Very low vacuum - what do I do?

[Print Dad 58]

Hello all,

I am having major trouble. I will try to be brief but as complete as possible.

Car is 70 Mach 1 with 351 cleveland. Car sat for 23 years but I prepped the motor and got her going.

Now I have 5.5 inches of vacuum at idle.

 

Before putting the car on the road about 2 months ago, I did a dry compression test. All cylinders where close to 165 so I left it alone.

 

I put in a new gas tank, fuel pump, blew out the lines, replaced all rubber hoses. On the ignition side, I installed new points rotor, condensor, cap, wires, plugs. I capped all ports. I disconnected the retard portion of the distributor and am running just 1 hose to the advance portion. I have tried manifold and ported sources for the advance.

 

The car has been giving me trouble with what I thought was carb issues. I have tried 3 new Holley's in the last 3 weeks.

Presently on it is a new 600 Holley smog carb with adjustable bowls and sights.

 

I checked and re-checked the timing, dwell. Today I pulled 3 plugs and checked the compression. They were the same as my first full test at 165.

 

I had never used a vac guage but hooked one onto the manifold port between the carb and distributor. At idle (800rpm's) I get a steady 5.

If I race the motor and rev to about 2000 the vac goes to perhaps 12.

 

I double and triple checked all vacuum ports and they are all capped with small plastic caps. I am sure I didn't miss any.

 

Next i sprayed starter fluid all along the edge of the intake in the hopes that the motor would speed up. Perhaps I should have been watching the vac gauge. I thought the rpm's would raise if the starter fluid found a leak.

 

I know this problem could be the intake gasket, but why didn't the starter fluid make the rpm's raise? I would feel much better pulling the intake if I thought that was pretty sure to be the trouble.

 

As for a bit more info, the oil looks very clean on the stick and no smell or foam. There are no bubbles in the radiator.

 

I am so lost....what do I do?

 

Sorry for the lengthy post but I need ideas........Print Dad

[S code 69 13]

First thing is cover the carb slowly with a rag while it's at an idle and see if it speeds up or dies, then find the leak if it speeds up, vac booster is a good place to look if you have power brakes.

 

If it's not a leak then check your idle mixture and play with the timing a little, don't pay much attention to the factory specs that's just a suggestion not an actual best setting. if all this fails your guides are probably shot or something more along those lines.

[Print Dad 58]

Hello S code 69,

THanks for the help.

I will try the rag on the carb.

 

The car uses no vacuum except for the advance on the dizzy.

No power booster nothing. All ports are capped with new caps.

 

It seems there must be a leak somewhere, but why doesn't the starter fluid cause the rpm's to go up? I was hoping when I spray..say the front of the intake, that the motor would rev up a bit., This would let me know the leak is there in the front for example.

 

Several people have said to replace the intake gasket. I would do this in a second if I understood why the motor doesn't race with starter fluid.

 

My fear is I will change the intake gasket and if not solved will have to remove heads.

 

The car seems to run too strong to have valve issues. I checked and re-checked the compression and it is good and consistent. Never pops thru carb.

 

I am stumped. I may try to do the intake this week-end, just wish i had a better feeling about the source of the leak.

 

I'll through this out there - - Any chance to intake (stock Ford) could have become porous or something weird? I know this is way out there but I am stumped.

 

Thanks again...... Print Dad

[SlimeGold 69 15]

Real big cam? Do you know the specs??? Big cams= less vac, rough idle etc.

Edited September 18, 2009 by SlimeGold 69

[Print Dad 58]

Hello SlimeGold,

Sorry I should posted.... stock cam.

The motor is all stock except for Holley 600 vacuum secondary. carb.

It is running old style (points distrib). Dwell perfect.

Timming...initial 10BTC - Has always run fine there.

 

I am stumped for sure..Thanks...... Print Dad

[LoneWolf2U 136]

Spraying starting fluid will only raise the rpm if it finds a direct leak, not a seeping leak.

Before you pull the intake off, pull out ALL the plugs and look them over.

is one or two of them oil soaked? This is a godd indication of the gasket leaking from the valley area into the intake ports.

 

If your plugs are all tanish colored, I would try tuning the carb mixture screws to raise the vac reading along with timing changes to 16 bftdc or till you do not have a pinging problem.

 

As for your other post, you can remove the intake with the dist in, but it is a bear to replace it and not have the gaskets slding around.

I pull mine out and use 4- 5/16 all thread rods in the center holes for guides to get ti straight down on the gaskets.

[SlimeGold 69 15]

Hello SlimeGold,

Sorry I should posted.... stock cam.

