Ignition Timing
 

This is my first foray into distributor recurving. I have a GM HEI on original 350 with Edelbrock Performer intake and RV cam, rebuilt .030 with 9.0:1 pistons and original heads. Ported vacuum advance that comes in with bare tip-in of the throttle. The engine ran well but was sluggish. I replaced the OEM center plate, weights and springs with Mr. Gasket parts. The smallest (black) springs are way too light, coming in at 900 RPM. The next size up (silver) work much better, coming in at 1650 RPM BUT I can only get 10 degrees mechanical advance at 2250 RPM. Initial advance is16 degrees so total all-in is only 26 degrees. The curve diagram from the Mr. Gasket kit seems to show the weight/spring combo only allowing 12 degrees or so. If I advance initial much more than 16, it is hard to turn over. Any thoughts or ideas how to get total timing up to between 34 and 36 degrees? By the way, it runs better than ever as it sits now.
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Re: Ignition Timing
 

Why do you say the Black springs are too light? To me that curve seems to be what you need for a stockish build. You'd have 16 initial timing and ~20 mechanical advance. Depending on what RPM this maxes out at (hopefully you get around 36* at 3000rpm.)

If you get pinging drop your initial timing to 14 and total advance will be 34. You can also mix/match the springs. IE one black and one silver to get somewhere in the middle between the two curves.

Lastly I'd hook your distributor up to full manifold vacuum. It will help immensely with the sluggishness you are experiencing and the engine will run cooler. Not to mention you can lower the initial to help with starting but still get the advanced timing right away to be more in the right torque range.
You may need an adjustable vacuum canister to dial in how much max timing the vacuum can adds at cruise. Run as much timing as you can during cruise without inducing pinging.

One more thing. I ran a Progression Ignition HEI for awhile and it was super awesome to be able to set my own timing tables from my phone (connected with bluetooth) without having to mess with any springs or weights at all. It's pricier that a standard HEI unit but so worth it.

https://progressionignition.com/

Re: Ignition Timing
 

Cue argument over Ported vs Manifold vacuum in 3...2...1..

Thanks for the reply and info. The black springs were coming in at 900 RPM, below the idle RPM I had set at the time and I could only get 12 degrees mechanical at 2200 RPM, not 20 as shown on the chart. Maybe the black/silver combo will get me more at a little higher RPM since I’ve since reduced idle to 850, but it doesn’t seem like enough.

I’ve read a lot of debate on ported versus manifold vacuum. I know by going to manifold I could reduce initial, but wouldn’t that defeat my quest for more total?

The Progression looks like a nice unit and would no doubt be fun to play with. However I’d rather not spend hundreds on a distributor right now and besides, I’m having too much fun learning and playing with the old-school analog one I already have.

Thanks again.
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Re: Ignition Timing
 

Next question is why is your idle set so high? I've got a street/strip cam in my 383 and it idles at 800 when hot. A tamer build with an RV cam ought to idle fine below that...

And you dont have to reduce the initial w/ manifold vacuum but you had mentioned you were coming into starter issues at 16. It just allows you to run lower initial but with the vacuum advance in play AT idle you will have a peppier engine.

If you cant get the advertised 20* in the chart i'd try another product, personally. That's your true issue. Unless for some reason you have limiter bushing in place that should be smaller to allow for more mech advance...

Yeah, idle was too high for sure. I have it set at 815 now, could go much lower and idle fine in Park but Drive brings it down 215 to 600.

My starter problems begin at 18, no problems so far at 16.

I haven’t seen anything that should limit distributor travel. I am going to try a different product. Morosso seems to make a quality kit. I’ll look deeper. Appreciate your help!
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Re: Ignition Timing
 

Oh gotcha. I was thinking you were in drive at that idle number. That make way more sense. I'd aim for a 600-650 idle in drive. I assume you have a carb as well? We're getting off topic (I like tuning talk haha) but when you tune the idle circuit for best manifold vacuum do it in drive with someone pressing the brakes.

