'71 Vacuum Advance/TCS Routing

[Charlie P.]

I would like to correctly connect and route the vacuum advance and TCS circuit on my CJK '71. Can anyone describe the proper routing? Also, can anyone provide a pic and dimensions on the steel line that's used? THANKS AS USUAL.

[Jim K.]

Re: '71 Vacuum Advance/TCS Routing

Dr. Rebuild has a nice diagram that shows the location and routing. I think he also has most of the parts. GO TO: http://docrebuild.com/dr-r-web/SPARK1.pdf

[Patrick H.]

Re: '71 Vacuum Advance/TCS Routing

Jim,

It's a nice drawing, but he does not have the parts. I've asked him. You can purchase the big block line and bend it to fit.

I had heard an outfit out of Ohio wanted to reproduce the line. I even talked to them about it back in 1996. It's not yet out, so it hasn't gotten far.

Also realize that few people are specialists in the 70-72 base motor cars. use the big block tube, bend it to match Dr. Rebuild's diagram, and I'll bet it will fly by most judges.

Patrick

[Chuck S.]

Re: '71 Vacuum Advance/TCS Routing

Patrick, Jim, Charlie,

Patrick's idea of using one of the Drs repro tubes bent to correct dimensions is probably the only way to get this part now. Patrick, maybe you can confirm if the 71 tube is the same as the 70; the 70 base/L46 tube was discussed in detail awhile back, and dimensions for the tube were posted in that thread. See the archive link below.




70 Base/L46 Vacuum Advance Tube

[Charlie P.]

Re: '71 Vacuum Advance/TCS Routing

I checked out the diagrams at Dr. Rebuild. Although it apparently isn't correct, he shows all '71 engines, including base, with the same tube. I think I will try the trick as suggested by Patrick, but I still hope to get a glimpse of a real one to copy.

Also, check out the TIM/JG on vacuum lines, pgs. 69 and 70. Read the first sentence under "Vacuum Advance Lines 1970 and 1972-Small Blocks", and "Vacuum Advance LInes 1971-Small Blocks". More confusion.

In the former section, it says steel line from carb to TCS on '71s (although paragraph is headed "1970 and 1972"). In the latter, it says a rubber line is used there.

Thanks to all for the help.

[Chuck S.]

Re: '71 Vacuum Advance/TCS Routing

Charlie, buy yourself a copy of "Fact Book of the 1968-1972 Stingray" by M. F. Dobbins. It's available here at the NCRS website store. It will cost you a few bucks, but it's darn well worth it for questions like this.

My fourth edition shows an excellent view of the small block vacuum advance tube on page 174. The 71 tube is nothing like the 70; you'll have to see it to see what I mean.

I am a little confused as to why the 70-72 TIM&JG indicates that 71 has a TCS solenoid...as far as I can tell from Dobbins' multiple pictures of different engine options, there is no TCS solenoid on the 71s...a metal tube extends from the front of the carb around to the rear of the carb parallel with the valve cover (clip shown on intake manifold bolt), then changes to rubber tube that runs directly to advance cannister on the distributor.

[Charlie P.]

Re: '71 Vacuum Advance/TCS Routing

Chuck- I am looking at the bottom picture right now. The vacuum line that runs over the fuel line goes to the TCS/CEC solenoid-at least according to the TIM/JG that's what that thing is. According to Dobbins, it's a solenoid to kick up the idle when the A/C comes on. The center picture must be of a different engine because the line routing is different.

I don't think the bottom picture is correct in any case because there should must be two lines to the solenoid, I think.

I'm getting more confused by the minute.

[Patrick H.]

Re: '71 Vacuum Advance/TCS Routing

Charlie,

All of the 71 base motor pics in Dobbins' book are incorrect with regard to the vacuum hose for the system you're looking at.

70 and 72 were rubber, 71 had the metal line wrapping around the front. It extends from the right side of the carb, actually next to the valve cover, over the front of the intake and over to the solenoid at the front of the carb.
Look at the page 175 of Dobbins' book. The correct metal hose is there, but routed wrong. Instead of leading to the carb, it should lead to the solenoid. Look at page 184 and 185 of the big block, and you'll see how it SHOULD be routed. In addition, I believe the small block metal hose should be almost long enough to reach the solenoid, not as short as it actually appears on 175.

I hope that helps.

Patrick

[Chuck S.]

Re: '71 Vacuum Advance/TCS Routing

Charlie, perhaps I oversold Dobbins...the one picture I cite appears to have been "Bubberized".

Patrick says the routing on the big block is correct. If you go back to Jim's posted link of Dr. Rebuild's catalog illustration (center bottom picture), you can understand the layout better.

There ARE two lines into the idle solenoid...one from the front of the carb to the solenoid (about 4-6" long in Dobbins' pictures), and a SHORT rubber connector hose (1-1 1/2") from the solenoid to the steel line that turns parallel to the valve cover and connects to the advance cannister.

One confusing feature is that the catalog illustration seems to show a clip or something in the middle of the 4-6" vacuum supply line that makes it look like two shorter hoses. From Dobbin's photographs, it is clear that the line from the carb to the solenoid is a single unformed rubber hose that simply "loops" to the solenoid.

[Jack H.]

'70=TCS; '71=CEC

And CEC stands for Combined Emissions Control. The system was 1-year only on Corvette and a theory of operations thumbnail is given in your 1971 Chassis Service Manual. The CEC solenoid does TWO things: (1) it toggles the carb's curb idle RPM, and (2) it controls vac advance to the distributor.

It has NOTHING to do with boosting RPM for A/C equipped cars. The only thing that was 'special' about A/C cars in '71 was the addition of two control relays under the RH dash to momentarily FORCE the compressor clutch ON during engine shut-down. Running the AC compressor on engine shut-down added drag to thwart the propensity for engine run-on due to a modestly higher curb idle RPM setting (NOT documented on the car's tune-up sticker nor in the service literature) to compensate for the extra load presented by the A/C compressor during normal engine idle.