L-71 vacuum diaphragm question

[Kenneth K.]

[Timothy B.]

Re: L-71 vacuum diaphragm question

Kenneth,

There is nothing wrong with your carburetors diaphragm from what you report if they held by covering the pick up hole inside the venturi.

[Kenneth K.]

[Chris H.]

Re: L-71 vacuum diaphragm question

No need for a mity vac in this situation. Just push up on the diaphragm rod, place your pinky over the hole in the Venturi, and see if it holds.

[Kenneth K.]

[Timothy B.]

Re: L-71 vacuum diaphragm question

Kenneth, FWIW, the Holley vacuum secondary blades open at WOT when a low pressure signal (vacuum) is created in the primary carburetor from air rushing through the reduced area in the venturi where the small pick up hole is located. The low pressure signal is further bled off by the small holes in the secondary venturi's until as the secondary blades start to open and air starts rushing through the secondary venturi's the bleed holes change from bleed holes to vacuum feed and further aid the signal that pulls on the diaphragms and opens the blades completely.

[Kenneth K.]

Re: L-71 vacuum diaphragm question

I tested the 2 end carburetors on the car by pressing up the diaphragm rod and using my pinkie to cover the hole in the Venturi. I also plugged the vacuum hole on the outside of the carburetor. Both carburetors did not hold. So now all 3 of my carburetors (2 ends on the car and spare on the bench) do not hold by testing this way and they no longer hold testing the way the instructions say to test for a leaking diaphragm.

I found the rebuild instruction sheet and interesting it says to remove the “secondary diaphragm housing assembly from the carburetor body” (then test for leaks as I mention a few posts above). So apparently the vacuum hole in the venturi has nothing to do with the test that the rebuild instructions says to do. The rod should remain up when the diaphragm is tested off the carburetor body therefor it should also remain up when tested on the car without covering the hole in the venturi.

I found a cutaway drawing of the secondary carburetor that shows the vacuum flow when the diaphragm is closed (rod up) under high vacuum. I just don’t see how the diaphragm can stay up when I do the manual tests if both vacuum holes are not covered. See upper part of attached cutaway drawing. Anyone know what I am missing?

That brings up another question - If the diaphragm is leaking will that cause the secondary to not close properly? It seems like the secondary throttle plate sticks slightly open on the rear carburetor causing a high idle. I can blip the throttle and get it to close sometimes. It’s not sticking but it just doesn’t always want to close. So is the diaphragm responsible for closing the throttle plate?

[Timothy B.]

Re: L-71 vacuum diaphragm question

Your picture should mention it's referring to venturi vacuum not manifold vacuum.

Get a small piece of putty and cover BOTH the small pick up holes in the secondary carburetors and get a air chuck and blow air down the center carburetor passenger venturi past the small pick up hole, the end carburetor blades should open.

The secondary shafts have teflon bushings to aid in the smoothness of the shaft movement. Manifold vacuum will hold the blades shut as they are offset when installed on the shaft. There is a secondary stop adjustment screw to keep the blades from sticking shut to the bore, the adjustment is 1/8-1/4 turn from completely shut.

[Chris H.]

Re: L-71 vacuum diaphragm question

Kenneth my apologies as I was incorrect on when I said no need to mityvac. It’s b en a few years and my memory is foggy. You do need a mityvac. Picture shows the set up with the diaphragm shaft fully retracted.

[Chris H.]

Re: L-71 vacuum diaphragm question

Secondary throttle blades sticking on my L71 has been problematic. First make sure your closing linkage is adjusted properly, meaning the blades are fully closed at idle. The procedure is in the chassis manual. The problem I was having is that they were sticking shut at WOT and then busting open. I wound up smoothing the blades very gently with some emory cloth and that’s seems to have done the trick.

Timothy, you mentioned secondary throttle stop screws and I wish the heck mine had them. Maybe 67s had them, but my original ‘69s do not.

[Timothy B.]

Re: L-71 vacuum diaphragm question

Chris, I am not as familiar with that detail on the tri power carbs but I would expect all vacuum secondary Holley carbs have a secondary stop screw. If the blades stick in their bores the diaphragm will never pull them open, look at the bracket where the diaphragm rod connects and see if the stop screw is present.

If for some reason it's not there then the linkage controls the at rest position of the blades.

[Kenneth K.]

[Kenneth K.]

Re: L-71 vacuum diaphragm question

IMG_6205.jpg

[Kenneth K.]

closes

[Chris H.]

Re: L-71 vacuum diaphragm question

Kenneth you now have proven the secondaries are working and now need to focus on idle speed. The adjustment procedure is in the chassis manual and must be followed to the T or you will just be chasing your tail.

Disconnect the secondary closing linkage and start the process.