79 350 with vortec heads setting timing

Hello everyoneI'm trying to get my 79 350 timing correct
I have a craftsman dial back timing light.
Motor is 350 bored 30 over
L 31 vortec heads
Comp 268x cam
edelbrock performer intake
freshly rebuilt quardrajet all parts came from cliffs high performance.
700r4 tranny
Well I got her running and got it timed by ear this was befor I picked up timing light.
Idled good and cruises good but when you stomp it it would ping like rocks in a can. Felt like barely any power so I decided to pick up timing light yesterday.
My timing tab is at the top 12 o clock spot and I've never set anything with a light this is my first motor btw ....

so my question is how do I read these tabs it has like a, tube or little ring then to the left has a wide V then two skinny v v kinda like vvVo... I found the groove on damper and put some white out on it I hooked up my light to it saw my groove was a little bit off of the tab sho I rotated distributor tilll it came to the wide V on tab dial on back of light was set to zero ..

I'm a lill confused on how to exactly read what theses, marks mean and what to set dial at for initial timing see where my centrifugal advance comes in anf total timing I read somewhere that vortec heads needs to be around 32° total

So if anyone can help me I would greatly appreciate it thanks for reading......


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The 12 o`clock pointers are a pain. If you want to keep it, then you`ll need to clean the tab so you can read the timing marks. However, with the stock tab you can`t set the full advance, I`m sure you can with the dial back timing light but I hate those things, I trust it set at zero and perfer to read the timing on the tab instead of the light. I suggest getting a balancer cover from proform, it gives full timing readings all the way up to 50 degrees.
Next go to the parts store and get a 2 o`clock bolt on timing tab.
Now we can set the initial timing.
Starting out with vacuum advance disconnected and plugged, set the base timing at 12 degrees before top dead center.
Reconnect the vacuum advance and make sure it`s connected to a full time manifold source, this is any source on the carb that pulls vacuum full time, including at idle. When you do so right away it`s going to idle up so lower the idle speed back down to normal. Recheck the timing, it should be 18 to 24 degrees before top dead center.
Next you`ll want to set the full advance. Usually around 36 degrees is the norm. While it`s true vortec heads don`t need as much timing, it`s still varible depending on the combustion characteristics. If the engine has a tight quench with Vortec heads, then less timing is needed, if it has no quench then 34 to 36 is the norm. Disconnect the vacuum advance when your setting the mechanical advance, this way it won`t influence the setting.
You may need to install a limiter in the mechanical advance to get it where you want it. Or, you can back down 2 degrees at a time on the base timing with vac advance disconneted, then reconnect it, test drive and leave it where the pinging stops. However ping is influenced by a few things. What pistons did you use in the build? are you running 93 octane fuel? You ordered a Q-jet kit but is it still using the same secondary metering rods that it came with? What engine did the Q-jet come off of? It could well be too lean at wide open throttle which also can cause ping. You can check this after you get the timing back on par by doing a plug chop which is take it down the road, floor it and hold it for about 6 seconds, take your foot off the pedal, then click the ignition one click back so the steering wheel doesn`t lock, then ease over to the side of the road and pull a spark plug and check it`s color, if it`s white it`s lean, if it`s black it`s rich. As for the mechanical advance did you install a advance kit? If you used the light springs it could be the timing is coming in too fast, I usually mix one light and one medium spring.
Quick edit: NEVER set the timing by ear, this is asking for detonation induced engine failure.

What I think I really need to know is which mark on thee tab its zero . Then I set my dial to whatever degree I need.


Does anyone know about the tabs marks

Thanks for your reply double vision I am running a serpentine setup anf the 2o clock tab I think would be covered by the brakets. The pistons are stock flat tops the q jet came off same motor beforw rebuild and cliff sent new secondary rods with kit. I've been running 89 octane. tgank you for your reply tho I think I'm gonna have tooooo use my 12 o clock tab tho just need to figure out what is zero the tube looking thing oru the wide v our what

Clean off the tab. you can run a 2 o`clock tab as I`m running one on my 4.3 V6 with serpentine system as is my best friend on his vortec 350.
Now as for the stock tab and mark here`s it`s layout:
Zero is the deepest mark on the tab which is more to the drivers side plus it`s mark is pointing to the rear of the vehicle. all the points pointing towards the front of the vehicle is 4, 8 and 12 degrees before top dead center.
To get a better idea here`s a picture.
While it`s not the exact one you have, the timing marks are the same.
http://www.perfprotech.com/store/Ap...ing-set-2-c.jpg
With flat top pistons and Vortec heads you`ll need to use 93 octane fuel. With 89 you`ll have to lower the spark timing to keep it out of detonation.

Oh one more thing I bought this hei distributor from summit cam with advance kit in it it had semi heavy springs I changed then to one light one medium spring,

I'm gonna mess with it today see what I can do. So a full manifold vacuum would be better, than ported, for cruising and economy? So my initial should be 12 btdc and my total 32 btdc at 3000 rpm? So that leaves me 20° advance with mechanical and the vacume advance how do I make sure I'm not exceeding my spark timing.

thanks double vision for your patients with me I wanna learn how to fix the problem so next time ill know what to do

Here ya go (66GMC posted this originally):

Yes thats the same tab . Ok lemme make sure I under stand this right. The line on the harmonic balancer is zero on crank .

