JBrooke825

Been through a lot of searches and I can’t find a solution.
I have a 3x2 400 horse Ian’s I can’t get those secondary’s to work.
Right know I have the center carb off at the rebuild shop. For a check.
I put a gasket kit on to stop some leaks ,did that but I still have trouble
With a warmed up idle. When I go through the adjustment with the vac
Gauge it is fine . Next morning starts right up ,leave it warm up hit the gas
The idle fall off ,it won’t idle again. I’ll wait and see what they come up with. In the mean time I go to the other carbs to look into this secondary
Problem . My Mighty Vac won’t open up the secondary’s. Could you tell
Meir it should or how I could make it,must be a vac. Leak,I think.
Thanks for reading through all this. There must be a solution.

crf311

Are they getting fuel or is the butterfly's not opening? I had a new card and the secondaries cloged up from not using them when driving. I would never beat on my vette. After this happening, if gives me a reason to drive it like a stole it to keep fresh fuel in the secondaries.

NeverTooOld

There is a small vacuum port inside the right venturi of the center carburetor that feeds vacuum to the two hoses that go to the diaphragms of the front and rear carburetors. Suck on each of those two hoses and the butterflies in those carburetors should open. The front and rear carburetors operate the same way as the secondary of a single 4-barrel works. The factory designed them that way to prevent an over-throttle condition which would cause a big bog at low rpm.

JBrooke825

That’s what I did with the hand held vacuum pump. Neither one
Opens up. What is missing? I took the diaphragm off still won’t
Hold vacuum. Buy holding the ports shut

NeverTooOld

Your hand held vacuum pump may be leaking. Is it a Mighty Vac? If so the little plastic cap on the top needs to be tight or it'll leak so bad it won't pull any vacuum. Try using your mouth to suck on the hoses. If they open when using your mouth they're opening when you drive it at a wide open throttle and a high enough rpm. Remember, the outboard carburetors won't open until the velocity thru the center carburetor is high enough to allow the outboard carburetors to open.

JBrooke825

I’ll try that thanks,I am thinking it must be a vacuum leak from
Somewhere even when the car is at idle,having a problem with that
Too. That’s a bigger problem than the secondary’s not opening.
But maybe I can fix them both.

bazza77

Usually its a vac leak with those tri-powers and maybe the spring(s) too heavy ?

The 2300 series is supposed to be as a 4150 4 barrel in two halves and I think the secondary springs were very light.


I recall there's a vac port in each secondary as well so you wont get them to open with a vac pump ,that's why you test on the car with the paper clip or similar to see if there's movement . Its been a few years since I played with my 427/400 but I think I'm right as I pulled the damn things off and on so many times

also the secondaries have a fixed idle fuel circuit which you have to allow for when tuning , it's there so the secondary bowls have fresh fuel and are full when you stick your foot down , and HANG ON lol.

someone who still has a unit should confirm or flame my statement soon

NeverTooOld

When air rushes thru the right venturi of the middle carb it creates a vacuum that is fed to the outboard carbs via the rubber hoses. By disconnecting the rubber hose from the center carb and sucking on it both of the outboard carbs should open because their hoses are joined together. To see how far they are opening when giving it a full throttle bend a small paper clip enough to firmly grip the lift rod. Slide the paper clip up until it hits the housing then drive it under a wide open throttle at a high rpm. When the rod lifts it'll push the paper clip down the lift rod. To get them to fully open you'll have to be spinning it at least 5500+ rpm at a wide open throttle.

JBrooke825

Thanks for that,I did what you said and with the linkage unhooked
And they opened. Great. I am now thinking the linkage wasn’t
Keeping the throttle plates closed enough. I hope. It’s all back together
I’ll wait till the morning to try it. I guess I better have a close look at
That mighty vac!

NeverTooOld

When the outboard carbs open you shouldn't feel any SUDDEN difference but rather a steady increase in intake noise and power. The shaft that the butterflies are connected to is not centered on the butterfly so engine vacuum actually holds the butterflies closed. But when the diaphragms get vacuum to them they over-power the engine vacuum that was holding them closed. To see what I'm referring to flip any carburetor upside down and you'll notice the shaft is ever-so-slightly offset to the rear of the butterflies and that's why engine vacuum actually holds them closed under a slight amount of force. Plus they also have a weak coiled spring around the shaft that exerts a small amount of closing pressure.

Oh yeah, and let your oil warm up to full operating temperature before giving it a full throttle and high rpm. I always drive mine for at least 15 to 20 minutes before getting crazy with it.