Vacuum Question

J

james singleton

Well-known member
OK, I bought a vacuum gauge today and wanted to check what kind of vacuum I was getting on the line that runs from my carb to the dizzy. (I have a newly rebuilt 200c.i.d with a Holley 1940 carb; I also have the orginial Ford-o-matic dizzy). I was getting a "0" reading on the gauge at idle and maybe up to 15 inches or so when I rev'd the engine. The engine runs well at idle and low to meduim rpms, but hestitates just a little at higher speeds; but not too bad! Does this sound like a carb problem as I suspect? I have good suction on my PCV valve when I take it out of the valve cover and hold my thumb over it (the PCV). Any Idea? Thanks, Jim
 
you likely have a load o matic dizzy, so your carb has a spark control valve on it, which uses vacume to change advance, but a specific amount at a specific RPM, to get a specific amount of advance, the vac signal is altered in the SCV
 
Matt, sorry, it is a 1964 Load-0-Matic dizzy off a 170c.i.d engine, not a "Ford-O-Matic"! Yes, the 1940 carb on my 200 has a spark-control-valve.
TJ, thanks for the references to earlier posts. It looks like there is a spring in the SPV according to the drawing. I am wondering if that is something that goes bad in the SPV; anybody familiar with this? I believe that I am suppose to be getting something like 17 inches Hg of vacuum at idle speed, and I am showing zero inches! I read Mustang Six's Sticky and he said that a "failed or stuck carburetor vacuum advance valves, clogged internal venturi vacuum passages, ......etc." "can lead to a lack of full spark advance, causing poor performance, poor gas milage, and potential overheating". I find his statement about "potential overheating" interesting since I had some overheating problems on the engine I pulled out (the 170c.i.d.) using this same carb & dizzy! The new engine (the 200) has not been driven enough to know whether it will have overheating issues. Around town the engine has been running rather cool. Thanks, Jim
 
Reconnect the vacuum control line between carb and dizzy. Fit a 1/8 NPT hose barb in the manifold's threaded hole just below and forward of the carb. Attach some vacuum hose to that, and then your gauge.

Readings will now be quite different. :wink:
 
You might be able to reduce some of the high speed issues by advancing the timing a few degrees, too. The load-o-matics don't really deliver enough advance at speed.

Another thing to check is the vacuum diaphragm itself. You have verified that you have vacuum, now you need to verify that the diaphragm is good and is advancing the timing as required. You can do that by applying vacuum to the diaphragm and watching the breaker plate. The plate should move and hold with vacuum applied. If the diaphragm is leaking or damaged, it won't move or stay in place, and it must be replaced.
 
Addo, I checked the vacuum at the base of the carburetor and I was getting about 17 or 18 inches vacuum. I took the spark control valve off; inspected everything, blew things out with air (except for the rubber diaphram), put everthing back together. No vacuum going to dizzy. Does it sound like a bad SCV, or possibly something plugged up? Any other thoughts; from anyone. I read the sticky; very informative. Thanks, Jim
 
Have you got airflow through the ports accessible when your SCV is removed? If so, then the SCV is your culprit.
 
HELLO JAMES.

.....JACK SAID TO CHECK THE DIZZY DIAPHRAM BY LOOKING AT THE BREAKER PLATE. THAT IS THE BEST WAY. THE ONLY THING TO MAKE CLEAR IS THE PLATE MUST MOVE AND THEN..., NOT MOVE BACK AT ALL. IT MUST HOLD TO THE VACUUM. THERE ARE SMALL LEAKS THAT WILL LET YOUR TIMING BE OFF. BUT I THINK YOU MUST BE GETTING ADVANCE BECAUSE YOU SAID IT DRIVES PRETTY GOOD. IT WON'T DRIVE WELL AT ALL IF YOU HAD NO ADV.

.....IF YOUR HIGH SPEED BOOG IS MILD IT IS MORE THAN LIKELY LATE TIMING. THE DIZZY'S BREAKER PLATE SHOULD BE VERY LOOSE. IF THE SPRINGS ARE VERY TIGHT THAT WILL CAUSE THE HIGH SPEED LATE TIMING. THE LARGE SPRING ON THE PLATE WILL LIMIT THE HIGH RPM TIMING. YOU CAN TAKE THAT BIG SPRING OFF ALTOGETHER. THAT WILL STOP ANY LIMIT YOU MIGHT HAVE. YOU CAN TRY IT.

