do the end carbs on the 427 tri-power has idle circuits??

[Michael F.]

above says it all, I didn't think they did..thanks.

[Stuart F.]

Re: do the end carbs on the 427 tri-power has idle circuits??

No, Michael. They seem to be just all in dump carbs. Regarding the dash pots (vacuum diaphragms), I tried to test them when I was reassembling them using a Mity Vac. When I pumped on them they would begin to open the throttle plates, but then the internal spring would slam them shut. It would take a full time application of vacuum (such as from the center carb @ WOT) to open them fully and maintain them open until the vacuum dropped and the internal spring over came the vacuum. That's the way my common sense saw it, but I may be wrong.

On my son's L-89, I intend to just verify the adjustment of the mechanical linkage to ensure it is not holding an end carb open, then install a new divorced choke mechanism and adjust that. The one on the engine now does not either close the choke for starting (even partially), or open fully upon warm up (probably the cause of the dieseling on shut down). With that working properly, I can then do a high idle adjustment (another stupid Holley design, I.e. Bending a linkage tab within a plastic cam, ugh!), and get a good set on full warm idle.

Stu Fox

[Timothy B.]

Re: do the end carbs on the 427 tri-power has idle circuits??

Hi Michael and Stu,

I don't have a end carburetor to look at but I believe there is a idle circuit on all the carburetors. The center carburetor is the only one that's adjustable with regard to the curb idle emulsion screws which can make you think it's the only idle circuit.

The end carburetors have a fixed air bleed/idle jet in the metering plate and a transfer slot which is feed through the idle circuit. This is necessary to start the fuel flow as the throttle blade opens until air flow through the venturi creates a signal to pull fuel from the booster nozzle (main circuit).

If I am mistaken I will be corrected but without it there would be a huge lean condition (bog) before the main circuit starts feeding fuel. The problem is when all these modular surfaces are not flat and vacuum wreaks havoc pulling air where it's not supposed to. I also believe there may be a small .020 hole below the transfer slot to keep fresh fuel in the bowl if it's not exercised.

[Wayne M.]

Re: do the end carbs on the 427 tri-power has idle circuits??

Just to refresh our memories on how this works. Notice they say that the 3 x 2 adjustment procedure does NOT appear in the 1967 or 1968 shop manuals .

Now, here's what's in the '67 Chevrolet New Product Manual (Aug '66). [repeats in Noland on pg 417], and which I don't really understand (other than to ensure that the gasoline does not go stale and gum up the orifices in the end carbs).

quote "... The secondaries have constant feed restrictions for corrosion-free operation from restricted use. Secondary carburetors will gradually begin to open around 2000 RPM, and are opened by a vacuum signal from the primary carb..."

Where are these constant feed restrictions ?

[Timothy B.]

Re: do the end carbs on the 427 tri-power has idle circuits??

Wayne,

The constant feed restrictions are little (piss) holes +- .020 in size just under the transfer slots on these carburetors. They are there to keep the fuel fresh in the bowls plus keep the transfer slot circuit primed with fuel so when it's called into use there is no fuel delivery problem.

The 4160 (3810) on my 300hp Holley also has them but the point I wanted to make above is that they are feed through the idle circuit as well as the transfer slot.

It's my belief that the secondary carburetors operate the same way as a vacuum operated four barrel. Air flow through the center carburetor creates a low pressure (vacuum) signal in the port that goes to the end carburetors. That signal overcomes the vacuum pod air bleed that is in the end carburetors and starts to lift the diaphragm. As air flow increases through the end carburetor venturi's, the vacuum pod air bleed changes from air bleed and a vacuum signal starts and contributes to the center carbs vacuum signal further opening the secondary's.

[Stuart F.]

Re: do the end carbs on the 427 tri-power has idle circuits??

Wayne and Tim;

Sounds good to me. Those pictures and instructions go a long way to clear it up for me. Thanks Wayne.

