Ignition Timing for First-Generation GM V-8 Engines

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Swearbody

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Ive concluded with help of a friend that my mechanical advance is bad as i have the distributor hooked to manifold vac at the moment and theres no change in running except maybe a very slight bit better. Im actually contemplating one of those complete skip white distributors and swap the whole shebang.

Mechanical advance has nothing to do with vacuum. It works off the springs and centrifugal force of the weights and rpm.
 

Vbb199

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I'm having some issues getting my truck to start. It'll crank and back fire (LOUD!!!)

One place I read to find TDC was to remove the driver's side valve cover. When the 1st rocker goes down then up, second rocker goes down then up it should be at TDC; but this would mean both valves are closed.
Am I reading one of them wrong?


This reminds me of a guy we had on the forum that installed his distributor 180° out, and somehow managed to get it to fire, which in turn caused both of his mufflers to explode (he posted pics)

We had many lolz that day.
 

rich weyand

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Ported vacuum, or timed vacuum, is emissions control nonsense and has nothing to do with how gasoline actually burns. At idle, you'll be dumping burning gas out your exhaust valves so the AIR pump can add air and burn up emissions in the exhaust manifold. No AIR pump? Then why retard the idle ignition?

I remember when they started that ported vacuum nonsense in 1968. First thing you did was rip off the AIR pump and swap the distributor vacuum advance back to manifold vacuum where it belongs.

Just follow carburetor manufacturers instructions as below.
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Ricko1966

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technically you'd want say a transmission modulator on the drivers side port, and your vacuum advance for distributor on the passenger side port.

No that is not correct. Ported vacuum( passenger side port on eddy 1406 )wasn't even thought of until emissions equipment was thought of. Before emissions standards cars were connected to manifold vacuum( drivers side on eddy 1406.) Ported and manifold vacuum are the same except for at idle. Another thing a lot people do not realize is if you set your idle speed using the throttle stop screw, instead of the idle adjustment screws, is many times you uncover the transition ports in the carburetor which screws up your fuel delivery at low end.
Not saying you can't tweak it a little but there's a fine line between a little and too much.

Damnit I wrote this before reading the post above mine.

Rusty, you didn't ruffle my feathers, I just didn't know if you knew. How people do you think will get tickets this week setting their timing?
 

QBuff02

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No that is not correct. Ported vacuum( passenger side port on eddy 1406 )wasn't even thought of until emissions equipment was thought of. Before emissions standards cars were connected to manifold vacuum( drivers side on eddy 1406.) Ported and manifold vacuum are the same except for at idle. Another thing a lot people do not realize is if you set your idle speed using the throttle stop screw, instead of the idle adjustment screws, is many times you uncover the transition ports in the carburetor which screws up your fuel delivery at low end.
Not saying you can't tweak it a little but there's a fine line between a little and too much.

Damnit I wrote this before reading the post above mine.

Rusty, you didn't ruffle my feathers, I just didn't know if you knew. How people do you think will get tickets this week setting their timing?


It's not totally incorrect, just not exactly optimum. If you only have two ports and an automatic transmission, you need manifold vacuum to operate the trans vacuum modulator correctly, so for the quick and dirty you could hook your distributor to the ported and trans modulator (it should already be) to the manifold port. It will more than likely require a timing adjustment for sure. best case would be to get a T fitting and tie them both in if you don't have another location for full manifold vacuum. Or just don't run junk edelbrock carbs to begin with. Lol I mean they are good in some applications but I won't have one on any engine that's modified much over stock. I agree with you though that too many people adjust the idle screws as a crutch to make their engine idle correctly and it only compounds the problem further. And then they wonder why! I set the butterflies and transition before installing the carb and then use a vacuum gauge to tune the carb once it's installed, that's the best way to do it I think. I run an 850 Demon carb on my Big Block and after about 3 trips around it with a vacuum gauge the throttle response is about as crisp as fuel injection. And that's part of the other problem, a lot of people don't want to invest the time into it.
 

Raider L

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@rich weyand,

In addition to how I have my dist. set as I talked about above, I failed to mention the next thing I go to is to set my carb. idle jets, according to where the rpm is after the timing is set. I have gauges installed my engine block just for timing. I have a small tach over on the inner wheel well so I know what the engine is doing. Then on a bracket installed on the front of the engine I have a vacuum gauge, and a front oil pressure gauge. The vacuum gauge is used so I can start turning the idle jets setting them for max intake vacuum on each side of the carb. I can tell what is going on after I have the timing light looking down at the harmonizing wheel when I'm at max advance as to what the springs are doing all the way to 3,000 rpm. I can do ignition and carb. settings on the side of the road. I hardly ever have to change the idle screw. Usually setting the idle jets does all I need to do with no waste of fuel.

I don't know why my engine won't run as well as it runs now with a vacuum advance on it, but I took that thing off long ago and haven't looked back. Here recently I took the cap off to change it, I could tell it needed it, and also changed out the springs for new ones to renew anything there.

