1970 LT1 carburetor vacuum connections without TCS or smog pump

[Kenneth H.]

First let me say that I spent a couple of hours searching the forum but didn’t find the answer to my question. I also spent a lot of time trying to find the names of some of the parts of the carburetor so I could more fully explain what I’m asking but again came up short.

A friend is installing a correct Holley List 4555 carburetor that I purchased a few years ago on my LT1 (the old carb was a List 4555 carb but with a 1973 date code). I do not have either a smog pump or the TCS system installed.

We were almost finished but ran into a gas leak issue and had to stop so that I could get the replacement fuel lines to allow us to continue. In the meantime I’m trying to connect the vacuum hoses to the distributor vacuum advance canister but am unsure of the configuration when installing the carb without the TCS or smog pump. There are three vacuum ports on the carb, one under the forward metering block, one on the passenger side at the top center and one coming out of a canister attached to the carb on the passenger side forward.

I can’t look something like this up because all of the manuals show the initial configuration with TCS and smog pump installed. I'm not a mechanic myself, but I know that I can do this with a little help and information. Can anyone tell me the correct configuration for the vacuum hoses from the ports on the carb to the distributor canister?

Thanks.

[Jim T.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

Run a full length of hose from the distributor vacuum advance to whatever carburetor port you chose, check them all out if you want and chose the one you want to connect to that increases idle rpm connected to the vacuum advance. Increasing rpm's with connection is full time advance when the engine is at idle.

[Timothy B.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

First let me say that I spent a couple of hours searching the forum but didn’t find the answer to my question. I also spent a lot of time trying to find the names of some of the parts of the carburetor so I could more fully explain what I’m asking but again came up short.

A friend is installing a correct Holley List 4555 carburetor that I purchased a few years ago on my LT1 (the old carb was a List 4555 carb but with a 1973 date code). I do not have either a smog pump or the TCS system installed.

We were almost finished but ran into a gas leak issue and had to stop so that I could get the replacement fuel lines to allow us to continue. In the meantime I’m trying to connect the vacuum hoses to the distributor vacuum advance canister but am unsure of the configuration when installing the carb without the TCS or smog pump. There are three vacuum ports on the carb, one under the forward metering block, one on the passenger side at the top center and one coming out of a canister attached to the carb on the passenger side forward.

I can’t look something like this up because all of the manuals show the initial configuration with TCS and smog pump installed. I'm not a mechanic myself, but I know that I can do this with a little help and information. Can anyone tell me the correct configuration for the vacuum hoses from the ports on the carb to the distributor canister?

Thanks.

Kenneth,

The vacuum port under the forward metering block is a full manifold vacuum port and it's connected to the canister on the passenger side with a short vacuum hose. The passenger side top center is a ported vacuum source, just plug it with a rubber cap.

To connect your distributor vacuum advance, use the full manifold port under the forward metering block by installing a tee in the short hose.

[Leif A.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

Refer to the attached picture. Though this is a List 3810, the vacuum connections should be similar. First note where the line is attached on the canister attached to the carb. The vacuum port directly below should be used for your vacuum advance can on you distributor. The one to the rear is used for you PVC valve.

[Terry M.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

A friend is installing a correct Holley List 4555 carburetor that I purchased a few years ago on my LT1 (the old carb was a List 4555 carb but with a 1973 date code). I do not have either a smog pump or the TCS system installed.



Can anyone tell me the correct configuration for the vacuum hoses from the ports on the carb to the distributor canister?

Thanks.

Ken

Not to add confusion to your question, but a correctly configured 4555 should have a port (not vacuum) at the rear top of the air horn. This is a vent to bleed fresh filtered air into the vacuum system when the TCS solenoid operates. Without that solenoid you do not need that tube, but to keep unfiltered air out of the carburetor you should cap that tube.

[Leonard M.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

Ken, The 4555 uses the front r-side vacuum tube on the base. It originally had a short hose to a nylon fitting with 4 tubes.
1. short hose from carb base 2. smog diverter valve 3. TCS 4. choke pull off. So, if you have the fitting (look at the picture forward of front fuel line) plug diverter valve tube and run a long vacuum line direct to the vac adv instead of the TCS which is what I have.
If you don't have the correct fitting, get a tee and run one to choke pull off and other direct to vac adv. You can see my TCS has a plug on the vac, but I do have the hose to the top of the carb.

[Kenneth H.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

Thanks everyone for the help. I configured the vacuum from what I believe everyone is saying. Below is a picture of the setup. I hope it's right. If not, please let me know.

[Leonard M.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

You got it.

[Duke W.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

Converting LT-1s to full time vacuum advance MUST include replacing the OE 201 15 VAC with a B28 VAC (Airtex 4V1053). The OE VAC does not pass the Two-Inch Rule and will result in idle instability. Once done go through the pre-emission idle speed/mixture adjustment procedure and target an idle speed of about 900, which should yield about 12" manifold vacuum. You should also install lighter springs in the distributor to bring the full 26 deg. centrifugal advance in earlier than the OE 5000... preferably about 2500, and advance the initial timing to the range of 10-14 degrees for 36-40 degrees total WOT advance, as high in this range as it will tolerate without detonation. You want essentially the 365/375 HP 327 spark advance map. Details are in my 2012 San Diego national convention tuning seminar, which is an easy Web search.

[Kenneth H.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

Leonard,

I have all of the correct fittings. I also have a correct original smog pump as well as a TCS unit along with all of the correct hoses and wires. The only things missing are the diverter valve and the trees. Once I get those, I'll go ahead and install and test the entire unit.


Thanks everyone for all of the help.

[Kenneth H.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

Duke,

Your comments explain a lot about the problems I've had in the past concerning the idling of the engine. It wasn't much fun dealing with it. While I'm not a mechanic, and most of what you've said in your post is Greek to me, I'm very confident in the mechanic who's working on the car, and will give him the info that you supplied so that he can set it up properly.

Thanks.

[Leonard M.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

Ken, email me at toylman@aol.com for a little info and tips I can offer about TCS and smog. Lenny

[Duke W.]

Re: 1970 LT1 carburetor vacuum connections without TCS or smog pump

In my 55 years of playing with cars I don't think I've ever found a professional mechanic who understands the concept of a "spark advance map" or why a base engine needs a different map than a SHP engine. Even Chevrolet didn't understand it when they equipped the distributor in my '63 340 HP engine with a full time advance 15" 201 15 VAC, when it only pulled about 12" at idle. It took me a couple of years to figure out why the engine would stall after a couple of minutes idling and buy the weights, springs, and 8" VAC that were on the 365/375 HP engines. Read my tuning seminar and send it to your mechanic. If you lived in SoCal, I could help as I have many others who gave up after spending God knows how much money on "mechanics" trying to get the engine to run right. A SoCal chapter member will be up here on Memorial Day with his '67 427/390 TH400 Coupe and I'm confident that I can get the "tune" optimized in a couple of leisurely hours of gathering and analyzing the test data I will take. He pre-purchased (about 15 bucks) a tee and vacuum tubing to convert the ported to full time vacuum advance along with the spring kit. The only issue is that the work order for the engine rebuild from about five years ago says "reprofiled" camshaft, so if it was reground with more overlap it might need a different VAC. We'll cross that bridge when he gets here and if a new VAC is needed he'll have to spend about another ten bucks.