.....ONE MORE THING. WHERE DO YOU LIVE? I'M IN CALIF. AND AT IDLE I GET 19.5 TO 20 ON A VACUUM . YOU SHOULD TRY ALSO SETTING THE TIMING AT 12* . TRY ALL OF THESE ONE AT A TIME. THAT WAY YOU WILL KNOW WHAT CHANGES EACH OF YOUR MOVES MAKE. THEN TRY THEM ALL TOGETHER AND YOU WILL KNOW WHAT TO CHANGE BACK IF YOU NEED TO.

....I HOPE THIS WILL HELP.

LIVE IN GRACE

LEROY POLL
 
Leroy & Jack, I am not getting any vacuum @ idle from the line going between the carb and dizzy! Zero vacuum @ idle and only about 5 or 6 inches, at most, when I rev the engine. Correct me if I am wrong, but if I have zero vacuum coming from the carb (going to the dizzy) then it shouldn't make any difference what the dizzy diaphram or breaker plates are, or aren't, doing since there is no vacuum. Correct? Or am I missing something here?? Don't I have to establish vacuum at the carb before I can worry about the dizzy diaphram or plates??? By the way, I have a pertronix l ignition and my timing is set @ 11 degrees!
ADDO, I didn't get a chance to check the airflow through the ports when I had the spark control valve removed. I sprayed with gumout and then blew the insides of the spark control body out with compressed air. I will check to see if I have air coming out the hole in the carb throat (above the accelerator plate) and also check the venturi hole (where the vacuum line that goes to the dizzy connects). Thanks Guys, Jim P.S. Leroy, I live in S.Cal
 
james singleton":hp8f3scl said:
Leroy & Jack, I am not getting any vacuum @ idle from the line going between the carb and dizzy! Zero vacuum @ idle and only about 5 or 6 inches, at most, when I rev the engine. Correct me if I am wrong, but if I have zero vacuum coming from the carb (going to the dizzy) then it shouldn't make any difference what the dizzy diaphram or breaker plates are, or aren't, doing since there is no vacuum. Correct? Or am I missing something here??
Click to expand...

That is correct sir. If you arent getting vacuum at the distributor, it doesn't really matter if the dist is leaking or not - its not going to work regardless. Fix what is obviously broken first.



I am also in SoCal. Sort of.
 
Here I go throwing my half-baked opinion into ring, but I thought ported vacuum is supposed to be zero at idle?

If I remember correctly, manifold vac is full at idle, ported is zero at idle. Then as the load increases, they essentially sync up to about the same. Now the difference may be the SCV and the Load-a-matic. I haven't had one of those on my car for years and maybe that acts differently...
 
cfmustang is correct; vacuum at idle is supposed to be zero. The throttle plate closes off the vacuum port. As the throttle plate opens, the passage should see manifold vacuum, but with an SCV system, it may not see full manifold vacuum because of the way it bypasses to venturi vacuum. Here's where it gets tricky; as the load increases, manifold vacuum drops, but as the airflow increases thru the venturi, the SCV should provide a greater signal to the advance diaphragm.

So far, with the way you're measuring vacuum, the SCV seems to be working correctly. The only way to ensure the vacuum signal is there under speed and load is to put a tee in the line and then drive it. You can't replicate the load and airflow by revving the engine.
 
I am still a little bit confused on exactly how the vacuum (and the spark control valve / SCV) works inside the carb; even after reading the Sticky and reading the posts. So let me see if I have this straight; On acceleration the manifold vacuum drops, shutting off the vacuum to the distributor and thus retarding the timing; but the Venturi vacuum provides a small amount of vacuum to prevent a full timing retard ? And the spark control valve (SCV) opens when manifold vacuum increases and the dizzy then advances the timing for a "more efficient engine operation"!???? So does this mean that at idle speed there is more vacuum to the distributor and thus advances the timing; and that on acceleration, and under load, there is less vacuum to the dizzy? Is that correct? So the big question: Does this me I should be getting a full vacuum reading (15-20 inches Hg) at idle speed; and should then decrease while accelerating under load? What kind of reading while accelerating in the driveway, but not under load? Again, I am not getting any vacuum from the carb to the dizzy at idle rpm's, and about 5 inches Hg when accelerating in the driveway. I should be getting a vacuum reading of 15-20inches Hg while idling, correct? Like I mentioned in earlier post, I got 17 inches Hg or so when i removed the 5/16" +/- plug below the carb spacer plate (in the intake manifold) and hooked up the vacuum gauge! I was busy today and didn't get a chance to blow out various ports in the carb. I still suspect the SCV, and would gladly replace this (it couldn't cost too much, right?), but I am not sure if the auto parts carry these SCV's!??? And the carb rebuilt kits that I looked at don't appear to have replacement SCV's in the kit! Has anyone tried buying these SCV's???? Maybe it's time for a new carb? Ha! Thanks, Jim
 