Yeah, Quadrajet. I’ve only tuned an idle circuit once, on a 72 Pontiac 350, but did it in Park. Why Drive? I wasn’t planning on tuning this one since I know it’s pulling 15” vacuum at idle, and there’s virtually no exhaust smell in the shop when the door is shut and the truck’s running. I’m a novice at tuning but am finding it enjoyable. Just ordered a Moroso kit, should see it before the end of the week.
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Re: Ignition Timing
 

So just a little 101 on tuning:

First for your carb you want to make sure the idle set screw is set so that your transfer slots look like squares when viewed from the underside. If you've played with this screw to set your idle speed you may induce an off idle stumble if the transfer slots are overexposed.

Then set your initial timing to achieve the idle rpm you want. Larger cams will need more initial.

Then you set your idle mixture screws in drive simply because that's where you need the right mix, in drive ready to drive the car, with a load on the engine from the torque converter.

Then set your idle mixture screws for best/highest vacuum.

All of this has less to do with exhaust smell and more with optimizing the AFR at idle and this is how you do that short of an AFR gauge.You may very well have the right tune on your carb/distributor but this is how you do it from square 1.

I knew most but not all of this. Great explanation and step-by-step. I’ll post back after I receive and install the Moroso kit. Thanks!
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Re: Ignition Timing
 

This write up is way better and more detailed than what I noted above:


One method of setting up your idle system is a follows:
To start with, invert the carburetor and check the position of the throttle
butterflies. Turn the idle speed setting screw to set the bottom edge of the
primary throttle butterflies about .020" from the bottom edge of the transfer
idle slot. Don't worry about measuring anything - your eyeball is good
enough. Positioning the throttle butterflies near the bottom of the transfer
idle slot at curb idle is absolutely critical for maximum acceleration.
Next, turn the idle mixture needles in until they are lightly seated. Excessive
force here will damage both the needles and metering block and make the
idle fuel mixture difficult to set with any accuracy. After seating them turn
them out 1 1/4 turns, which is a good baseline setting. Now you're ready to
reinstall the carburetor and setup your idle system.

Before you start the engine, examine the fuel bowl side of the throttle body.
Hopefully you'll see a little tube, covered by a rubber plug. This vacuum port
connects with a passage in the throttle body that 'sees' manifold vacuum.
Remove the plug, attach a good vacuum gauge to the port and position the
gauge where you can see it clearly. Don't forget to zero out the gauge.
Without touching the carburetor, turn the engine over until you have
pumped fuel into the bowls. Work the throttle a few times then start the
engine. If it dies, which is likely, you'll have the turn the idle speed setting
screw to increase the RPM to get it to idle while it's cold. Since throttle
butterfly position is critical, count the turns and fractions of turns so you'll
know exactly where you're at. The whole idea is to be able to return the
throttle butterflies to the position you originally set them at. As the engine
warms up it should gain rpm, so you should be able to reduce the throttle
opening at least somewhat without the engine dying. Now the fine tuning
begins.

With the engine idling, pick one of the idle mixture needles and turn it in 1/4
turn while you're watching the vacuum gauge. Give the idle a few seconds
to stabilize. If manifold vacuum increases repeat the process, letting the idle
stabilize each time, until it starts to decrease. If turning it in hurts manifold
vacuum then try turning it out. When you've found the 'sweet spot' (i.e. the
manifold vacuum is as high as you can get it) repeat the process with the
other idle mixture needle.

Presumably you'll be able to pick up enough idle speed by optimizing the idle
fuel mixture that you can close the primary throttle butterflies down to their
original position near the bottom of the transfer idle slots.
As a final check give each idle mixture needle a slight turn in then a slight
turn out. If any motion hurts manifold vacuum, you know that needle is set
properly. At this point if the idle is stable and the engine responds quickly
when you just crack the throttle, you should be good to go.
One final note: Make sure your timing is set correctly before starting this
process.

