q-jet vacuum question

[Johan]

Hi,
My 74 hesitates a bit when i floor it, so I got ready to tune it yesterday.
I started by checking the vacuum, connected the meter to manifold vacuum and the needle was steady at 18-19. So far so good.
Then I checked the connection to the distributor, which I thought was hooked up to ported vacuum - nothing!
I guess I will connect the distributor to full manifold vacuum, but is there some functionality that I'm not aware of that will completely shut off ported vacuum, or what's wrong now?
Connections look like this:
1-distributor (ported?)
2-misc
3-brakes
4-lights
5-pcv

 

[Johan]

Wait a minute - now I realize that ported is shut off at idle, i thought it was just lower all the way.
But anyway, is there any reason NOT to connect the distributor to full manifold?

 

[Vette79]

I believe part of the reason why it initially was connected to ported vacuum on the carb was for emissions reasons. From what I have read, full manifold vacuum will give you better throttle response but adjusting the timing and distributor springs may be necessary. I have not done this yet and am still undecided.

 

[markmark]

I believe part of the reason why it initially was connected to ported vacuum on the carb was for emissions reasons. From what I have read, full manifold vacuum will give you better throttle response but adjusting the timing and distributor springs may be necessary. I have not done this yet and am still undecided.

The way I was taught was to always run your distributor vacuum ported, if not, tuning your distributor advance becomes a nightmare. Remember, your advance works off of vacuum, so when you have vacuum readings that fluctuate at idle, where is your reference point for tuning?

 

[petes74ttop]

Correct Vacuum connections.

Hi Johan and welcome to the forum, Beautiful Vette, are those Western Wheels?

GM originally used ported vacuum on the distributor for emission purposes.

Here's how it should go for best performance.
#1 the distributor should get manifold vacuum- that should be the port with the white cap on it just left of the choke pull off.

#2 what you call misc- that is full manifold vacuum to raise and lower the headlights.

#3 that is correct-full manifold vacuum for the power brakes.

#4 what you call lights- that should be ported vacuum for the vapor canister, and would also go to the EGR if you had one.

#5 PCV is correct- full manifold vacuum.

Full Manifold vacuum is produced at idle or steady cruise,
ported vacuum comes on only after idle.

Hope this helps, PG.

 

[gmjunkie]

Hi Johan and welcome to the forum, Beautiful Vette, are those Western Wheels?

GM originally used ported vacuum on the distributor for emission purposes.

Here's how it should go for best performance.
#1 the distributor should get manifold vacuum- that should be the port with the white cap on it just left of the choke pull off.

#2 what you call misc- that is full manifold vacuum to raise and lower the headlights.

#3 that is correct-full manifold vacuum for the power brakes.

#4 what you call lights- that should be ported vacuum for the vapor canister, and would also go to the EGR if you had one.

#5 PCV is correct- full manifold vacuum.

Full Manifold vacuum is produced at idle or steady cruise,
ported vacuum comes on only after idle.

Hope this helps, PG.

I'll seconed that!!

 

[minifridge1138]

Johnz wrote a great article about ignition timing and vacuum.
Search the forum to find it and get all of the details.
But I'll provide a brief summary:
1) Ported vacuum was an environmental crutch. It increases engine temp and decreases performance.
2) A lean mix (such as you have at idle) burns slower and requires more time to burn, so you the spark needs to start sooner in order for the flame front to hit the cylinder at top dead center. The more advance, the sooner the spark. So at idle, you want an advanced spark.
3) A rich mix (such as you have when you floor it) burns faster and requires less time to burn, so the spark needs to arrive later for the flame to hit the cylinder at top dead center. The less advance, the later the spark. So when you floor it from idle, you want less spark.
4) at idle, your vacuum advance should be fully engaged.
5) You set your timing with the vacuum line plugged, so an erratic idle has no effect on timing (the same is true with ported vacuum because there isn't any at idle).
6) There are 3 factors to timing: base timing, vacuum advance, mechanical advance. Base timing is set by turning the distributor at idle with the vacuum advance disconnected and the vacuum line plugged (to prevent a vacuum leak). At idle you should have base timing + full vacuum advance. Mechanical advance is a separate system. It advances as RPMs increase. It should be fully engaged by 3000 RPM. At full throttle, you should have base + mechanical (at WOT there is little vacuum, so there should be no vacuum advance).

I highly recommend reading JohnZ's "timing 101" and "mapping advance". Search the forum for the link.

 

[Johan]

Thanks everybody.
Pete, thanks, and yes those are Western Wheels

Ok, I connected the distributor to manifold vacuum, but now I have som timing questions:

Base timing is now set to 8 @ 900 rpm.
According to my manual, maximum centrifugal advance is 20 @ 5000 rpm, and maximum vacuum advance is 14.
This seems pretty accurate with my car, max total excluding vacuum is 28 (@ 2500, maybe I have weaker springs), and with vacuum connected I get around 42.

