Is my Vacuum too low?

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rustyswinger

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Greetings all.

Once again, I am back with a question where I have just enough knowledge to be dangerous.... and can use some assistance from the astounding bench strength here on FABO.
sorry for the long post, i want to be complete.

I have a 1974 swinger, with I believe a 1974 318... (I installed the engine, but dont have much history on it....)
I am reading a "steady" 16 inches of mercury Vacuum on my gauge. The needle vibrates some, but does not deviate more than about a quarter of a degree on the dial. (could be my crappy gauge)

This is a large improvement from when it was waggling up and down the dial as the engine surged and hunted.

I have:
* Swapped intake manifold to a factory iron 2 BBL manifold that was cleaned and inspected by a local engine rebuilding shop
* Replaced timing set with a double roller kit from Rock Auto. (probably went cheap here)
*Replaced Harmonic Balancer ( dang thing had no timing marks, I scribed one, after using a piston stop to find TDC.
* Set Initial timing at 12*BTDC ( Vacuum canister pulled and Carb port plugged)
*Set Idle speed to 750RPM
*Rebuilt and fiddled the Carter BBD carburetor.
*Installed new distributor, PVC, Plugs, Wires, Cap, Rotor.
*Started with Mixture screws 1.5 turns out. They are backed quite far out, to the point they seem wiggly and too far backed off.
*drank lots of beer, and fiddled with different idle speed/timing/mixture screws etc.



Engine seems to idle sweet, and smooth. Puts out a fair bit of black soot onto my garage floor, ( could be just till the choke opens fully) and what seems to me to be a lot of water.

No matter what I try I cant get Vacuum to raise higher than the 16 inches I am seeing on the dial. Is this a concern?

I cannot road test this tune setting, because I live in the frozen north and I am months away from ice free roads :-(
 
Why would you need more than 16” at idle?

Jake
 
what he said
but at 750 rpm and 12* initial, IMO,it should be higher. Most STOCK teeners I have worked on are up at 18", at much lower rpm and waaay less timing.
So if it idles nice, I suspect a bad gauge.
The vacuum depends a lot on the ring and valve seal, so you might want to do a compression test.

The mixture screws are usually out about 2.5 turns, but that depends on how far up the transfer slots that the throttle valves are. I suspect the 750 rpm has something to do with it.
So my question is; why 750?
At 12* initial, I would think she would idle well at 650. That 100 rpm alone, could account for the number.

But with the mixture screws almost falling out, that's messed up. I suspect a problem in the low-speed circuit, and your solution was to crank up the rpm.
 
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Not bad.. could a tad be higher but you could have some worn valves/guides that are preventing it from going higher. I'd not be too worried until you actually get to drive it. Pull the PCV valve while idling and put you finger over the open end of the valve and see if the vacuum goes up much; if not, then the PCV sounds like it is right.

I'd also attribute to the soot and water being from warmup and cold temps. The choke might be set to stay on too long, but again, drive it to see it the choke setting is right. Back it off (leaner) a bit each day until it wants bog/stumble partway through the warmup cycle, and then enrich it a bit.
 
I don't get near that amount of vacuum on mine. Mine runs a bit over 11. I think you are worrying about it too much. I'm no expert, but here's how I set mine, and I then took to a shop to have it tuned on the chassis dyno. A/F mixture was good at idle when I got there, so apparently I did ok dialing it in myself.

I started about 1.5 turns out like you did. I then adjusted mine out while watching the vacuum gauge. When I hit max vacuum, meaning it was no longer climbing, or as soon as it dropped even a little, I stopped adjusting. If vacuum doesn't climb as you are backing them out, then dial them in a bit and watch for the needle to drop on the vacuum gauge and stop there. In my experience, you want them set right at where the vacuum peaks out and no longer climbs. I had my timing light running so I could closely watch RPMs for changes while adjusting too.

Granted mine was on a 750 cfm Speed Demon with 4 corner adjusters, but the concept is the same.
 
