Wanting to switch my 1967 L71 to manifold vacuum
I have read several posts about Duke Williams recommending switching from ported vacuum to manifold vacuum to reduce temperature at low speeds or idle. I confess that I am not a mechanic so I was hoping someone could post pictures showing how to switch my L71 to manifold vacuum. I probably won't want to modify the center carburetor as I have seen some do, so I am looking for the simplest way to accomplish this conversion. I realize that I need to swap out the vacuum canister on the distributor and would like to know how to do this without removing the distributor itself if possible. Thanks to all who contribute their knowledge and expertise to this forum! Chris Covinsky
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Re: Wanting to switch my 1967 L71 to manifold vacuum
More than one way to pick up the full-time vacuum source, I don't own a '67 but I seem to remember the aluminum intakes have a plug in the intake runner area toward the rear of the carb to pick up vacuum for power brakes. If you don't have PB, you can remove the plug and install a suitable fitting to give vacuum to the carb. For a car with PB, just add a "T" into the lne.Bill Clupper #618 -
Re: Wanting to switch my 1967 L71 to manifold vacuum
There are also variations of the power brake vacuum fitting for the intake manifold that have a second nipple for a distributor vacuum line. Paragon probably has them or maybe even GM.Comment
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Re: Wanting to switch my 1967 L71 to manifold vacuum
You don't have to remove the distributor to switch the VAC, pretty easy job. Use a B-28 if you can, or a B-26 if you can't find a B-28. And as for the full manifold vacuum, I put a "T" in the line coming from under the center carb, plugged the ported nipple at the center carb, then plugged the VAC line into the "T", also a pretty simple job.
Note: Beware of the judges, they don't like this vacuum rig, and when I told them "Duke recommended it for the L71", it just didn't matter to them…...
Big Tanks In the High Mountains of New MexicoComment
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Re: Wanting to switch my 1967 L71 to manifold vacuum
You could change the manifold fitting at the rear of the manifold to add another port, like this....
P2090054.jpg
Or my choice, and as Michael's suggestion, is to cut the line going to the choke diaphragm pull-off, insert a "Tee" in that line, and bring the vacuum advance hose connection to there, then cap off the ported nipple. This is exactly what I did on one. Here's a photo with the areas shown. Note, do not confuse this line with the 2 lines(black hoses with white stripes) used for the secondary vacuum actuation at the Front/Rear carburetors.
P1180030vacmod.jpg
In my case and most, a B26 works perfectly, which follows Duke's 2" rule well. But..........This was a manual transmission. You haven't specified if you have a manual or automatic. It will make a difference. (edit.....thinking about this statement a bit more, I'm not sure if the Powerglide was a available option with the L71 or not. I am leaning.....NOT)
To be sure you have the right choice select the right VA scientifically. Attach a vacuum gauge to the existing manifold vac port at the rear of the manifold. Take a reading with a normalized engine at idle, which should be around 900 RPM IIRC. Record that reading. If it's a manual trans, you may get around 14". Choose the Vacuum advance based on the 2" "Rule". It may very well be a B26, as in my case and mos L71's, but I would do this check to be sure. A comprehensive list of VA's here.....
Vacuum_Advance_Specs.pdf
FYI, if this becomes your choice, the B26 is available at Napa as Part# VC1765
As far as changing the actual VA can itself, this is what I do, mostly on non-TI distributors, but works on a TI unit too......
Use a Mity-Vac tool(rent, borrow, etc). Open up the distributor to gain access to the 2 VA attach screws. Use a magnetic tip screwdriver if you can. Remove the hose from the existing VA. Attach the Mity-Vac hose to the VA and pump it up to pull the advance lever to move the rotating magnet base plate. This will allow you to gain access to the rear screw much easier(and not drop it down into the dizzy
). Then remove the front screw. Reverse the operation using the Mity-Vac on the replacement VA to hold the lever to help get it lined up through the hole in the magnet base plate hole and attach the screws.
PC230001mod.jpgLast edited by Richard M.; July 5, 2014, 03:25 AM.Comment
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Re: Wanting to switch my 1967 L71 to manifold vacuum
The simplest way to tap into full time manifold vacuum is to tee into the choke vacuum break hose and route the base of the tee to the distributor. Buy a 1/8" two feet of 1/8" vacuum tubing from a parts store. If you have an original choke vacuum break hose, remove it and use the new hose to make the new harness.
Use the tee cross bars between the conections for the choke vacuum break, and use the base of the tee for the hose to the VAC.
The functionally correct VAC for all big blocks with full time vacuum advance is is B26. B28 is ONLY for small blocks with any OE mechanical lifter camshaft or any aftermarket camshaft that makes less than 12-14" manifold vacuum at normal idle speed, assuming you have a manual transmission. If THM use the Two-Inch-Rule.
As stated, the VAC can be removed with the distributor installed on the engine, but I prefer removing them because most need a disassembly, cleaning, lubrication, and end play shimming. both point type and TI.