So, how am I going to get to the magic 36 degrees (ex. vacuum)?? Should I just set it at 36 @ 2500 rpm, and don't bother about the advance at idle? (which I guess will be 16)
The only other solution I can see is rebuilding the distributor so it will allow more centrifugal advance?

 

[JohnZ]

So, how am I going to get to the magic 36 degrees (ex. vacuum)?? Should I just set it at 36 @ 2500 rpm, and don't bother about the advance at idle? (which I guess will be 16)
The only other solution I can see is rebuilding the distributor so it will allow more centrifugal advance?

Try setting your initial at 12* (which will give you 32* total) and see how your engine likes that setting; 16* initial is probably excessive. I'm not familiar with HEI distributor innards, but point distributors have a bushing pressed on the limit pin that protrudes down into the slot in the autocam plate (see photo below); the length of that slot and the O.D. of the bushing on the pin determines maximum centrifugal advance. Removing that bushing or replacing it with one of the smaller O.D. "advance kit" bushings will provide more centrifugal advance.

 

[JohnZ]

The way I was taught was to always run your distributor vacuum ported, if not, tuning your distributor advance becomes a nightmare. Remember, your advance works off of vacuum, so when you have vacuum readings that fluctuate at idle, where is your reference point for tuning?

Nope. Go here:

http://lbfun.com/Corvette/Tech/vettetech.html

Scroll down to "Distributor & Timing", then to "Timing101Article.pdf"; left-click to read it, right-click to download it.

There's no "nightmare" at all - the vacuum advance is completely separate and independent of the centrifugal advance system in the distributor, and doesn't affect it in any way. When you re-connect to full manifold vacuum from ported all you need to do is to re-set your base idle speed.

 

[Johan]

Thanks, I will look into this.
My distributor has a "Ignitor" breakerless conversion installed, but I guess that will make no difference.

 

[petes74ttop]

Now on to timing.

Hi again Johan, glad I could help with the hoses.

I also have a 74 T-Top with Western Wheels, I like the way you painted the slots, look great. I left mine rough cast.

Minifridge1138 pointed you in the right direction when he quoted johnZ. Your in good hands with JohnZ, everything about timing I learned from him. I think you have a stock distributor with the no points conversion, so do I. The stock springs are stiff so they limit full advance to close to 5,000 rpm if you want full centrifugal advance at 2,800 to 3,000 you may have to change one or both of the springs.

Just follow the link JohnZ provided and if you have any questions just ask.
Best to you, and enjoy your ride, PG.

 

[Vette79]

Nope. Go here:

http://lbfun.com/Corvette/Tech/vettetech.html

Scroll down to "Distributor & Timing", then to "Timing101Article.pdf"; left-click to read it, right-click to download it.

There's no "nightmare" at all - the vacuum advance is completely separate and independent of the centrifugal advance system in the distributor, and doesn't affect it in any way. When you re-connect to full manifold vacuum from ported all you need to do is to re-set your base idle speed.

John, are you saying that if I wanted to connect my vacuum advance to full manifold vacuum from ported vacuum on the carb all I have to do is ??? Please explain...

Is the bottom right front port on the carb full manifold vacuum? I don't think it is because it is not below the throttle plates. Where would be the best place to acquire full manifold vacuum?

 

[petes74ttop]

Switch to manifold vacuum.

Hi Vette79, yes you can switch your vacuum can hose to manifold vacuum and all you might have to do is reset the idle.
Look at Johan's picture, the right front side with the white cap on his carb is full manifold vacuum. I can't tell on your photo.
Or just use your vacuum gage on the differant ports, PG.

 

[BarryK]

Vette79, as already suggested use a vavuum gauge (or even just your finger) to identify which is ported and which is full manifold vacuum source connection on your carb.
IF, your stock vacuum advance connection is ported (I say IF because after '75 and the inclusion of the cat converter on the exhaust they didn't need to use ported vacuum for vacuum advance and emission requirements anymore. My 100% stock, 11,000 mile '78 L82 is connected to full manifold vacuum from the factory) to switch it to full manifold vacuum all you need to do is move the vacuum hose to a full manifold vacuum source and readjust your idle speed as it will increase with the increased vacuum advance at idle. You don't need to make any other adjustments to your timing or distributor because of the change.

 

[Vette79]

So, you think it will be that easy. Just connect to full manifold vacuum and adjust timing?

 

[Johan]

So, you think it will be that easy. Just connect to full manifold vacuum and adjust timing?

I just did, no problem. What you really have to adjust is idle wich will propably be a bit high, but it will not hurt to check timing also.

My initial timing is now set to 12 or maybe 13 degrees and total is 32-33, as suggested by JohnZ and it runs better than ever.

 

[Vette79]

You noticed a difference?

 

[JohnZ]

So, you think it will be that easy. Just connect to full manifold vacuum and adjust timing?

You don't need to touch the timing, as the change didn't affect it; the initial timing and centrifugal advance haven't changed. All the change does is pull another 15* of advance in at idle (which is why you need to re-set the idle speed).

 

[minifridge1138]

Yes, you will notice a difference.

Good Luck!