I see no immediate problem, but will say to WATCH advancing a stock distributor if that is what you have. All stock smog era distros have a very very long slow advance curve. This means that jacking up initial will result in OVER advancing at high RPM. Learn to check total mechanical advance, which should be (rule of thumb) 32--35. Back in the day with fuel and some combos we could run up near 40 BTC

For idle screws, "give it what it wants." Set timing FIRST, get the engine completely warm, and after "roughing in" idle mixture screws for max RPM / max vacuum, diddle the speed screw to get idle where you want for RPM. Then touch up the mixture for max RPM/ vacuum, and end up "on the lean side" just a touch, just that point where the vacuum/ RPM starts to barely decay

Also keep in mind that so far as vacuum, while you are fretting about nothing, that the gauge itself might not be all that accurate.
 
I think 18-21" is a normal vacuum to have on a stock cammed idling engine.

Low vacuum can be caused by worn piston rings, lousy engine tune, wrongly installed camshaft, leaky valves or guides, just to name some.
Most issues can be found just by reading the vacuum gauge.
Vacuum gauge 'issues' Charts can be found all over the net.

My own 318 idled with 21" in Neutral and 19+" with transmission in Drive.
 
AJ/FormS said:
what he said
but at 750 rpm and 12* initial, IMO,it should be higher. Most STOCK teeners I have worked on are up at 18", at much lower rpm and waaay less timing.
So if it idles nice, I suspect a bad gauge.
The vacuum depends a lot on the ring and valve seal, so you might want to do a compression test.

The mixture screws are usually out about 2.5 turns, but that depends on how far up the transfer slots that the throttle valves are. I suspect the 750 rpm has something to do with it. So my question is; why 750?
Click to expand...
I pulled the 750 from a post here on Fabo, and I believe that was the setting recommended in the carb kit instructions.

where would you set the idle speed?
 
BigBlockMopar said:
I think 18-21" is a normal vacuum to have on a stock cammed idling engine.

Low vacuum can be caused by worn piston rings, lousy engine tune, wrongly installed camshaft, leaky valves or guides, just to name some.
Most issues can be found just by reading the vacuum gauge.
Vacuum gauge 'issues' Charts can be found all over the net.

My own 318 idled with 21" in Neutral and 19+" with transmission in Drive.
Click to expand...


I have seen the charts, and corrected most of the issues I have had, and at this point have replaced parts, and tuned to the point I cant seem to improve from where I am at. A steady needle and smooth engine at idle is where I'm at.... itching to drive it and see how it likes my settings.

The chart I read said low vacuum can be caused by initial timing being wrong, I have been up and down the bend with that distributor, and carburetor and where it is now seems to be where it wants to be. Like I said.... I don't know much about the history of this engine. I do know for sure it had aftermarket timing set on it, so for all I know it has an aftermarket Cam shaft. I wouldn't know by looking at it.

Would not be surprised if it was a high mileage engine with the appropriate wear and tear..... doesnt smoke a big blue cloud like my last 318 did at least!
 
67Dart273 said:
I see no immediate problem, but will say to WATCH advancing a stock distributor if that is what you have. All stock smog era distros have a very very long slow advance curve. This means that jacking up initial will result in OVER advancing at high RPM. Learn to check total mechanical advance, which should be (rule of thumb) 32--35. Back in the day with fuel and some combos we could run up near 40 BTC

For idle screws, "give it what it wants." Set timing FIRST, get the engine completely warm, and after "roughing in" idle mixture screws for max RPM / max vacuum, diddle the speed screw to get idle where you want for RPM. Then touch up the mixture for max RPM/ vacuum, and end up "on the lean side" just a touch, just that point where the vacuum/ RPM starts to barely decay

Also keep in mind that so far as vacuum, while you are fretting about nothing, that the gauge itself might not be all that accurate.
Click to expand...

I'm pretty sure I took screen shots of a post you made some time ago with these instructions and more....., (it had step by step instructions at least 15 different points)

I used your road map ( I think) as close as I could and it greatly helped me get to where I am. Thank you for that, it is a huge improvement from where I started!

I am fiddle farting around with it, because it is 40 below zero outside and all I can do is tinker....
 
