Engine Timing and Carburetor Air/Fuel Ratio Adjustment 83-84 305 SBC

[Tynan918]

Well, I set initial timing at 6° BTDC and haven't set total timing yet, but the results are much better...

I also noticed I forgot to put the vacuum advance hose back on the vacuum advance but had the hose from the carburetor plugged off...which raises a question, do I need the vacuum advance ?

 

[57 Handyman]

Couple points:
1. All engines are different because there are variances that are inherently a part of manufacturing. Also, how these differences can impact a motors performance depends on what parts are installed. The combination of these variances either converge or diverge on a setting of timing and air-fuel mixtures.
2. The settings and procedures you are referring to above are a range and not cast in stone. Use these as a starting point to make things easier.
3. We, as drivers/owners, are also different. Our senses and reaction times are not the same. What is a hesitation to someone is a bog to another or a "falls flat on it's face". All of this is tied to how you drive your vehicle.
4. Bottom line: Basically, use what works best for you and your vehicle. Once you get your motor and car running fine, leave it alone and enjoy it!

With regard to vacuum advance, it depends. If your distributor is a vacuum advance distributor, you should not disregard that point and operate it as though it is a locked advance distributor. If I recall correctly, a distributor with a locked advance setting operates with that setting throughout the low-to-high RPM range; this approach is most beneficial in race cars that mainly operate at higher RPMs. For street cars, we seek an optimal setting that makes the motor run/perform efficiently/best in a wide range of RPMs.

Lastly, I may not have all of these points correct, especially those in the last paragraph. So, I suggest checking out some online sources regarding how advance works, especially the differences/pluses vs minuses of each Good luck!

 

[MrSony]

Vacuum advance can be hooked to manifold vacuum (suction at idle) or ported vacuum (only vacuum with throttle cracked open).
Manifold will give you extra timing at idle and help it run a little cooler. It will increase idle speed so you'll need to lower the idle speed with the idle screw.
Ported vacuum will add extra timing at highway rpm and will generally get a an extra mpg or two.

 

[Tynan918]

Vacuum advance can be hooked to manifold vacuum (suction at idle) or ported vacuum (only vacuum with throttle cracked open).
Manifold will give you extra timing at idle and help it run a little cooler. It will increase idle speed so you'll need to lower the idle speed with the idle screw.
Ported vacuum will add extra timing at highway rpm and will generally get a an extra mpg or two.

Mine is currently manifold, but sounds like ported is where I want it...

But why manifold if the vacuum advance is only advancing at idle...advancing at WOT makes more sense, especially getting more mpg too...

 

[MrSony]

Mine is currently manifold, but sounds like ported is where I want it...

But why manifold if the vacuum advance is only advancing at idle...advancing at WOT makes more sense, especially getting more mpg too...

Vacuum advance is separate from mechanical advance with the weights and springs. It just adds extra at idle for cooling if you have a high compression engine with a lumpy cam or as said extra timing at cruise rpm. They both disappear as you approach wide open throttle and vacuum drops to near zero. Your 305 had it set to ported I believe stock for emissions and mpg.

 

[Tynan918]

Vacuum advance is separate from mechanical advance with the weights and springs. It just adds extra at idle for cooling if you have a high compression engine with a lumpy cam or as said extra timing at cruise rpm. They both disappear as you approach wide open throttle and vacuum drops to near zero. Your 305 had it set to ported I believe stock for emissions and mpg.

Last compression test I did was last October (2021) and this were the results...is this considered high compression engine ?

Cam seems lumpy at low idle rpm 500-600, but I don't think it's other than OEM.

If I have a TH350 transmission, which needs manifold for the modulator.

Here is my set up...green line goes to the distributor vacuum advance and is on the ported side of the carburetor. Red line goes to the transmission modulator and is at the manifold port on the carburetor...

The diagram photo shows me a lot of making the decision of manifold and ported at the distributor, depends on if my engine is emissions controlled or not...

How do I know or check if my engine is emissions controlled ?

 

[Clone TIE Pilot]

I'm reading 3 different scenarios when it comes to initial and total timing on a SBC...

Scenario 1, Factory Settings: Initial is 6° BTDC @600 RPM in Drive, but doesn't specify the total timing...I'm guessing 34-36° total with the initial being 6°.
View attachment 201104


Scenario 2: Says here, with the vacuum advance hose disconnected and plugged at the hose, to:

1. Set the initial timing to 4° BTDC.

2. Connect the vacuum advance hose back to the distributor vacuum advance canister...timing reading should advance 18° plus or minus two.

3. Raise the RPM to 3000 and the timing reading should show 32° total timing, and if not, advance or retard the distributor to achieve 32° total timing.

How to Time a Chevy Small Block Motor

To establish the correct advance curve, the vacuum advance and mechanical advance in the distributor must work properly with no binding. When a small-block Chevy is started, the spark is retarded to 4 degrees BTDC. As soon as the engine starts and a vacuum is produced, the vacuum advance...

itstillruns.com

View attachment 201102

Scenario 3: 12-18° BTDC initial timing and 34-36° total timing...
View attachment 201103

Which one is correct ? I'm willing to bet my cam, lifters, rockers, and H.O. heads are stock.

Should I start with the factory settings of 6° BTDC @ 600 idle rpm and total it at 34° ?

The other options outside of the factory settings didn't specify what the RPM needs to be when setting the initial timing...does it even matter or should it be higher when setting the timing at higher degrees BTDC?

The factory settings only work on factory engines. Since you removed the factory computer Qjet and distributor the factory settings no longer apply to your engine.

 

[MrSony]

Your engine is no longer emissions controlled/compliant. As said above, you removed all the factory computer and smog stuff, so treat it like any other low power low compression small block v8. Your compression numbers are very good, that engine is not a powerhouse, but it seems to be in good health and the rings are sealing well.
Manifold vacuum, suction at idle, is where the modulator gets connected to. There must be a solid steel line with rubber on the ends for flexibility for the modulator.
The way you have the lines hooked up is fine. Make sure there are no vacuum leaks, it looks like you are using non vacuum rated lines, which is why you need hose clamps on them to keep them from sliding off. Vacuum lines are much smaller in diameter than those. Spray brake cleaner at those connections, if the idle increases or changes, thats a vacuum leak.