I did quite a bit of reading on various posts, some dating back a few years. There is a lot to learn and quite a few thoughts on vacuum as it's affects the distributor's advance & retard, and the carburetion! From what I have read, it makes a difference as to what type dizzy vacuum advance you have, and what year; what type of carburetor you have and what year, etc...Here is what I (believe) that I have: #1 I have a load-a-matic (from approx. 1964) with the single tube (not the double tube type of the later sixties) coming off of the bisquet/diaphram dizzy, and #2 I have a Holley 1940 single barrel carb that has a SCV (spark control valve). I tried several tests today and this is what I found. Test #1 I hooked up a vacuum pump & gauge directly to the dizzy with the dist. cap off. The pump moves the breaker plate while pumping, but does not hold the vacuum inches, so I really cannot tell you how many inches of vacuum it took to move the breaker plate. I then removed the gauge and sucked on the hose and the plate would hold! Does this sound like maybe the diaphram on the dizzy is bad? Test #2 with the vacuum gauge hooked up between the carb venturi and the dizzy I got zero inches of vacuum at idle. It runs well in idle, but hesitates when I punch the gas. Test #3 I hooked up the gauge between manifold vacuum and the dizzy and I had 18 or 19 inches of vacuum! It ran rough at idle speed, but ran real smooth when I punched the gas. Any ideas or comments?? Thanks, Jim
 
sucking on the line to the vacuum motor should hold vacuum. So I suspect you have a bad diaphram/vacuum advance cannister. That is not surprising after 40 years.
Doug
 
HI JIM


.....I'M BACK AFTER A REW DAYS. JIM IF YOU ASK FOR A 1960-1963 CARB. KIT IT COMES WITH A NEW SCV. I HAVE MANY THAT HAVE NEVER BEEN USED.

.....YOUR FIRST POST WAS CORRECT BUT THE SHUT OFF OF THE SCV COMES ONLY AT WIDE OPEN THROTTLE. (WOT)

.....TO TEST THIS CARB AND DIZZY WITHOUT A LOAD IS HARD. BECAUSE THE DIZZY REACTS TO THE MOVEMENT OF THE THROTTLE. WITHOUT A LOAD IT IS TOO QUICK TO CATCH.

.....IF YOU HAVE A AUTOMATIC TRANNY THEN TWO PEOPLE COULD TEST IT. ONE INSIDE WITHFOOT ON BRAKE AND HAND BRAKE FULLY SET. WITH THIS WAY OF TESTING YOU GET THE LOAD YOU NEED.

.....YOU CAN CHECK VACUUM AND ADVANCE ALL AT THE SAME TIME. YOU WILL BE ABLE TO SEE ABOUT 15 LBS OF VAC. AND SEE WHAT YOUR ADVANCE ACTUALLY IS.

.....I HOPE THIS WILL GIVE SOME HELP IN TESTING FOR YOUR PROBLEM.

LIVE IN GRACE

LEROY POLL
 
Leroy, thanks for post (and others as well). So I should be able to get a replacement SCV for my carb from a rebuild kit for the Holley 1940 that is from the 1960 to 1963 kit!? That is good to know. It would be much cheaper to buy the kit than to have to buy a new carb! By the way I have a standard trans. So if I do a load test with a vacuum gauge hooked up between my carb venturi and the dizzy, I should experience something like 15 lbs of vacuum at some point, and should be able to see what and when the advance actually kicks in?? Is that correct? This weekend I hooked up a vacuum pump directly to my dizzy and could move the breaker plate, but the plate did not hold. Wouldn't this indicate that I have a bad dizzy diaphram? It should hold the vacuum shouldn't it?? Thanks Again! Jim
 
Leroy, I forgot to mention that the Holley 1940 (1 barrel) that I currently have installed on my 200, was purchased around 1980 as a replacement carb for a 170c.i.d. engine that was previously installed. Does the fact that this was a Ford replacement carb back in 1980 have any bearing on the fact that you mentioned a 1960-1963 carb rebuild kit that has the SCV that I need??? The kit you mention, is it for a Holley (1940 model) or some other model? Thanks, Jim
 
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