Instructions that are included with advance kit I linked above: https://static.summitracing.com/global/images/instructions/sum-850535-1.pdf

Unfortunately there isn't one from the company. The Mopar Performance version of the kit had a partial list of spring rates.
That's why I measured all the springs and made my own spreadsheet with the calculated spring rates.
However as I learned the hard way, that's only part of the story.
Spring Rates control how quickly the advance increases (the slope of the graph).
But the advance starting RPM is controlled by the free length of the spring and how much it is stretched when installed.
For the long looped springs, the slack between free length and the spring perches determines the degrees out before it engages.
and that's what is misleading about the graphed advances in Mallory's instructions.

One of the flukes of the YH advance mechanism is changing the degrees of advance changes the spring perch positions.
This is where I and others with cammed up motors that need short advances easily get messed up.
Say we want 16 to 17* initial, and 32* max. So we set the mechanicsm for 15 to 16* advance.
The side effect of this is the first spring as very little tension on it, As a result the weight starts moving out at far less than 750 rpm.
The way to put tension back in it is to bend the spring perches out. But obviously that's something we don't want to bend so far and then back too many times.

Fortunately that's not your situation. You can still do the tab bending as needed to either put more tension in the first spring, or to get the secnd spring to engage earlier.

Yep totally agree. Over time as well, depending on QC, materials changing, etc. etc. it all requires some trial and error.

I'm afraid who ever put together those Mallory instructions didn't fully understand the unit.

My timing experiments with the YH distributor where all done on the car, with high idle, not on a distributor machine. That's how I finally started to get hang of the relationships. So yes, especially with all the variables, its definately something that requires trials and adjustments.
Once you see where your at, if you don't like the slope, I can tell you what springs should get a steeper or shallower slope. We'll have to count the coils and if you have a caliper, measure the diameters and length the ones you have.

I have a set of calipers. I'll measure what I have, and what comes in the new kit and throw it in a spreadsheet. I want to get the car to run better and starting with something closer to factor spec probably makes sense so I'll work it that way, then carb/fuel system if necessary.

I am going to leave the brown and put the purple spring back in. This is how the distributor was setup out of the box. I never ran it this way, I pulled the purple spring from the get go. This will give the stock "two step" advance curve that is described here: http://www.loco4fomoco.com/MalloryAdvanceCurveKit.pdf

The summit curve kit I linked to above was probably an unnecessary expense, but it was pretty cheap (20 bucks), and the keys are handy. We'll see how accurate they are.

I took some measurements and organized the data. That purple spring is really stiff! The above document says that all advance should be in by 3500 RPM. We'll see I guess. Hopefully this info is useful for someone.