dual diahpragm vacuum advance on DUI?

[falcon fanatic]

Now that the snow is finally melting I'm getting a chance to break in my new engine. (approx 10:1 with 264/274-12 and 1.65 rockers) I'm honing in on the right jetting, but I'm probably going to need to recurve my DUI distributor.

The issue is that the lumpy cam only give a good idle with a fair amount of advance at idle (presently getting about 15" vacuum, so with only 50 miles on engine ring seating may still be an issue?). i prefer using the vacuum advance to help with idle and overall responsiveness/fuel economy. However, with the idle advance where I want it, I get pinging at speed. So, I need less total advance, but I also think that I'm pulling some vacuum even at WOT and that is giving me a little more advance than I need at speed and under load.

The question: Recurving aside, I was wondering what wisdom was out there regarding using a dual diaphragm vacuum advance running off the ported vacuum port on the carb (weber DGES 38/38) to pull out vacuum advance as soon as I hit the gas.

 

[CZLN6]

Howdy Back FF:

I'm thinking that converting to a dual diaphram vacuum cannister is a long reach for a short gain- maybe. IIWIYS, I spend some more time diagnosing the problem. Is your vacuum line hooked to a ported source, or manifold vacuum on your H/W carb? Have you checked to be sure that the vacuum advance plate is returning to it's starting point once vacuum goes away. A problem existed on the old Load-O-Matic distributors when the inside got dirty and/or worn. It would advance, but returned to a "No advance" position very slowly. Problem solved by a good cleaning, lubing the pivot points and, on ocassion a new return spring.

A second issue- vacuum advance should decrease, or disappear under load and/or WOT conditions. So, initial advance and centrifugal advance should be all your engine is seeing "at speed". Your condition raises some questions- Have you verified your timing mark on your damper? Are you using a timing light? What is your initial adviance setting? What do your spark plug readings say? I forget, what trans are you using? I'd suggest that you hook up a vacuum guage so that you can watch vacuum levels as you drive, to more accurately assess what's happening, even at WOT.

I recall that you are using a DUI distributor so recurving the centrifugal advance is relatively easy. On the vacuum advance, with your cam and elevation you should be looking at using manifold vacuum for idle and fuel economy. I seem to recall several suppliers with adjustible vacuum cannisters for HEI type distributors. You might check to see if the DUI versions of the HEI doesn't already have an adjustible vacuum cannister. Going that way would sure be easier and make more sense than trying to adapt a dual advance cannister from an early ford distributor. The adjustment is easy, but it only allows for when and how much the advance arm moves.

Your vacuum level should increase slightly as your rings seat, but do something about the pinging. Either less initial advance, higher octane gas, colder spark plugs or some combo of all.

Keep the info coming on your progress.

Adios, David

 

[JackFish]

I have the same cam with stock rockers and am going through the same process.
Still have the Holley 1946 on top. I am going to switch to manifold vacuum and see if that improves things at idle. I am getting about 17" of vacuum so I'm not too bad there. Timing is at 12°.
I'm thinking this carb now needs a bigger jet.