Vacuum Advance and how to set total timing

[f2502011]

I'm trying to get everything tuned on a 76 Bronco. The vacuum advance diaphragm was broken so I put on a new one. I have initial timing set at 12 BTDC like it says on the valve cover decal. I would like to adjust and set the total timing including that affected by the vacuum advance, but I'm not sure what it should be and how best to measure it and make adjustments. The vacuum advance diaphragm is adjustable. Any tips on how to dial this in and get it running its best would be great to hear.

 

[f2502011]

Anyone know which way I turn the screw inside the advance diaphragm to advance timing, and which way retards timing? What should total timing be and at what RPM? I've seen 30-34 listed or 34-38 listed. What does that mean and is that correct? At what RPM? Do I just mark 34 marks beyond the 12 BTDC at idle and try to achieve that number at a certain RPM? Any tips, info or links to find the answer would be great to hear.

 

[gearida]

Really you need a timing light that shows advance. Spin the rpms up and watch the timing move as the advance works through the dizzy. Your timing gun will show you how high the advance has gone. No idea which way to turn the screw to increase timing advance, should be easy to tell.

 

[f2502011]

Thanks. I'll see if I can borrow one from the local auto parts store. I've got a timing light but not one like that.

 

[itsabronco]

just a quick pointer, make sure your vacuum line is hooked up to the correct vaccum port on carb, secondary, not primary.

 

[f2502011]

This advance diaphragm has just one port.

 

[Tasty Geezer]

he means the correct vacuum port on the carb...the diaphragm will have only one port...

 

[Explorer]

Read the vacuum advance section and sidebar to the right: http://www.fordmuscle.com/archives/2000/03/timing/index.shtml

 

[f2502011]

he means the correct vacuum port on the carb...the diaphragm will have only one port...

I see...but some diaphragms have 2 ports.

 

[f2502011]

Read the vacuum advance section and sidebar to the right: http://www.fordmuscle.com/archives/2000/03/timing/index.shtml

Great info thanks!

 

[Gerbow]

Thats all chinese to me.....

 

[broncnaz]

Ideally you want a vacuum advance with one the single port the dual ports are a PITA.
As for timing you need to know how much mechanical advance is in the dizzy there are seaveral different advance postions depending on what dizzy you have 8, 10 ,13,16 come to mind there may be others. I dont really factor in vacuum advance in my total advance numbers. But its said that you can usually run between 45-55 degrees when all added up. I just take my dizzy advance add it tio my initial and shoot for the 34-38 mark vaccum advance just takes up where the mechanical is lacking on a stock dizzy full mechanical advance may not come in until around 4000 RPM.
The onkly time you need to adjust the vacuum advance is if the engine pings while cruising. To adjust it for less advance turn the screw counter clockwise. Its recommended you do 2 full turns then test for pinging again.

 

[bad 68]

To explain further: vacuum advance is really just intended to be for part throttle cruising. The job of the vacuum advance is to further timing during part throttle cruising down a highway.

The theory is that the more advance timing you can give an engine without detonation the more power you can make with less fuel. It is intended for economy cruising.

Vacuum advance should be disconnected and plugged while setting base timing.
Full timing should be a product of base timing+mechanical timing. During wide open throttle (wich is the normal way of reaching max RPM) the vacume signal to the advance will be very week and unmeasurable while sitting in the driveway.
But depending on vacuum signal from the carb port, full timing will be variable with each engine/carb combination.

An adjustible timing light is needed to dial in your mechanical advance with a spring kit.