Manifold Vacuum

[Hugh Mongus]

I'm only getting 12lbs of vacuum at the manifold. Shouldn't it be higher? I have adjusted the idle screws a few times but I can't get it above 12.

[Coley]

...thats not overly high but there might be a little more at work here.
When you view the vacuum gauge is it steady?
(here is a great link to get started...scroll down the page to readings...very interesting and see how it relates to your condition...AND, note scenario 10 on the reading reference from the website.)
http://www.secondchancegarage.com/public/186.cfm
Next, how is your timing?....note where it is right now...and try both advancing it and retarding it. Both of these efforts will usually reveal a change in both vacuum and needle behavior. Also, plug the vacuum advance and recheck the readings)
The most important thing is to get the needle steady.
What is your engine rpm when you are checking.
Coley

[hamjet]

Hi, it probably should be a little higher, ( 17-22) leave the gage hooked up and move the timing back and watch the gage. see how it runs then...Does it have a hot cam in it?( a lot of duration) that would make it lower too...OOPS, Coley beat me to it....

[Hugh Mongus]

Originally Posted by Coley

...thats not overly high but there might be a little more at work here.
When you view the vacuum gauge is it steady?
(here is a great link to get started...scroll down the page to readings...very interesting and see how it relates to your condition...AND, note scenario 10 on the reading reference from the website.)
http://www.secondchancegarage.com/public/186.cfm
Next, how is your timing?....note where it is right now...and try both advancing it and retarding it. Both of these efforts will usually reveal a change in both vacuum and needle behavior. Also, plug the vacuum advance and recheck the readings)
The most important thing is to get the needle steady.
What is your engine rpm when you are checking.
Coley

Needle is steady at 12. The needle bounces anywhere else on the gauge. My timing is currently set at 16 degrees. My engine speed at idle is around 750-800. This engine was in it when I bought it. I don't know what kind of internals it has. When I was checking the vacuum I had unplugged the vacuum advance and used that port on the carb for the gauge.

[Coley]

...Ok...something in that is not quite correct Jason.
The vacuum port on the carb should NOT be providing any vacuum signal at idle....only at higher rpms...definitely not at 750/800 rpm. So it should not be useable for attaching a vacuum gauge to. So something on that vacuum circuit is incorrect.
Separately, you need to find a different port ideally for checking this.
We don't have any info on your motor or your carb...or any pics. Can you supply us with any of these?
Also, was that timing set with the vacuum advance line connected to the distributor? ....or not?
Coley

[Hugh Mongus]

Originally Posted by Coley

...Ok...something in that is not quite correct Jason.
The vacuum port on the carb should NOT be providing any vacuum signal at idle....only at higher rpms...definitely not at 750/800 rpm. So it should not be useable for attaching a vacuum gauge to. So something on that vacuum circuit is incorrect.
Separately, you need to find a different port ideally for checking this.
We don't have any info on your motor or your carb...or any pics. Can you supply us with any of these?
Also, was that timing set with the vacuum advance line connected to the distributor? ....or not?
Coley

I'm using the manifold vacuum port on the carb(DS), not the ported one(PS). I set my timing with the advance unplugged and the vacuum port plugged.

Motor is a 350 bored over .030. Supposedly rebuilt around 9k miles ago. Has HEI, Double hump 2.02 heads (1492s), Edelbrock RPM intake, and 1406 Edelbrock carb. That is the extent of my knowledge of the engine. I can post up a pic later when I get home if it would help.

[Coley]

Originally Posted by Hugh Mongus

I'm using the manifold vacuum port on the carb(DS), not the ported one(PS). I set my timing with the advance unplugged and the vacuum port plugged.

Motor is a 350 bored over .030. Supposedly rebuilt around 9k miles ago. Has HEI, Double hump 2.02 heads (1492s), Edelbrock RPM intake, and 1406 Edelbrock carb. That is the extent of my knowledge of the engine. I can post up a pic later when I get home if it would help.

Well...it looks like with the engine and some of the upgrades on it....sounds like you might have a cam too.
If so, and as Hamjet mentioned....a lower vacuum signal is to be expected.
How does the vehicle run? Is it running poorly? or?

C.

[Hugh Mongus]

Runs like a scalded cat.

My last truck had a lopey cam in it. This one doesn't sound like that. So I'm not really sure what it has in it.

Thanks Coley

[BMERDOC]

Can you bring the idle down under 700 and reset the idle mixture? Should be worth a couple of in. Hg.

[Hugh Mongus]

I can. It seems like if I mess with the idle screws the needle will just flutter until I adjust them back to basically where they are now. The needle is only steady at 12. Once I get the fuel pressure regulator installed tonight. I will try to reset the idle again.

[sKnNyPpY]

Check vacuum at the manifold itself where the brakes or transmission vacuum hook up.

[D.PASSMORE]

Originally Posted by sKnNyPpY

Check vacuum at the manifold itself where the brakes or transmission vacuum hook up.

x2 - Right, that's the place to get the most accurate reading.

