Hypothetical Question - 1967 L71 improper VAC plumbing

[Richard M.]

A friend's L71 was not running right after someone inadvertently made a mistake reconnecting vacuum hoses to the 3 carburetors.

When I restored the car I converted from Ported to Full-Time manifold vacuum for the B26 VAC I installed. I used a Tee in the Choke-Pulloff line for the Manifold vacuum. The car was running fine for a long time.

Recently the car was worked on by someone else. In the process, they mistakenly put the hoses back on but Tee'd the VAC into the Secondary Carburetor Diaphragm lines. Before I discovered this, the car was not running good at high RPM, and under load it would stumble and on one occasion going up a hill at high RPM upshifts it died. Would restart okay and run fair at cruise.

When I discovered the improper VAC plumbing, I suspected this was the problem. After correcting the plumbing to Tee the VAC back to the Choke Pulloff circuit, I drove the car for 30 miles under all conditions and it was perfect. BTW ambient temps were the same in most driving situations, in the 70's.

I'm sure now the the faults were caused by the error in the plumbing. Static Timing was still a 8* from my initial settings. Stock TI distributor advance springs.

I am trying to understand the theoretical reason of the poor engine operation. Can some one explain what would be expected with the VAC plumbed into the Secondary Diaphragm circuit?

Just trying to understand and learn. Thanks.

Rich

[Duke W.]

Re: Hypothetical Question - 1967 L71 improper VAC plumbing

I've been helping a local chapter member prepare his very low mileage original, unrestored L-71 for a PV at the Oregon Regional next week. During the WOT test I felt the engine wasn't pulling as strong as it should, and the first thing that came to mind was the end carbs weren't opening.

Unfortunately, the owner doesn't have a '67 CSM, just a '66 CSM. (I've strongly suggested that he needs a '67 CSM.) The '67 CSM should have the operational data along with any adjustments to check, but even without the '67 CSM it's clear that a vacuum line off the center carb is teed into vacuum hoses that run to the opening diaphragms, and it's my understanding that venturi vacuum from the center carb is the vacuum signal that opens the end carbs.

So I pulledl the vacuum hose off the center carb, attached my Mighty Vac thinking I could open the end carbs, but it wouldn't pump down, and without the '67 CSM I didn't want to go further down that path.

Moving to the driver's side I observed a linkage connecting the end carbs to the center carb, which appeared to be a way to positively close the end carbs when the driver lifts off the throttle. I tried to rotate the front carb throttle shaft, but it wouldn't move until I applied more torque and heard a SNAP, and it moved. Apparently the front carb butterflies were jammed in the throttle bore. On the right side of the end carbs there is a tang that appears to be a throttle stop, but it wasn't resting on anything, and there was nothing nearby to rest on... again, need the '67 CSM.

Subsequent drive showed a strong surge at about 3500 and lots of induction noise that wasn't present when the end carbs weren't opening. It pulled much stronger, but I felt there was more. The OE timing spec is 5 and we measured 3.5-4, but decided to leave it alone.

If he passes the PV it's off to Vegas next summer for a National Top Flight and a Duntov award. After that we'll convert to full time vacuum advance with a B26 VAC, bump the initial to 8-9, and then see if we can get the centrifugal in a little quicker without detonation.

BTW, the engine was suffering from idle stability problems on California's wonderful RFG II 91 PON pump gas. I figured it was percolation. Engine temp ran right about 180 (ambient was low to mid seventies) and the carb bowls were 140. Draining out the pump gas and filling it up with 100 LL avagas slayed that dragon.

If the guy who worked on this car teed the VAC into the center carb vacuum hose that connects to the end carb opening diaphragms then the only time you were getting vacuum advance would be at WOT, high revs, which would likely cause severe detonation. This guy must have no idea how the VAC and end carb opening works. Even without a CSM it should be pretty clear, and once my friend gets a '67 CSM and I have a chance to read it and make any necessary adjustments, I should be a trips "expert".

Duke

[Tim G.]

Re: Hypothetical Question - 1967 L71 improper VAC plumbing

Rich, I can't answer your questions, but whenever I set up a tri-power carbs I always check the operation of the end carbs opening with the engine off by opening the throttle full and inhaling on the vacuum line off of the center carb. The Mighty Vac work well, also. The end carbs should open without interference or binding.

[Richard M.]

Re: Hypothetical Question - 1967 L71 improper VAC plumbing

Tim, Thanks. Yes the Secondaries work fine and that's exactly what I do to check the diaphragms. If I don't have a Mity Vac handy I manually push the diaphragm lever(while holding main lever WOT) and hold my finger in the hose or nipple and if it keeps the lever tight I know it's good.

Duke, Interesting issue. I'm thinking you may have closed the Front Secondary throttle plates hard into the bore, and when you realized you had to open the Center Throttle lev er to allow activation, it then released with that "snap". In my experiences with the Secondaries they can in fact stick in the bores when forced. Normal operation keeps them slightly open and active by design to allow fuel to flow at idle and keep fresh in the bowls. Covering the front or rear air horns at idle will stall the engine as proof that their slightly open.

Yes Venturi Vacuum definitely is the source of vacuum for the secondaries. And you just reminded me of another reason why my failure mode, explained in my first post, likely occurred. No VAC operation at all until the Secondaries were told to open.

I don't have my '67 CSM at reach, but I had a document stored, see PDF file. See Explanation regarding Figure 8A for linkage adjustment. It may help. Also some pics of the linkage below. Most important aspect being to ensure there is no pressure on Secondary throttle levers/linkage at Pedal WOT.

Rich

PS Also, I understand maintaining Ported VAC for upcoming PV etc, but just keep a eye on that old MS201-15 VAC as they're unreliable. I've tested several originals and even the high priced reproductions. All are junk. Most don't start moving until about 16" of vacuum. Since most L71's stock engine setups run at about 14" at idle, not much happens to those VACs.

[Chris H.]

Re: 66 Parking Brake Warning Light

In the throat of the secondary there is a signal orifice I would presume for supplemental / maximum vacuum signal for the diaphragm. I check diaphragm operation by holding my pinky over the orifice and using the mity vac on the rubber vac line.

[Richard M.]

Re: 66 Parking Brake Warning Light

In the throat of the secondary there is a signal orifice I would presume for supplemental / maximum vacuum signal for the diaphragm. I check diaphragm operation by holding my pinky over the orifice and using the mity vac on the rubber vac line.

Chris, Yes I recall the first time I tested one on the bench I got no movement, then by putting tape over that hole in the air horn I got it to hold.

Rich

[Duke W.]

Re: Hypothetical Question - 1967 L71 improper VAC plumbing

That's what I missed. Apparently the end carb vacuum opening system is not "closed", but has air bleeds to the end carb venturis.

Thank for posting the pdf. I expect to see the owner at a C&C event up in Palos Verdes this morning and will make sure he looks at this thread.

Duke