|I'm having a problem with a carb I bought used and rebuilt. (Yes, I |used a Holley rebuild kit.) It's a Holley 4160 (750 CFM, vacuum |secondary), List #3310-2, with #2726 stamped below the list number on |the air horn. The problem I'm having is that the vacuum port on the |side of the primary metering block, which is normally supposed to be |"ported" vacuum for the distributor's vacuum advance, is in fact |manifold vacuum. I checked with my vacuum gauge - it reads about 18" |of vacuum at idle and drops when you goose the throttle, which is the |opposite of what you want for the distributor vac. advance. Other than |this, the carb seems fine. Interestingly, this is the ONLY vacuum port |on the entire carb, there are NONE on the throttle plate (which would |be manifold vacuum anyway). I took it apart again and followed the |passage from the port, through the metering block, and it goes right |to a hole in the body that comes out on the bottom of the carb, below |the throttle plates. Do you think the carb has the wrong metering |block, or the wrong gasket, or is this normal for this carb? If this |is the case, I guess I'm screwed for vac. advance. I'd like to be able |to have ported vacuum for my distrubutor (which I realize is a whole |'nother discussion, based on my newsgroup reading!) If it helps, the |numbers on the metering block are: L33102 5271 5275.
Why wouldn't that work? The normal 18 hg of vac holds the advance diaphragm in the normal position. When you dip into the throttle you get a momentary drop, which eases the diaphragm and advances the timing.
The main reason for the port as opposed to a tap in the manifold, is that being close to the throttle plate you get that pronounced dip in vac when the plate opens. Regular manifold vacuum drops, but not as quickly/sharply, so it's useless for the purpose.
As for manifold vac, you can either get a spacer plate like the older Fords used, or tap the manifold itself. I've also seen some thin spacer plates -
3/16" or so - with a vacuum tap.Good luck