[hugger6933]

Hugh, when you put in the cam[ you did go with the comp didn't you] did you line the timing marks up? If I remember you were talking about a 268 comp cam and they are know for making real good vac. usually in the 18 inch range.

[RichardJ]

As BMERDOC said, adjust the idle SPEED screw so that the idle is BELOW 700 rpm. 800 rpm is not a idle rpm. The throttle plates need to be closed far enough so that the mixture screw can control the fuel flowing from the idle port.



When the throttle plates are too far open. you are adding fuel from the transition ports above the idle ports and the idle mixture screws become less able to effectively control the fuel flow.

[Hugh Mongus]

Originally Posted by hugger6933

Hugh, when you put in the cam[ you did go with the comp didn't you] did you line the timing marks up? If I remember you were talking about a 268 comp cam and they are know for making real good vac. usually in the 18 inch range.

Jim, I haven't changed the cam yet. I am trying to get the engine smoothed out before i make any major modifications.

[Hugh Mongus]

I did not get a chance to do anything to my truck last night. I got the first phone call from my realtor, that is selling our old house, saying we had a water leak in the attic and there was a hole in the ceiling of the garage. After I figured out what caused that, I got the second call from my buddy (Ford owner, poor guy). He was stuck in the middle of town with a flat on his new F150. The wheel was seized onto the hub and would not come off. Long story short - I was out from the time I got home from work until about 9PM last night. I hope tonight will be a little more productive.

[Hugh Mongus]

Originally Posted by RichardJ

As BMERDOC said, adjust the idle SPEED screw so that the idle is BELOW 700 rpm. 800 rpm is not a idle rpm. The throttle plates need to be closed far enough so that the mixture screw can control the fuel flowing from the idle port.

When the throttle plates are too far open. you are adding fuel from the transition ports above the idle ports and the idle mixture screws become less able to effectively control the fuel flow.

Richard, I have a feeling my fuel pressure is the root of my problem. Since my carb recommends 5psi of fuel pressure and my stock pump is pushing 8-11psi, I have to adjust the idle a little higher to keep it idling.

I bought a pressure regulator yesterday with every intention of installing it last night but it didn't happen. I hope when I get it installed tonight and lower the line pressure I will be able to lower the idle speed then adjust the idle mixture screws again and get the vacuum up.

[geezer#99]

Fwiw 12 in of vacuum is plenty good as long as your power brakes work (if you have them).
Best to set your carb with the vac advance plugged in. THat way you can lower your idle as much as possible. You can buy vacuum tees at the parts store.
A vac source on the carb is exactly the same as on the manifold.
Here's a link to some vac tees. Not hard to find.
http://shop.advanceautoparts.com/p/d...354/22147871-p

[Hugh Mongus]

Geezer, my brakes work fine. I was just concerned that lower vacuum could affect the overall performance of the engine.

[Lattimer]

I have a fairly stout 355 crate motor, and it has a moderate high perf cam. Best I can get is 15" of vacuum at 700 rpm with timing at 14---thats what it seems to like best. With a little more cam I could easily see 12" being normal.

Just FYI, if you get below about 10" at cruise, you will want different rod springs in the carb or you will be running rich all the time.

[Hugh Mongus]

I have a feeling once I stop water-boarding my carb my vacuum will improve.

[Lattimer]

Originally Posted by Hugh Mongus

I have a feeling once I stop water-boarding my carb my vacuum will improve.

Possibly. I ran an Eddy 1406 for a while, and even with a regulator I could never get it to idle well. I gave up and bought a new carb and all my issues went away. I think the carb was just in bad shape (it came on the truck).

I have read that the higher feed pressure will overwhelm the float needle and push extra fuel, creating a rich condition, which can cause you to increase the idle setting and lean out the idle screws to get it all to work. That can through you way out of whack and make you chase your tail.

[piecesparts]

Things that I would do to get this adjusted. 1. get your fuel pressure where you want it. 2. check your cylinders for compression---to see if you have a valve sticking issue. 3. Adjust the carb idle settings by using a vacuum gauge to get the optimum setting for your combination.

However I run about 12 to 13 on my stroker engine, but I have a fairly active Comp cam setup on mine and the idle indicates a fair lope.

[Hugh Mongus]

I have already checked the compression on the engine. The cylinders average 150lbs. All within the acceptable differences of each other.

It would be easier if I knew what was inside the engine. I would know better what to expect from it when trying to tune it.

[D.PASSMORE]

If you need to check the condition of cylinders and valves. You can do a leak down test with a leak down tester. That will let you know if you have a problem or you can eliminate the combustiom chambers.
I had to troubleshoot some some issues with my motor: mild stumble, hard start... I used a direct manifold vacumn source. Eliminated / plugged of brake booster. My issues ending up being late timing and a lean running carb.
Being a novice mechanic, The problem I had with testing from carb was knowing which was ported or manifold vacumn and my readings seamed to be always erratic.