The motor is all stock except for Holley 600 vacuum secondary. carb.

It is running old style (points distrib). Dwell perfect.

Timming...initial 10BTC - Has always run fine there.

 

I am stumped for sure..Thanks...... Print Dad

 

OK. You should have plenty of vac then. Could be the intake gaskets but I would try and rule out any other causes first. Does it have a carb spacer with a EGR valve? Those kind of spacers can cause some major leaks. Do you have a PCV valve? Have you checked for a good seal between the carb and intake? Does it have a factory cast iron 4V intake or a aftermarket aluminum? Sorry for the long list of questions, just getting a idea about your set-up.

Edited September 19, 2009 by SlimeGold 69

[Print Dad 58]

Hello all,

Thanks for helping. It is appreciated.

The intake is cast (stock). There is no smog stuff EGR or anything just a PVC line. The spacer is a 1" phenolic.

I changed the PVC just to be sure it was OK.

 

I ordered an intake set today and will start pulling it apart Sat.

 

I feel confident there is a leak under the intake.

 

I pulled plugs, they look good. I re-did the compression test and Numbers were great.

 

I'll keep you people all up to date.

 

Once again, I thank you all. As a note I too use the rods to align and if the dizzy is in the way...out she comes.

 

Thanks....... Print Dad

[S code 69 13]

As Mach1 says if you have a manifold leak bad enough to drop your vacuum that low you're going to run lean, if not melt down. I would spend some time ruling everything else out before I screwed with the intake.

 

Are you sure your gauge is good and it really is that low? to be that low I'd think you should hear the leak if it is a leak. what happens to your vac reading when you rev it up and then close the plates back down fast?

[Print Dad 58]

Hello All,

I am really stuck here. Getting strong opinions of what is wrong.

I have sent PM to several members in hopes I can figure this out.

Sorry for posting but this is where I am.

 

My friend who teaches automotive in my school came by to hear the car.

The covered the carb throat with his hands and the car would stall.

Next he unplugged the hose that goes to the PVC and covered the carb throat.

The car stayed running. To him there is no intake leak.

 

It doesn't explain why the low reading. I think I am going to buy another guage to be sure of my numbers. I borrowed the guage and it is pretty new.

 

My friend did hear a little miss and thinks perhaps a cracked plug.

 

I was almost certain the intake manifold gasket was leaking underneath. I ordered a new set last night.

 

The car is running rich even though the carb is adjusted.

 

The car seems great under load. No smoke ever out the exhaust.

OIl looks great. the points are right on as is the timing.

I just don't know what to do. I was planning to put on the gasket set but don't know what to do. The compression numbers (dry) were all very good.

I didn't do a leak down test, never done one. The car seems to run too good to have a valve issue.

 

Help...I am stuck.

[LoneWolf2U 136]

If you think the intake is leaking from underneath, plug the pvc hose and Hook your vac guage onto the dipstick tube and run the motor rpms up to 2500.

If it has a leak from the vally area your guage will rise.

 

My thoughts are timing could be reset for better vac readings. the factory setting are best used for getting it in a running state and then fine tuning it to what the motor likes best.

The book on mine calls for 6* btdc and it runs much better with it set at 12*

The C motors are a breed all there own and need to be tuned as such.

[S code 69 13]

Running rich will lower your vacuum, so will retarded ignition, take care of that stuff beforer you try anything radical.

 

You can have bad guides and or poor valves and still run ok, if you can't tune it out that would be my next area to look at.

[SlimeGold 69 15]

Other than a guage telling you that the vac is low, how does it run overall? Does it idle good? Stuff like this is near imposible figure out over the net. If the engine has set for 20 years like you said, many things could be the problem. Bad valve seal either caused by seats/guides, it could have jumped time, etc, etc. You could do a leak down test and might get a better idea of the condition of the valves. Has the engine ever been rebuilt in the past? If so,was it by you?

 

Also I noticed you said you hooked the guage between the vacuum port and vacuum advance on the dizzy. Did you tee into this line? You need to hook the guage up to a manifold port. Have you checked the vacuum advance and made sure the diaphram inside is not leaking? Like others said tune will effect vac. So it's going to be a case of trouble shooting many possible causes, and it may turn out to be several things causing problems.

Edited September 19, 2009 by SlimeGold 69

[Print Dad 58]

Hello all,

Here is where I am at. I bought a new vacuum guage just to be sure.

The guage reads about 6. It is pretty darn steady.

 

Fro some info and clarifications:

1. - The motor has never been rebuilt but many years ago I replaced the lifters.