So when that line matches to the zero on, tab the number one pistons is, at top dead center?
Is that correct?
If so what I need to do start motor let warm up disconnect vacume advance plug port on carb. Set base timing to 12 btdc. Then set idle screw yo 3000 rpm make sure total is around 32-34 ? Then connect vacuum back to manifold vaccine source then lower idle dowm to 750 or so
Is amy of this wrong the vacume confused me I have an adjustable canister

Thank you fellow hottrodders

Ok I went out their got motor warmed up set idle screw to 3000 rpm I shot the timing tab I set dial back to 32 I had to rotate dizzy a lill tilll zero mark on crank matched up with zero on tab then I dropped idle back down to 700 shot tab again the slowed moved dial down tilll marks lined up it read 10-12 I drove it didn't ping any but didn't seem to fire off like a rocket seened slow .

While idling I was shooting tab while I slowly raised rpm to see where advance was coming in and any little raise in rpm advanced timing even when I had it idling so low it was barely staying running. right now out has one light spring one medium spring could that be ok our could it be coming in to soon maybe put to medium spring ote does that not even matter?

If you didn`t connect the vacuum advance it`s not going to have any power. 12 degrees with vacuum advance disconnected and plugged. Reconnect vac advance to a manifold source, recheck timing, it should be 18 to 24 degrees before top dead center at idle speed.

At the moment its hooked up to a ported vacume I think it was the little port just above fuel inlet on qjet. What is a good manifold vacume source for the advance?

So even tho I have vortec heads I should just set base at 12° and wherever total is leave it our should it never excede 32-34° btdc

Manifold vacuum is any source that is below the butterflies, as below them there is always manifold vacuum when the engine is running. A ported source is above the butterflies so it only pulls vacuum when the throttle is opened.
As I explained in my before post, Set the mechanical advance with the vac advance disconnected and plugged. Just because you have Vortec heads doesn`t mean the timing has to be 32, 34, 36 or 38. That`s why you have to experiment with them to see which it runs best without detonation. If you tried to set the mechanical advance with the vacuum advance connected it will pull in as much as 50 degrees advance at part throttle, why is because it`s adding together mechanical advance and vacuum advance.
So lets go over this one more time.

1. Disconnect vacuum advance and plug it. Set base timing to 12 degrees before top dead center. Snug distributor back down.

2. Reconnect vacuum advance connected to a manifold vacuum source. When you do so it`s going to idle up. So lower the idle speed to the desired idle speed. Recheck timing. It should be anywhere from 18 to 24 degrees advanced at Idle.

3. Disconnect Vacuum advance and plug. Bring the RPMs to 3000 RPM and slightly above. Check the spark timing. To get it to the desired range you will likely have to adjust the mechanical advance with bushings to get it where you want it. Once you`ve gotten the mechanical advance where you want it, reconnect the vacuum advance and test drive. You want full advance all in by 3000 RPM.

Many have said why is vacuum advance needed at idle? And I used to think the same thing. It`s needed because idle and off idle cruise mixtures are lean.
Lean mixtures burn slower than do richer mixtures, so the extra advance is needed to set the burn off sooner to compensate for it`s slower burning characteristics. When 12 degrees base is used with no vacuum advance, the idle speed on the carb has to be opened up more to compensate. When a larger cam is in use, to get it to idle it has to be opened up more and what happens is the throttle blades are off the idle circuit slots on the primary throttle body side of the carb. This forces the engine to get it`s fuel from the boosters so they boosters drip fuel at idle. I refer to this as "Runny nose"
Also since the leaner mixtures burn slower, with 12 degrees base only, off idle torque is way down as is throttle response. When I did my build in the early 2000`s I was running 12 degrees base timing. Vac advance was plugged into a ported source. I was running a 350 with camel heads, dual plane intake, RV cam, flat top pistons, headers, Holley 600, HEI, TH350 and 3.23 gears. The car was a complete slug. You couldn`t make it even burn a tire. Throttle response was lazy, power was sluggish and it drank gas. I learned to connect it to a manifold source and when I connected it the car was transformed. It felt like it picked up 50 horsepower. Throttle response was nice and crisp, I could smoke the tires off from a dead stop and gas mileage went way up.
Lastly on this book for a post I`ve wrote. Many guys buy a big cam and other goodies, but they can`t get the cam to lope at idle and they don`t
understand why. This happens because they aren`t tuning it right. On small high performance cams vacuum advance can be used, but on the large cams that have low vacuum it has to be set differently. So they have a street performance cam with a lopey idle, but it won`t lope. Why is because of the throttle blades being opened too far at idle due to the base timing being too low. One symptom of this is when it`s off the idle circuit slots, the idle mixture screws on the carb will have little to no effect on the idle quality.
With the base timing around 18 to 24 degrees, the engine idles up. so you lower it on the carb which puts the butterflies closed so it`s pulling the desired fuel mixture from the idle circuits and the car idles with a lope as it`s supposed to.

Ok thank you double vision I'm going to do that right now I will post back let you know how it went.