glroland

Hi all,

I'm having some vacuum troubles with my 72 LT1 and noticed the vacuum hose connected to the bottom of the distributor isn't connected to anything. From googling and repair manuals, it looks like it should be going to the carburetor or manifold but I'm not seeing anything to connect it to. Most pictures online show the hose going up into the carburetor but few pictures show where it goes without the air filter installed. The engine is original with the original Holley 4-barrell. I used a vacuum gauge to confirm there is no pressure.

Any one able to take a picture of their C3 LT1/Holly without the air filter so I can see where it should be going? Or any thoughts overall? I'm a rookie and would love to hear feedback!

Thanks!

694speed350

Hook it to manifold vacuum.

BLUE1972

I agree use manifold vacuum.

There should be a port on the bottom right of the carb - passenger side - that has vacuum when the car is idling.
Use that port. you can use any port that has vacuum when the car is idling on the carb.

Bikespace

Do you have a timing light? If the car was made to run with that hose disconnected, it may need to be re-timed now. It may even ping if the total advance is too great with the vacuum advance engaged. Look for Lars' timing papers on this site, or Google, for the "right" way to do this, which also explains the difference between ported and manifold vacuum.

Make sure you plug, or route properly, any open ports on the carb. I used a bunch of these when I was getting my two cars squared away. Also available at any auto parts store.

http://amzn.to/2lvZSch

Alan 71

Hi g,
Welcome!
First Post!
And a 72 LT-!. VERY nice!

Originally the rubber vacuum line leading to the distributor's advance can came from the solenoid that is part of the emissions system (TCS) that all 72 small block engines had.
The rubber line that comes off the front of the solenoid leads to a small 4 port vacuum fitting.

How you route the hose on your LT-! will depend on whether that solenoid is still in place on the intake manifold.... and if you have the TCS operating.

These photos are from a 72 base motor but the hardware, electrical connection, and hoses, would be very similar to what was originally on you 72 LT-! motor.

Happy New Year!
Regards,
Alan







The hose routing diagram from the Dr. Rebuild site. You can see the 4 port fitting.

toylman

Here's a '70 LT-1 with a similar Holley. The vacuum comes from the front right of the Holley. and (originally) was split through that off-white fitting and fed the smog diverter valve, TCS solenoid and the choke pull off.
I am guessing that your smog and TCS are gone or disabled.
In my picture the smog vacuum hose is blocked and the TCS is Not hooked up so the 3rd hose from the fitting goes direct to the distributor.
So, you want your carb/manifold vacuum to feed your distributor vacuum advance and your choke pull off with the other 2 feeds blocked for best results.
If you do not have them, you may want to get the assembly manual (AIM), NCRS Judging Guide and Richard Prince's book on the C3. The information will pay you back more than the cost of the books.
Good luck.

glroland

Wow! Thank you so much for all the replies! Let me take a closer look and get back to you today. Happy New Year!

7T1vette

Forget hooking the dist vacuum can line to the TCS system. That was a rudimentary system for emissions that allowed the engine to run retarded (and hot) at idle. Not what you want to do for the health of the engine.

Hook it directly to a manifold vacuum port: fitting in the intake manifold; line to the vacuum reservoir tank; manifold vacuum on the carb; "T" into to power brake vacuum line. Do not connect it to the PCV system or the fuel vapor canister 'purge' line going into the carb base.


Happy New Year!

ralphspears

Toylman, Is that your car in the picture ?

toylman

Hi Ralph, It was, but went to N CA recently. I think I have one more "project" in me, but nothing major.

F4Gary

My vacuum advance hose comes off the fitting on the front of the carb base. I have my 72 LT-1 timed to 16* btdc.

beekppr

I agree 100% here. Unless you need a certain stock appearance, you should find a vacuum source directly off the intake manifold rather than the carb. Put it a t connector if necessary.

For years, I used no vacuum advance on my engines and while they ran good and strong at full throttle, I got terrible mileage and the overall driveability was merely okay. I hooked up vacuum advance to my 64 and my mileage went up by 4. Driveabilty got so much better that it is a real joy to go cruising again.

Do not worry about pinging. It won't. The reason is when you put the engine under heavy loads, manifold vacuum drops so the distributor won't be getting much vacuum advance if any at all. Once your back at light loads (cruising down the freeway or idle), the vacuum is very high and gives the distributor more advance. It may seem counter intuitive but the engine actually wants this badly. Why? Because under light loads, the air fuel mixture burns much slower and the spark needs to start sooner so that peak power is produced at the proper moment of the power stroke. Timing needs today are different from factory setups because of the poor quality of today's fuels.

Ideally, you should set initial advance where the engine wants it to be. It could be 12-15 degrees for mild cams and as much as 22 degrees for big cams with lots of valve overlap. Then, the weights and springs inside the distributor should be curved to achieve around 34-36 degrees total. This is all without the vacuum advance connected. Then when everything is done, connect the vacuum advance. The rpm's at idle will go up significantly. Lower the idle speed on the carb until it is where you want it.

I've read this advice from great tuners for years and years and ignored it because I thought I had it all figured out. I wrongly believed vacuum advance was not for high performance engines. I would play with carb accelerator pump cams and nozzles to make it run nice when all along, I was simply covering up poor distributor setup. Recently, I gave in and tried it and was very happy with the results. I've helped a couple friends recalibrate things this way with great results. You'll be happy too.

glroland

Thank you all for the great feedback! It looks like a previous owner removed the smog pump and in the process, chose not to connect the distributor back to intake manifold. I basically added a tee to bring it back into the system and then re-tuned the idle mixture on the carburetor. System is running well at idle right now. I bought a timing light but need to do some reading before adjusting that part. I'll reply back with an update when I get to that point. Appreciate it!!!

F4Gary

Which vacuum port did you use for your distributor?
I was using the front port on the carb base with a tee for the choke pull off.

glroland

That’s the one I used as well. Idle was noticeably better with than without.

beekppr

Hopefully you got the timing light with the dial at the back. Those make the process SOOO much easier. If you don't know this already, you simply read the mark at the harmonic and twist the dial at the back until it shows zero. Then you read the dial at the back and it tells you exactly what you have. Where it gets nice is when you are trying to determine how much total advance you have at say 2500-3000 rpms (always with the vacuum advance disconnected and blocked off). A helper sits inside and brings it to that engine speed and you take a quick reading with your timing light. With the dial, it only takes a second. Also, I like to use "white out" to mark the line on the harmonic. It stands out super bright under the timing light for quick positive measurements.