If your distributor has a mechanical advance system there is a much better,
though more involved, way to setup the idle system.
Position the throttle butterflies and idle mixture needles as described above,
attach the manifold vacuum gauge to the vacuum port and start the engine.
Turn the idle speed screw to increase the RPM, again taking note of exactly
how much you have to turn the screw to open the throttle butterflies
enough for the engine to idle while it's cold. Allow the engine to warm up,
the close the throttle butterflies as much as reasonably possible without the
engine dying. Attach a timing light, check to see how much initial ignition
advance you have and make a note of the figure.
Next, loosen the distributor hold down clamp and turn the distributor so as
to increase the initial ignition advance. When the initial ignition advance is
increased the RPM should rise as well, allowing you to reduce the throttle
butterfly opening. Simply turn the distributor to increase the initial ignition
advance and continue to reduce the throttle butterfly opening until they're
in the original, correct position and the engine is idling at the desired RPM.
Lightly snug the hold down clamp to make sure the distributor can't move,
then adjust the idle mixture needles for best manifold vacuum. Once they're
properly set if the idle RPM is higher than desired, loosen the hold down
clamp and turn the distributor slightly to achieve the desired idle RPM.
Recheck the idle mixture needle position then tighten the hold down clamp.
Once the idle system is setup you'll need to correct the distributor's
advance curve. The first step is to attach a timing light and recheck the
initial ignition advance. Let's say, for example, that it was originally 15° and
now it's 22°, a 7° increase. If your total ignition advance was originally 35°,
in order to keep that figure the advance curve will have to be shortened by
7°. Assuming you have a centrifugal advance system you'll have to limit how
far the advance weights can move outward, which will limit the total
advance. The method required will vary from distributor to distributor, so I
won't get into that here, but any competent technician with a good
distributor machine should be able to do it for you.

If you don't have access to said technician/distributor machine and you can
come up with a way to limit the outward motion of the advance weights,
you can do the same thing using your engine as form of distributor machine.
Limit the motion of the weights somewhat, make sure you have the correct
initial ignition advance then check to see how much total ignition advance
you have. If the total ignition advance is still too high, just continue to limit
the motion of the advance weights until you achieve the desired total figure.

Just wow. Back to school. Wonder what I’ll be doing during an upcoming rainy period?
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Today I swapped out the Mr. Gasket curve kit for a Moroso. I immediately had 39* total. I ended with 14* initial and 34 all-in at 2500 RPM. It’s never run better. I’ll be satisfied with this until the next period of adverse weather, then dive into tuning the idle circuit. I hear a very low “puff” from the drivers side exhaust and and am hopeful to tune that out. Thanks for all your help!
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Re: Ignition Timing
 

nice. Just watch for pinging at cruise and WOT. Personally I think all in at 2500 might be a little early for a heavier truck. You could correct it at cruise if your vacuum canister is adjustable but for WOT you might need a heaver set of springs.

Following my previous post I took your earlier advice and moved the vacuum advance source from ported to manifold. And like you said, peppier response, and not just off-idle but at all speeds. The truck used to feel maxed out at 60 miles an hour with little pull after that. It now pulls hard up to 75, where I back down due to high engine speed. No detonation under any conditions I could induce including WOT, highway cruise, and pulling inclines at low RPM. I had to back initial down to 11* due to hard hot-turnover and ended with 33 at 2800 RPM. also adjusted the idle AFR using a vacuum gauge. I am very happy with the results of the total tune!
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Re: Ignition Timing
 

Awesome. That's what I like to hear! :metal::metal:

Re: Ignition Timing
 

Before I was old and deaf I'd loosen a distributor and keep advancing timing until I could make it spark knock. Then keep backing it off a little at a time 'til it stopped knockin'. Usually then, they wouldn't turn on the starter. I put a button in several cars and 1 truck that would kill(ground) the ignition circuit. Push the button, get it turning on the starter & turn loose of the button to start. Kinda crude, but my wife says I am.