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Vacuum Question...

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asoupley

Jedi Hopeful
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Ok, so I seem to have gotten all of my vacuum leaks fixed, and think I have the carbs tuned pretty well... anyway, I disconnected the vacuum line at the distributor for some unknown reason with the engine running, and the RPM's climbed quite a bit but did not cause it to run rough at all. Now I'm confused... when I do the piston lift test, the RPM's do not climb like that. Also, if I disconnect the vacuum line from the manifold, it starts running rough (I figure this is because it is too much extra air). I guess I am wondering why it seems like it runs smoother with the vacuum line leaking at the distributor.

Thanks
Tony
 
Likely just 'cause it'll pick up a few RPM and seem "smoother". Otherwise you may be running a bit too rich.
 
I thought that I was running rich, but doing the lift pin causes it to borderline on stuttering.
 
When you tune the carbs the vacum advance hose should be disconnected and blocked off.
 
Hello Tony,

when you disconnect the vacuum tube, the timing will retard by a certain amount. I would say that is why the engine speed rose rather than any marginal change of mixture strength. Maybe you ought to double check your timing as it may be too far advanced.

Jack,
"When you tune the carbs the vacum advance hose should be disconnected and blocked off."

Aren't you confusing this with setting the ignition timing?

Alec
 
Keep in mind that you don't get any significant vacuum advance at idle. I had to replace the vacuum advance unit in my distributor last week, so I did some measurements. There is virtually no advance until the vacuum hits about 5", and on a free-running engine (OK, at least in MY free-running engine), this is about 2500 RPM. So, at idle, there is virtually nothing. Sources on-line which I have consulted generally confirm that this is how they work.

As to the original question, why a leak at the distributor doesn't affect things as much as one at the carbs: (1) the line itself, being long and narrow, provides some degree of flow restriction, perhaps enough so that the leak rate is not too bad; (2) are you sure that the vacuum line to the distributor isn't plugged or kinked?
 
Hello Steve,

to the best of my knowledge there is significant vacuum at idle as the throttle is closed. That is why it should be disconnected when doing a timing check with a strobe light.

Alec
 
Don't forget that there are cars set up for manifold vacuum advance and ported vacuum advance. This makes a big difference as to the vacuum range that the advance module on the distributor operates.
 
Ok, what is "ported" vacuum advance? I am assuming that "manifold" vacuum advance is where the vacuum hooks to the manifold?
 
Ported is from the carb body its self.
 
Ok. I have a little port on the front of the front carb. Is this the vacuum port? Why would I have both? Should I be plugging this one? It seems like there is quite a bit more vacuum from the manifold than from this little port.
 
Should not have two, lol.

Yep, folks here can tell you which is correct for your car, I have no idea but know air leaks are not good.
 
A simple little rubber vacuum cap (from your local auto parts place) should take care of the extra one! I would try capping each in turn and see which gives the better result! /bcforum/images/%%GRAEMLIN_URL%%/wink.gif
 
Ported vacuum is taken off the carb BODY in the area at or just b-lo the carb butterfly. At idle there should be little or no vacuum present at the nipple/fiting on the carb. The amount of vacuum depends on the butterfly opening angle in the carb bore. At say a low idle of 700 rpm you will get no discernable vacuum...but when you open the throttle to say 1500 rpm you will get maybe 3" of vacuum.

Now manifold vacuum is sorta the opposite of ported spark. At idle you have high vacuum.....700 rpm might be 17 or 18" of vacuum...mayb a bit more. Then when you open the throttle the vacuum decreases slightly...and under load the vac really decreases....under full throttle in second gear for example...the vac drops to zero or almost so.

What to use....ported or manifold vac.

Depends on the vac canister on the dist. Most use ported vac. You can use a MiT Vac (pistol type vac gun) and watch the dist breaker plate and plot a curve with a protractor or just guess. Watch the vac gage on the gun and your there.....simple huh?
 
JerryB said:
Ported vacuum is taken off the carb BODY in the area at or just b-lo the carb butterfly. At idle there should be little or no vacuum present at the nipple/fiting on the carb. The amount of vacuum depends on the butterfly opening angle in the carb bore. At say a low idle of 700 rpm you will get no discernable vacuum...but when you open the throttle to say 1500 rpm you will get maybe 3" of vacuum.
Click to expand...

This is very close to what I measured.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]
N...... You can use a MiT Vac (pistol type vac gun) and watch the dist breaker plate and plot a curve with a protractor or just guess. Watch the vac gage on the gun and your there.....simple huh? [/QUOTE]

If you do this with a bugeye distributor (and from the data I saw, probably many later cars too) there is no movement of the breaker plate until you hit about 5" vacuum. The vacuum unit on the distributor seems to have a preload to it, so there is no motion until the force on the diaphragm reaches that point. Somewhere I came across a plot of the vacuum advance for A-series engines, and it showed this as well. I'll try to find it when I get back to my main computer.
 
Hello all,

this business of ported versus manifold vacuum is interesting, I find it hard to understand that the vacuum in the manifold at idle is high whereas a tapping in the carburettor body has none or very little. (I'm assuming from the description that the tapping is downstream of the throttle butterfly?) The vacuum in the manifold surely can not just disappear the short distance from the manifold to the carburettor. By the way my Triumph doesn't have a vacuum advance at all so this is academic for me.

For general information there are a huge variety of Lucas vacuum units, and although similar in appearance are not in operation. From my data sheets, the first number is the vacuum (inches of mercury) at which it operates, the second is the vacuum for maximum advance nad the third is total advance in engine degrees. A quick check of the data some units starting to advance with one inch and an other with eleven inches, so there is quite a variation.

Alec
 
The ported vacuum is on the atmospheric / filter side of the throttle butterfly. When the car is idling there is little vacuum there, when the throttle is opened the vacuum increases. Since the tap is on the high pressure(less vacuum than the manifold) side of the butterfly the advance module operates at lower vacuum levels in the 5 to 8 inHg range. On a lot of later smog equip'ed cars they used manifold vacuum in the 7 to 14 inHG range with greater total mechanical advance.
 
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