You can set the idle speed as low as you want to,until it stalls going into gear, or you end up with oil-pressure issues. The factory numbers are for emissions readings. If she bangs when going into gear, the idle-speed is too high.
Typically, the Rs will drop 50 to 100 rpm from neutral to engaged, and typically a teener will idle at 600/550 in gear with the anemic factory timing. So with 12* it could idle even slower. So in your case, the idle speed is sortof a personal preference. But if the Transfer slot exposure gets too far out of whack, you will have driveability issues such as bogs, hesitations,and soot. With a stock cam, she should have no problem idling down to 550 in gear. If it won't, then suspect a low-speed circuit issue. And with the falling out mixture screws , Ima thinking this.
 
AJ/FormS said:
You can set the idle speed as low as you want to,until it stalls going into gear, or you end up with oil-pressure issues. The factory numbers are for emissions readings. If she bangs when going into gear, the idle-speed is too high.
Typically, the Rs will drop 50 to 100 rpm from neutral to engaged, and typically a teener will idle at 600/550 in gear with the anemic factory timing. So with 12* it could idle even slower. So in your case, the idle speed is sortof a personal preference. But if the Transfer slot exposure gets too far out of whack, you will have driveability issues such as bogs, hesitations,and soot. With a stock cam, she should have no problem idling down to 550 in gear. If it won't, then suspect a low-speed circuit issue. And with the falling out mixture screws , Ima thinking this.
Click to expand...

Appreciate all your help. at the current 750 idle speed in Park, it drops to about 625 in gear.( from memory... will confirm)

I'm going to be honest and tell you I don't know what a transfer slot is, or what adjustments I may have made to get it into, or out of whack.

Once Jack Frost relaxes a bit and I can open the garage door without freezing solid ( you are a Manitoba man, I know you understand that bit!) I will happily drop the idle speed, maybe back off the timing a bit and fiddle with the mixture screws with the vacuum gauge in place. I am "mostly" confident that my Carburetor is assembled and internally adjusted correctly. I really took my time with it, and have done a few carbs over the years, not a pro, but I can usually follow directions.


thanks again to you, and to all the patient souls out there offering advice.
 
The transfer slots are the main fuel delivery system for the low-speed circuit. The mixture screws are the idle trimmers. If they get out of sync, then you have low speed problems.
The principal ways they get out of sync are;
1) too much idle timing,which causes the idle-speed to be too high and so you close the throttle valves, and boom there goes your sync. And
2) another way is not enough idle-timing. This causes a slow idle, and in compensation you speed it up.. and boom, there goes your sync. And
3) of course vacuum leaks wreak havoc with the sync too, as do
4) plugged low-speed orifices like air-bleeds and emulsion holes, and
5) a faulty WET fuel level in the bowl..
So the first place to start, assuming your carb is in fact clean internally, is always the WET fuel level, and the T-slot sync.
The best way to set the T-slot sync is to pull the carb off, make sure it is at curb idle and stays there. Then flip it upside down and eyeball the slots under the valves. They should be about square to a little taller than wide. Set them there and adjust the mixture screws to 2 turns from lightly seated. Then verify the float level and then put it all back together. After this don't touch the carb.
Set the idle speed with ignition advance/retard. Once the speed is close, you can twiddle the mixture screws for best idle quality.
If you find the best idle quality is with some adjustment other than 1.5 to 2.5 turns out, then the transfer slot is not right. If you had to richen the mixture screws, then put them back at 2turns out, and increase the transfer slot exposure under the valves just a tiny bit . If you had to screw the mixture screws in , then the Transfers are supplying too much, so drop the valves down, just a smidge.
After you finish that, you might have to change the idle speed BY ADJUSTING THE TIMING.
If nothing seems to work; go look for a vacuum leak, or if the quality of idle is bad, then firstly check the WET fuel level in the bowl, and if that is good, then the carb has to come apart again and the low-speed circuit cleaned out.
If you find the idle timing ends up TDC to 5*Advanced, don't be surprised;that is the factory setting.
The above assumes the engine is mechanically sound,and the valve timing, is right.
 
