Adding a Vacuum Secondary Pickup tube

Tech article time!

I’ll have comments with each photo, so if interested, read each one. I would have made a video, but it would have been pretty difficult to do honestly, so pics will have to do.

Almost any 1 5/16 or larger venturi has trouble opening the secondaries.

Holley originally had a small slash cut tube for vacuum pickup.

Basically as air flows past the tube the moving air draws on the vacuum secondary housing, it overcomes the spring pressure and opens the secondary throttle plates.

There is a bleed off on many carbs to the secondary venturi. If present it is often .052 in size.

With the bleed and without the tube, it is often very difficult to open secondaries.

This is most common with 3310s.

3310, 3310-1, 3310-2 all have the pickup tube, 3310-3 and later do not seem to.

Image01 shows brass pickup tube

Image02 shows the Venturi without the pickup tube

Image03 shows 1/8 brass tubing

Image04 we need to cut a piece of the brass tubing .750-1.00 inch long

Any longer and it can block the other feed holes.

Image05 I cut a dozen of these at once, I use a drill to chuck the tubing into and a tubing cutter

Image06 and 07

Freshly cut brass tubing. Grind at a 45 degree, It needs to be cleaned out with a drill bit and deburred so it can be installed smoothly.

Image08 and 09

This shows the lead plug into the vacuum feed and removing it with a drill bit.

Image 10

We confirm the tubing is .125

Obviously we need to make sure the passageway is the same size or close to it.

Image11 we are cleaning out the passageway to .125 or a few thousands greater.

Since I do a lot of these, I prefer larger so the tubing goes in easily. Obviously this leaves it loose, so I use a retaining compound. Be very careful here as the drill bit will want to grab and you can easily hit the booster.

Image12 and 13 shows the passageways

Image 14 and 15 shows retaining compound and installing tube with a punch.

Image 16 shows the tube pushed in far enough to not block any other passageways. Image17 shows tube installed.

Image 18 and 19

You can plug with lead as originally done or tap for a set screw.

If installing a set screw make 100% certain it doesn’t go in very far as it can plug the feed.

Anyway, this is a common modification I make, it may seem a little intimidating but it is essential if you find your 3310-3 or other carb like it cannot open the secondaries.

Good luck

Drew