TheSamba.com :: View topic - Tuning Vacuum advance distributor using air fuel gage

Mon Sep 22, 2014 5:20 am

I'm still saving money for parts, but I was thinking about saving to get a vacuum advance distributor instead of my currently only mechanical advance distributor.

John gave a pretty simple procedure for getting a mechanical only advance distributor tuned up...what more is involved when doing it with a vacuum advance distributor?

What's the best or method you use to tune in a set of carbs when running a vacuum advance distributor?

Thanks
Tim

Mon Sep 22, 2014 8:00 am

Using your wideband, you do your carb jetting while driving flat to the floor. When doing that you don't have vacuum, so it is not a factor. Do a wideband search, there is a lot of info out there.

Tue Sep 23, 2014 9:43 am

That's part of why I am asking this question, it seems like sometimes the venture size will be tuned in based off of this.

If you get the correct AF ratio when wide open and it stays the same even with a vacuum advance distributor, what does that mean? Or will it always jump up to 16 or more?

I would also assume that some performance motors might not pull as much vacuum as a stock motor, so adjusting the distributor would probably be critical on some motor combos as well.

Tue Sep 23, 2014 9:53 am

I've owned one of John's SVDAs, but they are no longer available. Even with an SVDA, there isn't much you can do for fine-tuning. This is why I'm looking to add a Black Box to my 009.

You have to tune for WOT in either case since that's your worst case scenario.

Stan

Tue Sep 23, 2014 10:08 am

stan_tichomirov wrote:
You have to tune for WOT in either case since that's your worst case scenario.

Stan
??? you can tune for whatever rpm band you want as long as the 02 sensor is located at least 20" from the exhaust opening.
High duration cams WILL however make the lower rpm (sub 25-2700ish) somewhat more difficult due to the larger overlap and therefore more air and unburned in the exhaust.

T

Fri Sep 26, 2014 6:26 pm

Does a W120 in a 2109 qualify as high duration? If it does what is the best way to determine an idle jet size even with the af gage?

Sat Sep 27, 2014 8:28 pm

Casting Timmy wrote: That's part of why I am asking this question, it seems like sometimes the venture size will be tuned in based off of this.

If you get the correct AF ratio when wide open and it stays the same even with a vacuum advance distributor, what does that mean? Or will it always jump up to 16 or more?

I would also assume that some performance motors might not pull as much vacuum as a stock motor, so adjusting the distributor would probably be critical on some motor combos as well.

Carbs are not as forgiven as FI...one thing you'll likely see is a rich condition when you have a light-no throttle condition while coasting around town & decel. Very normal with carbs.

High decel or shifting is when you'll see the AFR go lean (as high as 20:1 possibly). Again, this is normal and no cause for worry duet to the 'No load condition' on the engine.

You want to jet for worst case scenario...WOT acceleration under load. With a vacuum advance distributor shoot for ~ 13.0 - 13.5 or so. Your AFR readings will never be dead steady with carbs either. Pay attention to what it shows during steady state conditions as well.

With a centrifugal only distributor you jet for 12.8 - 13.2 or so under all conditions (load or no load).