AJ/FormS said:
The transfer slots are the main fuel delivery system for the low-speed circuit. The mixture screws are the idle trimmers. If they get out of sync, then you have low speed problems.
The principal ways they get out of sync are;
1) too much idle timing,which causes the idle-speed to be too high and so you close the throttle valves, and boom there goes your sync. And
2) another way is not enough idle-timing. This causes a slow idle, and in compensation you speed it up.. and boom, there goes your sync. And
3) of course vacuum leaks wreak havoc with the sync too, as do
4) plugged low-speed orifices like air-bleeds and emulsion holes, and
5) a faulty WET fuel level in the bowl..
So the first place to start, assuming your carb is in fact clean internally, is always the WET fuel level, and the T-slot sync.
The best way to set the T-slot sync is to pull the carb off, make sure it is at curb idle and stays there. Then flip it upside down and eyeball the slots under the valves. They should be about square to a little taller than wide. Set them there and adjust the mixture screws to 2 turns from lightly seated. Then verify the float level and then put it all back together. After this don't touch the carb.
Set the idle speed with ignition advance/retard. Once the speed is close, you can twiddle the mixture screws for best idle quality.
If you find the best idle quality is with some adjustment other than 1.5 to 2.5 turns out, then the transfer slot is not right. If you had to richen the mixture screws, then put them back at 2turns out, and increase the transfer slot exposure under the valves just a tiny bit . If you had to screw the mixture screws in , then the Transfers are supplying too much, so drop the valves down, just a smidge.
After you finish that, you might have to change the idle speed BY ADJUSTING THE TIMING.
If nothing seems to work; go look for a vacuum leak, or if the quality of idle is bad, then firstly check the WET fuel level in the bowl, and if that is good, then the carb has to come apart again and the low-speed circuit cleaned out.
If you find the idle timing ends up TDC to 5*Advanced, don't be surprised;that is the factory setting.
The above assumes the engine is mechanically sound,and the valve timing, is right.
Click to expand...
simply awesome. Thank you for this wonderful explanation.

I have this saved and will have a go at her again as soon as the weather breaks.

thank you
 
Fuel-level is , maybe not critical exactly, for the most part, but at idle it kindof is. This circuit is exceptionally sensitive, and the whole deal was designed to run at a certain WET fuel level. So you can get the other circuits to run pretty good even with a wrong level, but if it ain't real close at idle, you will have problems. And if it ain't stable, then you'll have even more problems.
I've never seen a WET fuel level for a carb like yours. The instructions in the kit,spec an upside down float height, that with the factory spec fuel-pressure, is supposed to get to the design WET level.
Jus saying.
If the WET fuel level is too high at idle, and assuming your T-slot sync is right,the engine will have an easy time of pulling the fuel up the idlewell and thru the discharge ports; maybe too easy, and the lowspeed circuit goes rich and there's nothing you can do about it.
Conversely, if the WET fuel level is too low, the engine may have a hard time pulling the fuel over, she goes lean, and the trimmers are all the way out, and you still have a rough stinky idle.
Jus saying.
These are your clues,after the T-slot sync is set, and if the trimmers won't come in at 2 turns +/- .5turn.
If you want to see how sensitive your particular carb is; just pinch the fuel supply line shut, while the engine is idling. One of three things is gonna happen;
1) the engine will continue to idle at the same rpm for a while,like maybe up to a minute, then slow down, idle rough, and finally stall, as pick-up becomes too difficult. This is normal
2) the engine will gain rpm as it stops being pig-rich, then continue as #1
3) the engine will almost immediately slow down, run rough, and stall, cuz it just can't pull up the fuel.
4) and a bazzillion degrees of in-betweens.
With every thing set right and synced up and a healthy warm engine, you should be able to reach in twist the key and almost as fast as it's done the engine is running. click-Vroom. I tell you that warms my heart every time it happens. And it always always reminds me of the little 260s in the early Falcons; click-Vroom. and maybe the 68 302s also; click-vroom. Install a mini-starter and BadaBoom, you never again have to listen to the patented Mopar; I think I can,I think I can,I think I can..

We had -39C and change in my town last weekend; BEFORE windchill lol. For our American friends;
-40C = -40F; awful daymn cold. And we won't even mention the humidity.
 
