Just curious. Everyone seems to like manifold.

 

I just got my HEI distributor for 74 C-3 from Summit and the instructions say to USE ported vacum...

 

Did they give an explanation?

I really don't want to open up an old debate but just curious.

 

Here's what the instruction sheet says:

11. Connect the vacuum hose to the vacuum advance canister. The vacuum line for the vacuum advance should be connected to a ported/timed vacuum source. To properly identify this port, you will need to have a vacuum gage in hand and test and small (3/16") vacuum ports on the carb. The port that does hot have vacuum at idle is the ported/timed source and is the correct port for the advance of the distributor to be connected to.

 

Ported vauum was used to reduce emissions. That's it. No other reason. Generally speaking, hooking vacuum advance to a manifold source will help the engine run smoother and cooler at idle with less throttle opening needed.

Here is some more info:

 

Good grief, how often is this topic going to keep popping up?


What's the point of having a Timing Sticky if it appears half the members won't read it?

 

900 RPM at idle seems kind of high.

 

Paul I think the key word is "work" , does it work ? The benefits of manifold vacuum are usually substantial, but some combination's just don't work with manifold vacuum. My first choice is manifold but in the end if ported works best I use that, its only an "idle" thing , once off idle everything is the same.

 

Here is an article posted by an ex GM engineer that explains the vacuum advance system. It is a great read

http://www.camaros.net/forums/showth...ght=timing+101

 

At the suggestion of Forum members, I changed my vac advanced from ported to manifold and it solved my low RPM overheating issue.

Gary

 

No one let you in on it Paul ?

 

Sure it 'works' and was standard configuration on many cars even in the late 60's, but much like smashing your thumb with a hammer, it's just a dumb thing to do.

 

OK, there is more than meets the eye here and I have negligent in not providing more details.

In recent days I have been fighting with the distributor. After I finally sorted out the wire set fiasco, and the car was running, I set the dwell and timing. No problem. I did notice that when I connected/disconnected the vacuum advance there was no change in idle RPM as there should be. Odd but the idle was fine at ~750 in P and 550 in D. And the car actually seemed to run well. The vacuum advance was hooked up to manifold vacuum as shown in the pic (near choke assembly).

I got to thinking about that lack of change in idle so yesterday I checked vacuum at the spout in the pic. There is none! It is plugged. So basically I was driving lately with no vacuum advance. I hooked up ported vacuum and again the car runs well (idle is as above unchanged as with ported there is no advance at idle and given the blocked spout there was no vacuum before).

I hit the spout orifice with carb cleaner in case some debris was doing the evil deed. That did not work. So I am at a loss to understand the lack of vacuum?? I thought of running a fine wire through the hole to manually dislodge the blockage and may try that today but I don't know where the far end of that passageway is to see any success. My other thought is perhaps an incorrect bowl gasket was used but again I don't even know if a gasket is associated with the spout internals.

Any ideas on this would be appreciated. If need be I can use ported vacuum (as per the original question) and ignore the defective manifold vacuum.

 

Paul,

I don't know the purpose of the port you're hooked the rubber line to. It may indeed be ported/timed vacuum or a plugged passage but you'll never know by testing it at idle. Hook up a vacuum gauge and see what happens when you raise the engine speed above idle.

 

No vacuum at any RPM. My spare Q-Jet in the basement shows a free flow at that spout as a comparison. The car does run well with ported (driver's side upper) and that spout does pull a lot of vacuum above idle. That's by finger test as my gauge is "somewhere" in the basement.

 

My guess would be the gasket between the base and the bowl is wrong , don't you have more ports in the front of the carb ?

 

It probably is the gasket. There are so many configurations of the Q-Jet.

don't you have more ports in the front of the carb ?

Too many.... Counter-clockwise from left top.

spare
choke pull-off
EGR
PCV
fuel evaporative canister
ported vacuum advance

I have pretty much decided to leave things as is with ported advance. The car idles and runs well (much better now with vacuum advance). Overheating at idle does not seem to be a problem (sits at a nice 180*F). I need to drive and fiddle less. It is August already and I have put on only 320 miles this year.

 

I had the opportunity to drive today for about 60 miles, both in-city and at highway speeds. Ambient temp was about 80*F. The car ran very well with ported vacuum. No idle over-heating (180*F). Having vacuum advance function with a B26 can does make a difference. I did not have that as described above.

Ideally I should pull that Q-Jet apart and find out why that manifold vacuum spout is non-functional (I suspect a wrong bowl gasket). But realistically the car is a pleasure to drive and I know darn well what happens when I open up a Q-Jet: gross confusion and foul-ups. I wish that I had the talent of people like Lars and Cliff Ruggles. Not much time left in the season here so I will just drive as is.

For those who are interested my rebuilt points/tach drive distributor (my 1974 self-destructed a couple of weeks ago; another issue) it is just fine. It came from my 1967 days (basement inventory). I will be looking for a new spare soon.

People like pics so here is the old 1974 ready for duty this morning. And even with ported it hummed!

 

Like u say Pual u need to sort out the ports. And its alot easier with the carb off the car. What i use is WD-40 as it has enough pressure and spray volume to make it through the carb and out the internal slots/ports inside the throat. That way u see which goes to what. Also some fine SS safety wire can be very helpful. But again u still need to drive the car and verify good results.

BTW since i read Cliff Ruggles book on Qjets i will never look back to my stock Qjet since i can choose better late Qjets to use. Better tuning and electric chokes came on Qjets after '74. U may want to take a look before carb shopping.

This has become a frequent topic and u should find plenty of posts with a search.
Good luck Paul,
cardo0

 

The car will run fine on 'ported' vacuum. That's the way it came from the factory. The purpose for hooking to 'ported' is to eliminate vacuum advance when sitting at idle. That 'ported' outlet changes to manifold vacuum as soon as the throttle plates move a small amount. You can check this out with a vacuum gauge: hook the gauge to that outlet--it should read "0" vacuum at idle but full vacuum advance amount when you just crack the throttle.

The 'manifold' outlet on the right side of the Q-Jet should be OK to use. But, the casting could have some flash/blockage from the casting operation; or the wrong gasket could have been used; or the right gasket still has the 'punch-out' in place which is blocking flow.

The engine idles cooler and more efficiently with 'manifold' vacuum. But it has less emissions at idle with 'ported' vacuum. I prefer to use 'manifold' vacuum, but it's not a big deal to run on 'ported' since your carb has a problem supplying manifold vacuum at that port.