69 L68 Secondary carb linkage

[Grahame M.]

Hi all,
Finally got my car out on the weekend after a very lengthy time in the garage. I took it for a run to test the refurbed carbs. When I bought it back to my garage I adjusted the secondary carb linkages as per the AIM.

My question is around the understanding of this adjustment. It says, the rear linkage is supposed to be half the rod diameter short of the rear secondary throttle lever hole. Is this measured at the front of the hole (centre carb side) or from the rear of the hole (Firewall side).

My thinking is if it’s the front of the hole that could open the throttle plates ever so slightly? Where if it’s the rear of the hole, that would ensure it’s closed?

I have seen some so called experts on YouTube say just make sure they are adjusted to fit directly in the hole, front and back?

Any clarification would be greatly appreciated as I don’t have any real experts to call on here in Oz. The shop that did the refurb are Holley agents but have only seen two sets of these carbs and they’ve both been mine.

Thanks as always.
Grahame

[David D.]

This description from the service manual explains it. The secondary rod should "fall short" of reaching the carb hole by 1/2 rod diameter.

[Duke W.]

I can't improve on the instructions in the above post. I guess you have to read them a few times and follow them step by step... like assembling something from Ikea.

I'll add for the benefit of those who don't understand the 3 x 2 system that the secondary carbs are opened by vacuum diaphragms actuated by venturi vacuum from the center carb - just like the secondaries on a Holley four-barrel. The mechanical linkage is there to ensure positive closure of the secondary carb butterfly valves when the driver fully lifts off the throttle pedal.

A few years ago I was acting as "judge" helping a friend prepare a '67 L-71 for a PV. During the WOT test the engine just wasn't revving like it should, so we aborted. Back in the garage I found that both secondary carb butterfly valves were jammed in the bores. It took some effort, but I finally got them freed-up and checked that the mechanical linkage was properly adjusted. Hard blips of the throttle showed that both end carbs were starting to open.

A L-71 with a two-barrel carb is like a NASCAR restrictor plate engine. It's my understanding that restrictor plate engines pulls about 7" Hg manifold vacuum at 7500 compared to unrestricted engines that pull less than 1" at 9500.

Duke

[Grahame M.]

Thank you David and Duke, your help is very much appreciated. I think i may not have done it exactly as the service manual states. I may have adjusted the idle (which is high) while the rods were connected. Next weekend i'll redo the full checks with the rods disconnected and also check the the float levels because it lost power momentarily when i was driving around corners and roundabouts. There is about half a tank of fuel so it may be the float levels or fuel pressure?

Also, should i remove the vacuum hose to the secondary carbs while doing this or because it's ported there's no vacuum at idle?

Thanks again for your help.

Grahame

[David D.]

Thank you David and Duke, your help is very much appreciated. I think i may not have done it exactly as the service manual states. I may have adjusted the idle (which is high) while the rods were connected. Next weekend i'll redo the full checks with the rods disconnected and also check the the float levels because it lost power momentarily when i was driving around corners and roundabouts. There is about half a tank of fuel so it may be the float levels or fuel pressure?

Also, should i remove the vacuum hose to the secondary carbs while doing this or because it's ported there's no vacuum at idle?

Thanks again for your help.

Grahame

There is no need to disconnect the vacuum hoses to the secondary carbs. If you are setting the idle speed, there is no vacuum to the secondaries at idle. If you are setting the secondary linkages, the engine should not be running at that time!

[Duke W.]

Also, should i remove the vacuum hose to the secondary carbs while doing this or because it's ported there's no vacuum at idle?

I think you're confusing the vacuum system that opens the end carbs with the ignition vacuum advance system. The former uses center carb venturi vacuum to open the end carbs. At idle and low load there is insufficient air flow through the center carb to provide enough venturi vacuum to overcome the springs to open the end carbs. At WOT once air flow is sufficient to create enough venturi vacuum to overcome the spring force, the end carbs will begin to open. This begins at about 2500 revs, WOT.