Another train wreck. Does the thing RUN good? Yes? Then fuggetaboutit.
 
RustyRatRod said:
Another train wreck. Does the thing RUN good? Yes? Then fuggetaboutit.
Click to expand...

Train wreck?

Like I mentioned it RUNS pretty good, but does it drive good? No idea, its about 40 below, and there is about 3 inches of ice on the roads, not to mention a big frigging snow drift behind the bay door the car is parked behind.

Ask me how it works once we get to a month with no "R" in the name of it.........
 
AJ/FormS said:
Fuel-level is , maybe not critical exactly, for the most part, but at idle it kindof is. This circuit is exceptionally sensitive, and the whole deal was designed to run at a certain WET fuel level. So you can get the other circuits to run pretty good even with a wrong level, but if it ain't real close at idle, you will have problems. And if it ain't stable, then you'll have even more problems.
I've never seen a WET fuel level for a carb like yours. The instructions in the kit,spec an upside down float height, that with the factory spec fuel-pressure, is supposed to get to the design WET level.
Jus saying.
If the WET fuel level is too high at idle, and assuming your T-slot sync is right,the engine will have an easy time of pulling the fuel up the idlewell and thru the discharge ports; maybe too easy, and the lowspeed circuit goes rich and there's nothing you can do about it.
Conversely, if the WET fuel level is too low, the engine may have a hard time pulling the fuel over, she goes lean, and the trimmers are all the way out, and you still have a rough stinky idle.
Jus saying.
These are your clues,after the T-slot sync is set, and if the trimmers won't come in at 2 turns +/- .5turn.
If you want to see how sensitive your particular carb is; just pinch the fuel supply line shut, while the engine is idling. One of three things is gonna happen;
1) the engine will continue to idle at the same rpm for a while,like maybe up to a minute, then slow down, idle rough, and finally stall, as pick-up becomes too difficult. This is normal
2) the engine will gain rpm as it stops being pig-rich, then continue as #1
3) the engine will almost immediately slow down, run rough, and stall, cuz it just can't pull up the fuel.
4) and a bazzillion degrees of in-betweens.
With every thing set right and synced up and a healthy warm engine, you should be able to reach in twist the key and almost as fast as it's done the engine is running. click-Vroom. I tell you that warms my heart every time it happens. And it always always reminds me of the little 260s in the early Falcons; click-Vroom. and maybe the 68 302s also; click-vroom. Install a mini-starter and BadaBoom, you never again have to listen to the patented Mopar; I think I can,I think I can,I think I can..

We had -39C and change in my town last weekend; BEFORE windchill lol. For our American friends;
-40C = -40F; awful daymn cold. And we won't even mention the humidity.
Click to expand...



Thanks again! will add the fuel line pinch test to my tinkering once the North Pole goes home.

At the risk of sounding even more like a Noob..... WET fuel level = actual float bowl fuel level???

I do have a bit of the little starter that could syndrome when the engine is cold and has been sitting a while, but its very nearly Click-vroom when fully warmed up.
 
Wet fuel level is the bowl level when the engine is running at idle; i.e., it is where the float and needled valve actually regulate the level, as opposed to setting the float by other means to some level when dry. When you look through the sight glass or port in the side of a bowl and set the level a certain way with the engine running, you are setting wet fuel level. Some other carbs have different ways to set wet fuel level, and I don't believe all carbs have provisions to do this.
 
You want more vacuum? Recurve that smog dizzy. 16inch is low for that stock cam and motor, FBO has that limiter plate easy drop in, but if your handy weld the governor slots up and measure it. More power more mileage. Trust me it's worth the time and money.
 
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Slantsix64 said:
You want more vacuum? Recurve that smog dizzy. 16inch is low for that stock cam and motor, FBO has that limiter plate easy drop in, but if your handy weld the governor slots up and measure it. More power more mileage. Trust me it's worth the time and money.
Click to expand...
I'm pretty handy with a mig welder, and have a spare distributor I dont mind fiddling with at all!
I will check into this...

thanks for the idea!
 
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