New to Tuning, How to read vacuum

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Projectile Dart

Projectile Dart

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Please don't chop my head off, I am still trying to learn the ropes of tuning a classic car and I don't have anyone around me that I can learn from
Set up:
I have a '68 Dart with a 360 out of a '76 Dodge RV. It has headers and an Edelbrock Performer RPM aluminum intake but is otherwise stock. Ignition system is a stock style points system with the basic style coil. Carb is a Holley 3310-1 with 66 primary jets and 72 secondary. Spark plugs and wires are relatively new (both less than 500 miles on them) Rotor and cap both seemed to be in great condition. It has been running far too rich and I've noticed a "hiccup" or "stutter" at part throttle cruise and the car has been running far too rich.
I finally got a vacuum gauge and decided to start from the beginning this past weekend. I found TDC, marked the balancer, and fired it up. I played with everything to maximize vacuum and topped off at about 19 lbs of vacuum at idle and the engine seemed to like about 14* of initial timing (where I was able to get max vacuum) Idle is great but I haven't driven on the street yet.
Question:
I have a 727 with unknown TC and when I drop it in gear the car wants to jump forward and vacuum drops to about 15 lbs of vacuum. Is this kind of drop normal? What is making the engine want to launch forward when I drop it in gear?
 
Not sure. Next thing in my shopping cart is a tach. I just set the idle as low as possible (in gear) where the engine ran smooth
 
Google engine vacuum chart. Notice all of them say steady 19 is very good. Pic out one that will print readable and print it.
 
RedFish said:
Google engine vacuum chart. Notice all of them say steady 19 is very good. Pic out one that will print readable and print it.
Click to expand...

Found a couple of charts, these are great. I will give an update on rpm when I get a tach in there. Is it normal for the vacuum to change that much when in park vs when in gear?
 
Projectile Dart said:
Found a couple of charts, these are great. I will give an update on rpm when I get a tach in there. Is it normal for the vacuum to change that much when in park vs when in gear?
Click to expand...
Without knowing the idle speed and the amount of RPM drop in gear it's hard to answer your question.
my guess is your not idling on the idle circuit.
my vacuum only drops a pound or two
 
I think throttle pressure set high could cause the forward surge too.
 
I agree with the assesment Fishermans67 provided.
I don't have any info about the cam or specs for a '76 RV 360 but can specualte it pulled hard down low. While it probably had some tweaking of the exhaust lobes for emmissions (mentioned in the early 70s Master Tech booklets) a timing curve that looks something like MP used will probably be a decent starting point.
Even though it seemed to like 14* in neutral, it probably will actually be stronger in gear with a little less.
When you get the tach, after its warmed up, see if it will idle decently with 12* around 700 rpm.
If you find its at 14* but 1000 rpm, leave the timing alone because it will drop when you reduce the rpms. Then remeasure.
Only if you find its at 14* around 600 to 750 rpm, then try a couple degrees less with a slightler richer idle mix.
It will take several tries going back and forth between adjusting the idle speed screw downward (counterclockwise) and the mix screws.
When turning the mix screws inward (clockwise) there should be a point where there is noticible drop in rpm. Turn back a fraction so the rpm recovers, then turn them 1/8 to 1/4 turn richer. Then check in gear. The least drop in either rpm or vac indicates the strongest idle.
If turning the idle mix screws in doesn't cause the engine to die, then the throttles are too far open.

Once you get the idle good, taking it for a short gentle drive to test is OK. Do not floor it. (We know nothing about the curve in the distributor, so moderate load, and especially full load, situations need to be approached with caution.)
 
Projectile Dart said:
Please don't chop my head off, I am still trying to learn the ropes of tuning a classic car and I don't have anyone around me that I can learn from
Set up:
I have a '68 Dart with a 360 out of a '76 Dodge RV. It has headers and an Edelbrock Performer RPM aluminum intake but is otherwise stock. Ignition system is a stock style points system with the basic style coil. Carb is a Holley 3310-1 with 66 primary jets and 72 secondary. Spark plugs and wires are relatively new (both less than 500 miles on them) Rotor and cap both seemed to be in great condition. It has been running far too rich and I've noticed a "hiccup" or "stutter" at part throttle cruise and the car has been running far too rich.
I finally got a vacuum gauge and decided to start from the beginning this past weekend. I found TDC, marked the balancer, and fired it up. I played with everything to maximize vacuum and topped off at about 19 lbs of vacuum at idle and the engine seemed to like about 14* of initial timing (where I was able to get max vacuum) Idle is great but I haven't driven on the street yet.
Question:
I have a 727 with unknown TC and when I drop it in gear the car wants to jump forward and vacuum drops to about 15 lbs of vacuum. Is this kind of drop normal? What is making the engine want to launch forward when I drop it in gear?
Click to expand...
You in all likely hood have a stock torque converter. What folks on here would call "tight".
It will drop your idle speed when engaged in drive.
 
