tri power vaccuum secondary

[Daniel G.]

When working on the carbs on my 67 427/435 I noticed the front and rear carbs had large vaccuum actuators and when I sucked on the vaccuum line the actuator would work but it does not hold when I plug the line but needs constant vaccuum applied to hold. Is this normal or is the vaccuum actuator leaking and in need of a new diaphram?
Daniel Grunwald

[John H.]

Re: tri power vaccuum secondary

The secondary diaphragms should hold vacuum when each one is tested all by itself from the fitting at the front of the diaphragm housing.

[Wayne M.]

Re: tri power vaccuum secondary

When working on the carbs on my 67 427/435 I noticed the front and rear carbs had large vaccuum actuators and when I sucked on the vaccuum line the actuator would work but it does not hold when I plug the line but needs constant vaccuum applied to hold. Is this normal or is the vaccuum actuator leaking and in need of a new diaphram?
Daniel Grunwald

Daniel --- if you got the symtoms you describe with the vacuum pods ATTACHED to the main body, then this is normal. Just to satisfy myself, I took the pod off a 3659 I have handy, pushed in on the diaphragm shaft, put my finger over the hole with the cork gasket, released the shaft, and it was completely air tight; diaphragm held (shaft didn't move), indicating no leaks.

Then, [main body] with your finger on the hole that connects to the pod, squirt a little carb cleaner in the nozzle where the vac hose attaches; you'll notice the liquid shoots out at the narrowest part of the throttle bore venturi on that side. This pinhole channel is where the vacuum signal from the outboard carbs (when throttle plates are open), augments the vac signal from the center carb. So yes, the diaphragm would close if you stopped sucking, due to bleed through this secondary channel.

So when you lift your foot off the throttle, the primary vac signal dies, and the secondary signal alone is not enough to override the diaphragm spring, so outer carbs close.

The linkage from center to outboards is only a back-up (safety) to ensure the outboards don't stay open.

[Clem Z.]

Re: tri power vaccuum secondary

there is a bleed hole into the carb venturi that causes extra vacuum to the actuators as the carbs start to open

[Stuart F.]

Re: tri power vaccuum secondary

Having the end carbs stick open on a tri-power is fun only if you enjoy watching tach needles bury themselves. I had that happen with a cheap mechanical linkage on a 283, did one fast key off. Thing ran better than ever after that. Just like an overheat loosens things up too.

Stu Fox

[Daniel G.]

Re: tri power vaccuum secondary

Thank you gentlemen. The carbs always seemed to activate well enough but there is always that dream that there is a bit more power lurking out there in some defective component. I am really glad to know all is well. NCRS is the best forum in the world!!
Daniel