On my first car, I had three Holley 2 Barrel carbs (replacements for Stromberg 97's) using a simple mechanical progressive linkage. The end carbs, of course, had accelerator pumps so there was no hesitation or bog of any sort. I don't recall exactly how the late 50's and early 60's Chevy 348's and Pontiac 389's tri-power set ups were like, but I think they too came with vacuum controlled front and rear carbs, but we're quickly changed to mechanical linkages by everyone.

Stu Fox

[Ron W.]

Re: do the end carbs on the 427 tri-power has idle circuits??

I have a '70 Challenger T/A 340 6 Pak. Same series carbs as discussed here, 2300C and 2300's. The end carbs do have idle adjustment screws in the baseplates.
From my Mopar literature...
"Practically all 340-6 Bbl. setups run in a lean condition. The car will probably not idle well at times and may stall frequently. Unbolt the front and rear carburetors and locate the two soft lead plugs at the extreme front of the base. Remove the plugs so that a screwdriver may be applied to the previously hidden idle adjustment screws. One-eighth of a turn out (counter-clockwise) on these screws should solve this problem."
"In order to ascertain whether the carburetors are running rich or lean, place the tip of your finger (this pertains to the end carburetors only) on either of the outermost air idle bleed holes. If the engine "falls off" (decreases in rpm), the fuel mixture is too rich; conversely, if the engine seems to "pick-up" and sound stronger, the fuel mixture is too lean."

[Ken A.]

Re: do the end carbs on the 427 tri-power has idle circuits??

Ron, The Corvette carbs are totally different than your Mopar & your advice is incorrect.

[Michael F.]

Re: do the end carbs on the 427 tri-power has idle circuits??

thanks stu, you found with your mightly vac what I found so I am happy now, thinking my carb to intake gasket is leaking thus issues with idle etc. ordered some from fel pro and will replace all. I agree ken, mopar different from chevy....been there done that in the day. thanks all for information, all useful.

[Ron W.]

Re: do the end carbs on the 427 tri-power has idle circuits??

Thanks for the correction.
Sorry for the misleading info.

[Stuart F.]

Re: do the end carbs on the 427 tri-power has idle circuits??

That's ok Ron. I for one appreciate the variations in applications, specially in carburetors.

Thanks for posting.

Stu Fox

[Timothy B.]

Re: do the end carbs on the 427 tri-power has idle circuits??

I have a '70 Challenger T/A 340 6 Pak. Same series carbs as discussed here, 2300C and 2300's. The end carbs do have idle adjustment screws in the baseplates.
From my Mopar literature...
"Practically all 340-6 Bbl. setups run in a lean condition. The car will probably not idle well at times and may stall frequently. Unbolt the front and rear carburetors and locate the two soft lead plugs at the extreme front of the base. Remove the plugs so that a screwdriver may be applied to the previously hidden idle adjustment screws. One-eighth of a turn out (counter-clockwise) on these screws should solve this problem."
"In order to ascertain whether the carburetors are running rich or lean, place the tip of your finger (this pertains to the end carburetors only) on either of the outermost air idle bleed holes. If the engine "falls off" (decreases in rpm), the fuel mixture is too rich; conversely, if the engine seems to "pick-up" and sound stronger, the fuel mixture is too lean."

Ron,

That's a neat set up and what a maze of plumbing from the emissions era, thanks for posting that picture. The end carbs must have a curb idle discharge port with the adjustment screws plugged. Just curious if the center carburetor has the same adjustment.

The "finger the idle air bleed" test mentioned in your post is a good way to test the idle circuit to see if A/F mixture is in the ballpark. You can also do this with a propane bottle and a piece of rubber hose attached to the end. Warm the engine and run it up to approx 2000-2500rpm and slip the hose into a primary venturi then crack the valve to see if the engine speeds or slows as this will tell you if the engine wants more or less fuel. It's also a good way to find a vacuum leak.