Manifold vacuum is used to command the ignition system VAC. L-71 (and L-72) have "ported" vacuum advance. The small opening in the throttle bores that signal the VAC are above the throttle plate at idle, so there is no (or not enough) manfold vacuum to the VAC for it to add any advance at idle. Your engine will achieve better around town fuel economy and have less tendency to run hot if you convert to full time vacuum advance, but since the OE 15" 201 15 VAC doesn't pass the "Two-Inch Rule" you will need to swap it for a 12" B26 VAC.

This mod to provide full time vacuum to the VAC is simple and easily reversible. Obtain a couple of feet of 1/8" vacuum tubing, a 1/8" tee, and a 1/8" vacuum cap. Remove the VAC vacuum hose from the carb to pipe and install the vacuum cap over the carb nipple. Remove the choke vacuum break hose. (Save it and the carb to pipe hose) This circuit provides full time manifold vacuum to the choke vacuum break. Use the tee and hose fabricate a new harness that connects this full time manifold vacuum source to both the choke vacuum break and VAC.

A B26 VAC should be available from NAPA, part no. VC-1765. If a NAPA store is not available, any parts store should be able to cross reference the part no. to whatever brand they sell. Since there are reports that recent VACs don't meet spec, buy locally and use a vacuum pump to check that start and stop points (start at 6" fully deployed at 12") are at least within 1" of spec.

To swap out the VAC, pump it down to maximum deflection, then remove the two screws and carefully wiggle the pin out of the stationary pole piece base plate. Save it as the OE 201 15 VACs are rare. Pump down the replacement and install.

With the additional idle advance the curb and fast idle speeds will increase, so you need to go through the idle speed-mixture adjustment procedure and also adjust the fast idle per the service manual. IIRC it's 2200 (hot ) that I think is higher than necessary and a few hundred less should be acceptable unless you plan on doing a PV. For example the specified hot fast idle speed for my 327/340 is 1750. and on a cold start in mild weather it stabilizes at about 1500 within a couple of seconds of firing up.

For L-71/72 I recommend setting the curb idle speed at 900, and this should pull 14" Hg manifold vacuum with the OE cam. The curb idle speed must be set high enough to keep the VAC "locked" at full advance. Otherwise, idle instability and stalling may result.

Additional technical information on ported versus full time vacuum advance is in my tuning seminar that you can find in the "restoration documents" thread near the bottom of the "sticky" section.

Duke

[Grahame M.]

Thank you gents, back in the garage on the weekend and will do it correctly.

Thank you one again for sharing your valuable knowledge.

Grahame

[Duke W.]

I post #6 I mistakenly talked about L-71. Since you have a L-68 that discussion would not apply to your L-68.

I looked at the AMA specs and since your '69 L-68 is an emission controlled engine the centrifugal curve is very short (start @ 900, 26 @ 3800 max), and with only 4 degrees initial recommended, total WOT advance is only 30, which is well short of the optimum 36-40, so you are giving up significant power and fuel economy.

As you know, the only difference between L-68 and L-36 is the induction system. I expect your L-68 has ported vacuum advance, and I still recommend changing it to full time vacuum advance. The OE VAC specs are start at 6", 12 @ 12" max, and since with the OE cam idle behavior is 14-15" @ 600, the VAC meets the Two-Inch Rule, so no need to change it assuming it meets spec. The ID of the OE VAC is MS 360 12.

I recently helped an owner troubleshoot an inconsistent idle speed issue on a '66 L-36, a non-emission controlled engine. The problem was a sticky 360 12 OE advance. Since an exact replacement is not available we installed a B-26, which has 4 more degrees advance at 12". We also installed lighter springs to bring the 30 centrifugal in by about 3500 rather than the OE 5000.

Test driving revealed some low speed/load driveability issues that I suspected might be caused by too much low speed/load advance with 6 initial. (Big blocks don't like as much low speed/load advance as small blocks with similar valve overlap characteristics.) The three solution paths were reduce initial, which would place total WOT advance below the optimum range, limit vacuum advance, or increase total centrifugal.

I preferred the latter and we found a '67 L-36/68 distributor cam assembly that offers 32 degrees total centrifugal, two more than OE. With 4 initial to provided 36 total WOT advance the low speed driveability issue went away. The engine was strong up to redline, and did not exhibit any detonation on CA 91 PON "premium", so the conclusion was the the spark advance map was about as optimized as possible with available parts.

Duke