Need Vacuum Secondary Help

[Atlfan65]

So I've searched and can't find any posts with the same problem. I have a 1965 C10 with a stock vortec 350 out of a 1996 Suburban with a Holley 18650 carb. I've been chasing a lean bog for a while now and now know it's my vacuum secondaries opening way too early. I know that the secondaries shouldn't open at all in park, but even with the black spring (stiffest), the secondaries are starting to open at 4000rpm.

I took everything apart and my diaphragm isn't punctured, but when I test it by putting my finger over the hole, the rod returns acting like it is. I thought that when that happened, they fail so that the secondaries don't open at all. What's going on here??

[PGSigns]

The secondaries will open anytime there is enough vacuum signal to pull them open regardless of the gear the truck is in. The lean condition could be accelerator pump cam selection, pump adjustment, squirter size, float level, ignition timing and a host of other things. Tell us more about when it happens.
Jimmy

[Atlfan65]

I know that vacuum secondaries shouldn't be opening in park at 4000 rpm with the heaviest spring installed. It's not a matter of "IF" the secondaries opening too early is causing the bog. It is a matter of HOW. With the black spring they should barely be starting to open at 4000rpm when driving, let alone with much less load on the engine in Park. In gear the secondaries are almost fully open at 3000rpm, but the black spring isn't even supposed to crack them open until around 3500 according to Holley.

[Captainfab]

Since you are asking about vacuum secondaries, when they open will also be dependent on how much vacuum your engine creates at any given RPM. I would suggest making sure your timing is set correctly first and then see when the secondaries open.

[PGSigns]

The holley chart is a guide. The system is totally vacuum dependent. So the cam, intake and timing all work to drive manifold vacuum. A vacuum gauge will show you what you have. As manifold vacuum drops the vacuum secondaries will open when there is enough differential pressure to overcome the spring. When I put the L31 in my 66 (the replacement engine for your donor) on of the things we did was a cam swap. The factory grind is an EFI grind and they don't tend to produce strong manifold vacuum compared to a cam ground for a carbed combination. It may only be a few inches of manifold vacuum but strong vacuum signal is what makes a carb work. You timing curve also can drive both power and carb signal. So it all has to work together.
Jimmy

[Oydllay]

Try disconnecting the secondary link from the throttle shaft. Take it for a ride. Bog still there? Not saying your early opening problem isn't possible but its not likely that you are developing too much vacuum. You're right about the black spring. Its pretty stiff and renders the secondary nearly useless. Holley chart is based on stock vac. I'm sure you have at least that assuming your in tune. Any other mods, large cam etc. would drop the vac. and delay opening. Also, don't put a lot of faith in the 4000 rpm in park thing. I can't remember for sure but the secondary servo may be fed ported vacuum developed in the primary venturi. With no load on the engine you may very well be able to get them open at that point. Try diabling the secondaries and verify your problem is not elswhere first.