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Where should I be pulling vacuum... this is probably a silly question for most so forgive me... I'm learning.
I am under the impression my 78 L82 uses manifold vacuum for distributor advance however my vacuum line from advance canister is attached to this air cleaner port which then goes into this sensor/switch... there is no connection to manifold or carb for vacuum.
I am reassembling everything after everything was removed but this is how it was connected before i started and im thinking its not right, or is it... shouldnt it be connected to a manifold vacuum and where would that leave this connection at the air cleaner.
Last edited by Golfobsessed; Aug 19, 2024 at 07:03 PM.
With these cars, there is often a difference between the way GM set them up, and what we currently understand to be the best practice.
If you are following @lars' tuning tips, which call for manifold vacuum, there is a port on the front of the stock Q-Jet that provides this. It should be connected via a single hose to your distributor vacuum advance, with no T, and no valve in the way.
Don't trust an online diagram (unless Lars sends it to you, or you find it in Cliff's book). Check with your finger which port is manifold vacuum. I messed up once, and my car barely ran.
Thanks to you guys, I've been reading Lars archive responses on here for the past few days... I feel like I owe him something... I've spent the past few days getting the manifold stripped and painted correct, rebuilt the carb complete and re-installed everything today, I marked everything with paint pans as well as tagged and photos, I'm certain this is how it was before I started... this is this morning and this afternoon pics and everything is connected correct. I actually have one port on the rear of the baseplate capped off so should be my manifold vacuum source... you can see my last tagged vacuum that went to that air cleaner.
here is the dumb question... what do I do with the air cleaner empty port now... it goes through that vacuum sensor... do I just leave it open ?
I am going to do a total timing once I can start it tomorrow.
I'm on the home stretch
Last edited by Golfobsessed; Aug 19, 2024 at 08:06 PM.
It's looking good with the restored intake manifold!
Be sure to email @lars for his latest white papers.
There are a few threads on what the dual-snorkel CAI air cleaner assembly was intended to do.
In my cars, I bypassed all of it, knocked the flappers out of the snorkels, blocked the holes with aluminized tape, removed the vacuum switches, and covered any unneeded hose nipples with rubber caps. For your orignial 78 Pace Car, you may want to read up on it. It mostly matters if you drive in the winter, but it is another nifty original system that you can keep functioning.
But run manifold vacuum to the vac advance can. No one will notice under the air cleaner.
It's looking good with the restored intake manifold!
Be sure to email @lars for his latest white papers.
There are a few threads on what the dual-snorkel CAI air cleaner assembly was intended to do.
In my cars, I bypassed all of it, knocked the flappers out of the snorkels, blocked the holes with aluminized tape, removed the vacuum switches, and covered any unneeded hose nipples with rubber caps. For your orignial 78 Pace Car, you may want to read up on it. It mostly matters if you drive in the winter, but it is another nifty original system that you can keep functioning.
But run manifold vacuum to the vac advance can. No one will notice under the air cleaner.
thanks bikespace, appreciate it and i will, im in Arizona and will do winter driving but not too cold here, i kept the crossover open but used a restrictor, I want to keep a stock appearance but not concerned about vacuum routing appearance under the air cleaner, I will get a game plan before start up tomorrow, as I said I do have an open vacuum source but need to know how to handle the flappers in the air cleaner assm, I'm reading now.
It would help you to read up on the Early Fuel Evaporator EFE system. It helps a stock engine warm up quickly. You have 2 thermal vacuum switched systems on your engine. One for the EGR Exhaust Gas Recirculation system and the previously mentioned EFE system.
Are you wanting to eliminate both of those or maybe just one of them? Are you wanting to stay stock?
It would help you to read up on the Early Fuel Evaporator EFE system. It helps a stock engine warm up quickly. You have 2 thermal vacuum switched systems on your engine. One for the EGR Exhaust Gas Recirculation system and the previously mentioned EFE system.
Are you wanting to eliminate both of those or maybe just one of them? Are you wanting to stay stock?
thanks for responding, i have been reading up on it all... most important is stock appearance under the hood.
after reading some things I am convinced something is not correct, my vacuum advance was only connected to the air cleaner flapper lines (one through the temp sensor in the air cleaner) and nothing else, I am assuming it needs to get vacuum from carb/manifold port or temp/vacuum switch on the t-stat housing but it does not... so first delema is vacuum advance getting vacuum then what's activating the flappers and perhaps these are both the same vacuum source.
I can just remove flappers (I live in Arizona) and grab vacuum advance from carb base but would prefer correct configuration
I am researching as best I can the correct routing.
You should have this sticker under your hood. Might help.
I do not... thank you, I have ordered a service manual, that routing tees off to the TVS on the t-stat housing which mine does not do.
again thanks, that helps
Would there be any benefit in connecting my vacuum advance directly to the rear base plate manifold vacuum port and the air cleaner flappers directly to the TVS on the t-stat housing?
I took off all the EGR, AIR and EFE stuff off and put it in a box. This is a manifold vacuum port directly to the vacuum advance canister. That might be all you want? Canister needs to be selected per Duke Williams white paper.
First, you should decide if you want to keep the engine in "stock" configuration (or not). Much of the 'jplumbing' on the engin e is strictly for EPA and emissions control purposes. If your state REQUIRES you to have this in operational configuration, the decision may already be made for you. If not, and you don't care about the strict originality stuff, keep the functions that you want and discard the rest. Some of the 'quick warm-up' functions, the fuel vapor recovery system, and the PCV system are beneficial (IMO).
In any event, it is best to plumb the distributor advance can directly to manifold vacuum. Connecting it to 'timed'/'ported' vacuum only causes the engine to run hotter at idle and lose efficiency for emissions benefits. The engine MUCH prefers manifold vacuum for the advance can.
I took off all the EGR, AIR and EFE stuff off and put it in a box. This is a manifold vacuum port directly to the vacuum advance canister. That might be all you want? Canister needs to be selected per Duke Williams white paper.
First, you should decide if you want to keep the engine in "stock" configuration (or not). Much of the 'jplumbing' on the engin e is strictly for EPA and emissions control purposes. If your state REQUIRES you to have this in operational configuration, the decision may already be made for you. If not, and you don't care about the strict originality stuff, keep the functions that you want and discard the rest. Some of the 'quick warm-up' functions, the fuel vapor recovery system, and the PCV system are beneficial (IMO).
In any event, it is best to plumb the distributor advance can directly to manifold vacuum. Connecting it to 'timed'/'ported' vacuum only causes the engine to run hotter at idle and lose efficiency for emissions benefits. The engine MUCH prefers manifold vacuum for the advance can.
perfect and after all the reading it was how I planned to move forward, i read huge debates about the manifold vacuum vs ported vacuum for advance and frankly hard to believe how much people are split on this subject all over every forum.
I want to keep the stock equipment and appearance for sure but I can run my advance direct to manifold vacuum and my air cleaner flappers direct to the TVS.
Thanks again gentlemen, got it all back together... ran vacuum like stated above and it fired right up and runs excellent, gonna pick up a dial adjust timing light in the morning and do a total timing but she runs great and no leaks 👍
You guys really need to do your own research on the origins of vacuum advance, why was used, and how it was implemented. This tread is full of misinformation.
Vacuum advance question