Put that vacuum gauge back into the toolbox, it will just make your life miserable.
That gauge is a pretty good diagnostic tool.but
IMO it is useless as a tuning tool.
While you are throwing things away, I highly recommend to upgrade from points as soon as you are financially able. Not that there is anything wrong with points other than they are just one more thing to complicate your life.
As to your rich-running; Ima thinking one or more of; the PV diaphragm is blown, the choke is on,the accelerator pump circuit is siphoning, or the float-level is too high/unstable.
As to the tip-in stutter; Ima thinking the transfer slot exposure under the primary throttle valves is too short.
A stutter/hesitation not on tip in is likely a lazy accelerator pump.
If your engine has been running rich for all 500 miles then, likely the ignition system will not be able to clean up the plugs. If they are black and sooty,throw 'em away, and while the plugs are out, do a compression test.
Ok, with the baseline now prepared;

The first thing you need to do right now is to;
Install a NEW PCV.
Start with fresh from the pump, whatever gas you will be using, and want to tune for. Your engine should be fine with 87gas. Higher octane gas will not make any more power ...... unless your engine detonates on 87.
Remove your squirter nozzle and prove the checkvalve is in there.
Then,set your minimum coolant temp to as high as your location will comfortably allow. This temperature must be stable at all times. If it is not, then fix it! You will have no tuning success if your engine temp is dancing around between 180 and 220, so fix it. I run mine at a minimum 205*F, and it maxs at 207*F,lol.Get this done.
Then remove the carb, flip it upside down, and set your transfer slot exposure underneath the primary throttle blades to between square to a hair taller than wide. Then make sure your secondary blades are closed up tight but not sticking. Shine a light up from the other side and make sure they are absolutely both closed. Next,reset your mixture screws to 1/2 turn from lightly seated, and, and, and, after this leave all those settings alone.
Finally set your accelerator pump preload. then
Reinstall the carb, prime it and start the engine. Now verify the float levels are correct. This is critical for your combo; it will not tolerate a setting very far from the Holley Spec.And make sure the choke blade is vertical and stays there.
Ok, now;
To set your idle speed;
you will back up the timing.
Without a tach; set the park brake and put it into gear; then just back up the timing, back it up,back it up, don't even look at the timing light, just back it up until the engine wants to stall; then put a little advance back in. Forget the light/ forget the vacuum gauge.
With the engine idling, stick your index fingers on the boosters, and make sure the idlespeed does not change. Then stick a shoprag into the secondary opening and make sure the idle speed changes not or changes very little.
Now, put it into Neutral and and back into gear a few times, noting how badly it clangs into gear and how much higher the neutral rpm seems to be. If you think you can tolerate a lil more jump, pull in just a lil more timing; do not adjust the curb-idle speed screw.
DO NOT TOUCH the curb-idle screw.
Ok having fixed the N-D jump and the high idle, take it for a roadtest. You are only checking the low-rpm response, do not change the curb-idle screw.
Watch for a tip-in sag, or hesitation, a flat-spot or an outright bog.
You should have none of those.
But if you do get a tip-in hesitation, NOW, you can screw in the curb-idle speed screw one half turn, and repeat the roadtest. You can do this a total of two or maybe three times, to solve a tip-in hesitation, which is from curb idle to just off, in gear and leaving a stopsign. If it does not go away with a turn and a half, something is wrong, put it back 1 turn, and report back.

Then; Ima thinking; 66/72 jets are gonna be too lean/time will tell.
After you solve your rich problem, you are gonna need to swap out whatever PV is in there , for a 10.5, and
probably upjet to at least 68/74.You may as well order those now. Put the 10.5 in with the 66s and roadtest that.

That's all for now.
 
Last edited:
AJ/FormS said:
Put that vacuum gauge back into the toolbox, it will just make your life miserable.
Click to expand...
I disagree, a vacuum gauge is a valuable tool when fine tuning an engine.
You need to know how to read it and what it's telling you.
I learned how to tune with a vacuum gauge at a young age in 1979. I was taught by my dad. also learned how to get initial timing with points using a volt meter or lamp.
A vacuum gauge is right beside a wideband in the cockpit. lol
If you only knew what I know. jk
 
Vacuum gauges are very useful. Especially if installed on a mighty-vac. Found more leaking vacuum pods with that thing than I care to admit running. Chuck it up to your distributor line and advance the distributor while your car is idling in P. See what it does or how it reacts with more advance. Youbwqnt to shoot for higjest vacuum at idle with lowest idle rpm, back and forth. Adjust timing to highest vacuum, then reset idle, lather-rinse-repeat.
 
Projectile Dart said:
Carb is a Holley 3310-1 with 66 primary jets and 72 secondary.
Click to expand...
This is an aside and has nothing to do with the setting up the initial timing and idle/off idle that we've been focusing on.
3310-1 came with 72 primary main jet (PMJ) and 76 secondary main jets. 10.5 Power Valve in the primary and 8.5 PV in the secondary.
Looking down the carb you may notice tabs sticking out from some of the boosters. If so, those are to direct fuel for the most even distribution on a factory 396 cid Chevy (such as came in a '65 Chevelle).
Anyway, if the 66 PMJs are too restrictive, the engine will surge and try to die when cruising at interstate speeds. If so, go up at least two sizes.

Projectile Dart said:
It has been running far too rich and I've noticed a "hiccup" or "stutter" at part throttle cruise and the car has been running far too rich.
Click to expand...
Your RV 360 has stronger (higher) vacuum at idle than that hot chevy engine. So what may be happening is the stronger vacuum is resulting in more fuel for the same amount of air flow in low throttle positions. The 'idle' circuit supplies fuel at all low throttle positions. Cruising at 35 mph or 45 mph take very little power and so a low throttle position.
 
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