SHP big blocks should make about 14" at 900 RPM idling in neutral with a manual trans. Since many of you may not know if bubba has installed a high overlap aftermarket cam that has less than OE cam manifold vacuum, you should check manifold vacuum before buying the new VAC and make sure it's in the 14" ballpark at 900. Full time vacuum advance will usually increase manifold vacuum at the same idle speed.
Once you have implemented the change to full time manifold vacuum advance you MUST go throught the idle speed/mixture adjustment procedure in the service manual, and for manual trans SHP big blocks I recommend 900 idle speed in neutral.
DukeComment
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Re: Wanting to switch my 1967 L71 to manifold vacuum
I am about to set up my son's 69 L-89 for which I rebuilt the carbs (already installed carbs, waiting for him to install the battery). I tried to talk him into a switch to manifold vacuum, but he wants to retain its Top Flight status. In the process of installing the carbs, I discovered another source of manifold vacuum (of sorts) - the bolts used on the carb mounts at cylinders no. 3 and no. 5 go into thru holes into the runners (bolts are used at those positions due to the close proximity to the individual steel fuel lines). If it were not for the fuel lines, one could use a hollow stud at either location - a stretch I know. With outa doubt, the choke pull off is the best choice. I used it for a temporary "T" connection for my vacuum gauge.
Stu FoxComment
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Re: Wanting to switch my 1967 L71 to manifold vacuum
You missed an opportunity to easily convert to full time vacuum advance in a way that can't be visually detected.
It requires drilling one small hole to move the vacuum advance signal port to below the butterfly valve and plug the passage that leads to the port above the throttle valve.
It's pretty easy to see how it can be done with a good visual inspection of the passage layout when the center carb is disassembled.
DukeComment
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Re: Wanting to switch my 1967 L71 to manifold vacuum
Thanks for reminding me Duke, I had to use the B-28 with my L71 due to the aftermarket cam to get the two inch rule, I forgot the B-26 was correct for stock.Big Tanks In the High Mountains of New MexicoComment
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Re: Wanting to switch my 1967 L71 to manifold vacuum
Duke;
That's about the same way I modified the Quadrajet back in 72 on my Nova SS - all internally. We were more afraid of the emission police back then. Living in the Chicago area, we were subject to emission tests and inspections. They would look for any sign of modifications, so I did my changes sight un seen. There was a solenoid in the line as well to lock out the vacuum advance until a number of seconds past in high gear (or some thing like that). I used a manual switch to activate the solenoid full time, and switched off once a year for the test.
I could have done something similar to the center Holley on the 69 L -89, but I was more concerned with getting them to work as designed w/o any leaks. The kits I used were generic for all three carbs, so it is nice to have the spare power valves and gaskets. My son wants to prime the engine for startup, but I'm afraid of having a back fire through the center carb. I don't know the condition of the ignition system other than it was running ok before he noticed the center carb leaking. I suspect it has points so I will at least check the dwell. This is just one of the cars he has in inventory, and I believe he has some buyers lined up for it.
Stu FoxComment
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Re: Wanting to switch my 1967 L71 to manifold vacuum
Richard Mozzetta,
Could you please take another picture of what all your vacuum hose routing, tee and connections look like after everything is connected as it should be for manifold vacuum. Thanks for your two prior pictures, but I am a little confused if I need a manifold fitting as in your first picture and a tee installed as the second picture mentions. Also, does the timing need to be adjusted? As the car sits, it starts, runs, idles perfectly, it just tends to warm up at low speeds and idle. Thanks again for all your help and time spent posting those pictures! Chris CovinskyComment
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Re: Wanting to switch my 1967 L71 to manifold vacuum
Hi Chris,
I looked and looked for that photo of it all done but couldn't find it last night. I recall taking a pic there and posting here but I can't find it.
The only one I found was that one before completion.
So........ you only need one of the 2 methods I show on that photo. Either get another manifold fitting with the small extra port to the VA, or Tee off that line coming off of the choke to the VA.
If you don't want to cut that hose off the choke diaphragm can, just pull it off the fitting and add another short hose to it and the Tee. Then add the new hose over to the VA can.
The other way Duke was talking about.........was modifying the center carb. It can be done easily but when I restored that L71 in the pics, the owner opted not to drill the carb as he wasn't going to get it judged..........in Norway!
So I can't take more pics of it.
The instructions and pics came from Timothy B. Here.....
in Post#11 and Post #13
RichComment
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Re: Wanting to switch my 1967 L71 to manifold vacuum
I knew they were somewhere....Post #27 in the above referenced thread, and again here for you.
Rich
P4020017.jpg P4020018.jpgComment
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Re: Wanting to switch my 1967 L71 to manifold vacuum
Thank you Richard Mozzetta for you pictures as they are very helpful! Thanks to all of you who contribute your knowledge to this website to help keep these works of art on the road! Once I complete the conversion to manifold vacuum, I will report back with my results. Thanks again, Chris CovinskyComment
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