DUI, h/w 5200, vacuum?

[Anonymous]

This might be obvious, but I'm not sure what to do. I just replaced my Holley/Weber 5200 32/36 carb with a newer one because the old one was messed up. This newer one has different vacuum ports, and I'm not sure how to hook it up.

On the carb, there are two ports: a larger one near the choke idle set screw, and a smaller one right at the base. I have a DUI distributor. The carb instructions say to connect the upper vacuum port near the choke to the distributor vacuum port, but that assumes a stock load-o-matic. Should I hook up the smaller vacuum port at the base to the intake vacuum and seal off the upper port?

My instructions for the DUI don't seem to specify what it wants.

I'm completely confused.

-marcus

 

[wallaka]

I'd probably put manifold vac to the DUI, and seal off the ports on the carb.

 

[H555]

got a similar issue - I'm not pushing back but why manifold vac?

 

[CZLN6]

Howdy Marcus:

I'd address that question to Mike. The HEI in GM vehicles were designed to use a ported vacuum source from the carb, but in this application that would just be a guess.

IF you have a longer duration performance cam, then the full manifold vaccum source might be the ticket. SO, you could try both and tell us which worked pest in your application.

Adios, David

 

[Anonymous]

Will do. I do not have a performance cam as of yet. All I really have is a set of nice headers on an otherwise stock 200, along with this two barrel carburetor.

I'm also dealing with some nasty hesitation that I had with the old carburetor also. Looks like .. a vacuum problem. Sigh.

 

[wallaka]

got a similar issue - I'm not pushing back but why manifold vac?

Why not? LOL. If he doesn't know which one is ported vac, then might as well go with one that he knows what it is.

 

[CZLN6]

Howdy All:

For H555- "why manifold vac?"

Because frequently ported vacuum sources don't work the same in a different application. Such as- a longer duration cam, a different intake or carb, or even improved exhaust.

The biggest differences are at an idle and/or at highway cruise speeds. Trial and comparison is usually the best way to determine which works best when mixing and matching parts.

Adios, David

 

[H555]

thx David, switched our DUI vac back and forth today, ported definitely seems to work best but I think I've got amildly leaking head gasket. (low comp between 2 cylinders and mild oil weeping on the side of the block. ) thinking about getting a later model head, having it milled, 3 angle valve job etc. car does runmuch much better.

 

[66 Fastback]

Why manifold instead of ported vacuum?
A manifold vacuum source as opposed to a ported vacuum source will yield more advanced timing, smoother idle and lower combustion temperatures. At idle, the fuel charge is more diluted with exhaust gases and needs more advance due to the slower flame propogation under these conditions. For maximum snap in performance you want to use all of the advance that you can short of causing detonation. Ported vacuum was not developed for performance enhancement, but rather as an emission strategy to reduce the advance at idle and increase combustion temperatures. The higher temps and leaner jetting of the carb helped reduce the hydrocarbon emissions. That being said, in order to take advantage of a manifold vacuum source, the vacuum cannister needs to be tailored to the engine vacuum, such that all of the advance from the vacuum can is fully applied at a vacuum level 2" below that of the idle vacuum. If the vacuum advance is not fully in below the idle vacuum, the idle will be erratic.

Another note, distributors from emission era cars that used EGR valves etc, may have more ignition advance designed into the timing curves than what a non-emission vehicle may need. The EGR valves dilute the combustion mixture, which required more ignition advance. They also typically had a larger portion of the timing contributed by the vacuum advance curves. These distributors may need to be re-curved to your specific application.
Doug