And also let me mention this, the weight of the counter weights makes a big difference when you are using the springs and weights for all the advance you are going to have. The counter weights come in grams. Some allow the "all in" to come in to fast no mater what stiffness spring you've got on there. I know I've tried them all. And everyone makes them, Moroso, Mr. Gasket, Accel, MSD, Mallory I think, etc., etc. I'm using the set I got with my MSD distributor, I don't remember what they weigh but I know they are a lot heavier than the Mr. Gasket's. The counter weights are matched with the springs that come with them. Each mauf. thinks they know what works with their counter weights. I know because I've mixed them up before to see what would happen, you are better off staying with the one's that come with the counter weights. I have papers and charts for particular cam, engine hp/torq cylinder bore, rod length (piston speed), etc. to get everything set like it should be. Although it still needs a bit of tweaking and just a tad of best guess to get it just right. It's fun, interesting, and a challenge to do this and have what you've done work.
 

13matsc

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Ported vacuum, or timed vacuum, is emissions control nonsense and has nothing to do with how gasoline actually burns. At idle, you'll be dumping burning gas out your exhaust valves so the AIR pump can add air and burn up emissions in the exhaust manifold. No AIR pump? Then why retard the idle ignition?


Just follow carburetor manufacturers instructions as below.
You must be registered for see images attach
Hi guys,

Been working on getting the timing and carburetor adjusted correctly on my 1985 C2500. The truck originally had the California emissions package but this has been removed and replaced by an Edelbrock 1406 and performer intake + long tube headers.

First: the timing marks only goes up to 12, how should one be able to set the timing to 16˚ when its off chart? Or is this different on all the 350 engines? The marks were 12, 8 and 4 BTDC.
I have mine at 8˚BTDC now and it is way peppier then when i got it (it was at 4˚)
It was adjusted without the 3/16 hose connected from the picture above.

Im not quite understanding how the different 3/16 ports affect the engine when RPMs rises, could someone please elaborate? The truck is quick and willing from 0 to about 30-35mph, but then slowly loses momentum.
 

75gmck25

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If you want to measure timing beyond what shows on the pointer, you can install a timing tape on the balancer, which will have degree marks all the way around.

However, the easier solution is to use a dial-back timing light.
- You can just set the dial on the light to the timing you want (for example 12 BTDC), and when you point the light at the marks it should line up with zero on the tab if the engine is set to 12. If it does not line up, then turn the distributor until you get the light to line up on zero.
- The opposite method is to rev the engine up to the rpm where you want to measure total timing (maybe 3000 rpm), and turn the dial on the light until the mark on the balancer lines up with zero. Read the dial to find out the total timing reading.

Cheaper dial back lights have a physical dial like this one. https://www.summitracing.com/parts/ino-3555
More expensive ones have a digital readout, and usually also have a tachometer. https://www.summitracing.com/parts/ino-5568

Always start out with the vacuum advance line (from the carburetor) plugged, and the idle set down to about 600-650. Measure your base timing at that rpm. Then slowly rev it up to about 3000 rpm, keeping track of how fast the timing advances and the max value it reaches for total timing. If the mechanical comes in quicker (maybe by 2000 rpm) it will improve acceleration. Subtract base timing from total and you have your mechanical timing.
Then finally let it go back to idle and reconnect the vacuum advance. Measure timing with advance connected, subtract base timing, and you have vacuum advance timing. It should be about 18-20 degrees on most stock distributors.

The full manifold vacuum port on the carburetor will pull on the vacuum advance all the time, but of course vacuum will still vary with engine throttle opening. Since idle and light throttle cruise both have high vacuum, you will get max advance at idle or at light cruise. The advance at cruise helps a lot with improving gas mileage.
- The "timed" vacuum port will have no vacuum at idle, but will start to have high vacuum as you slowly press the accelerator. Once you get a little past idle it will operate the same as a full vacuum port. It was used for many emissions controls because some devices needed zero vacuum at idle.

You can usually tune your engine to use either port, but may need to make some idle adjustments if you switch. Full vacuum at idle means the timing is also advanced and the idle speed goes up. If you use timed vacuum you may have to adjust the idle speed higher.
 

williestreet

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I bought my truck last fall and have been busy dealing with a ton of other problems with the truck.
Well yesterday I finally decided to check it and was confused with the results.
Vacuum disconnected initial timing was 11 degrees at 750rpm.
Revved it up to 2800rpm and it was still 11 degrees???

Connected the vacuum back up, 11 degrees at 750rpm, 2800rpm=28 degrees??

So I was thinking there must be an issue with the mechanical advance. Took the distributor cap and rotor and found the weights were not sitting correctly.
The little bushings where the weights pivot are almost completely worn away. Maybe the slack allowed the weight to move out of place or whoever had been in their last didn't take care to make sure everything was back in place correctly.

Reset everything correctly and now my initial is 12 degrees, mechanical adds another 22 degrees for a total of 34 degrees without vacuum.
Connect the vacuum and it adds another 14 degrees to show a total of 48 degrees at 2800rpm in the garage.
Truck feels a lot better now. Will drive it a bit and see if I need to make some adjustments to the base setting.
I will start looking for a bushings kit or a way to fix what I have.

Pic of how it was sitting when I took the rotor off.
 

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