2. - Rockers have fulcrum, positive stop.

3. - Distributor advance is working and no leaks

4. - When I checked the vacuum I went from several ports. One was the port on the intake, between the carb and dizzy. It is manifold vac. To be sure, I also tried a line on the back of the intake where the PVC hooks up. There is a fitting that sits up about an inch and has a 1/2'port for the PVC and a smaller fitting that was capped.

 

5. - When I was trying things for the surging, I used manifold vac and ported vac. to the dizzy. I have capped everything else and have capped the retard portion of the dizzy.

 

The only 2 things using vacuum now are, 1 line to the advance and the large hose to the PVC which I just replaced.

 

The timming is where it always has been which I run at 10BTDC. The car always ran well there.

 

Overall the car runs great. The problem first started showing up on gentle coasting. The car would buck. If I was in first gear going 15 and took my foot off the gas, the car would soon start to surge or buck.

This was the first signs of trouble. From there I started to have a little idle problem.

When I set the carb (idle mixture screws - Holley 600 vac sec) the settings don't seem to hold. It is hard to explain. if I start 1.5 turns out and adjust 1/8 turn on both sides, i get the car close. Then in a few moments the car will idle rough again.

The carb is new and is my third in 3 weeks.

 

Dwell is perfect. I know this motor and I know when she is running right. This motor has a problem.

 

The car seems to run strong and no popping ever.

I forgot to try the guage in the dipstick tube.

I bought another set of plugs and plan to put them in tomorrow just in case. Also I will check the condition of the plugs that come out. They only have about 250 miles tops.

 

Because i have never done a leak down test...I am planning to replace the intake gaskets and change the plugs. if I still have trouble I will have someone help me do a leak down test.

To have such low vacuum I would think the motor would be rougher at idle if i had valve issues. I don't know. I wish i could explain how the car feels strong but not right.

 

Thanks for all the tips and ideas. This has me stumped but not totally beaten yet.

 

As Slime suggests it could be many things causing the problem but I would like to think I have checked many of them. I started from scratch and re-checked the F.O., then I checked the initial timing. Today I did another compression test and get 165-170 in all.

I checked dwell and am running 28 which was always good for her. All vac ports are capped with new caps. I have done a lot of re-checking and am just spinning my wheels.

 

I'll keep you all updated. Thanks I truly appreciate the help. Print Dad

[jimmystang 13]

A leaky booster will change the vacuum reading

[buening 63]

Is this an auto or stick car?

 

I'm not to familiar with Clevelands, but isn't there a port towards the back of the intake on the drivers intake runner that feeds the vacuum tree on the firewall? If this is indeed a 70 car, all cars that year had the vacuum tree on the firewall directly behind the motor.

[Jsams 10]

Bump the timing to 12 or 14 degrees and make sure the distributor is connected to manifold vacuum.

Generally if a cleveland intake gasket has a leak, it will smoke like hell out the exhaust.

What is your fuel pressure? Sometimes the bypass in the fuel pump will stick and you will have high fuel pressure that can cause a rich condition.

Also the balancer ring can slip giving a false timing reading. You might want to try to bump the timing up until the motor is hard to start, and then back off a little.

Edited December 30, 2009 by Jsams

[Max Power 74]

Some ideas?

 

http://www.gregsengine.com/vacuum.htm

[69cobraman 10]

print dad,

You should have between 14 to 18 (I believe) inches of vacuum. I know there are several ways to check for vacuum leaks. I will try to give a link to one if I can. http://www.crankshaftcoalition.com/wiki/Finding_vacuum_leaks

[Print Dad 58]

Hello All,

First Max Power, thanks for the great link. I have bookmarked it.

 

And to all the other recent responders Thanks.

 

The problem is fixed. I am not certain how this thread started back to life but I should have responded with a cure.

 

I had a 2 part problem at least I did 2 things to fix the trouble. The car is a standard and the vacuum tree on the firewall is unused.

 

What I had to do was

1. - Replaced the intake manifold gasket and valley pan.

2. - Used a thicker, different gasket for the carburetor. The gasket I used was for a 1984? F-250 Ford with a 351. This gasket is about 1/4 inch thick and has 4 holes.

 

One of the troubles I had was the carb was sucking exhaust from the cross-over channel on the stock intake. I think the spacer I had put on was part of the problem as well. The spacer was a 1 inch phenolic (plastic like) spacer but was open at the bottom. This allowed exhaust to get sucked from the channel into the carb.

 

In any case...I changed the intake gaskets and the base gasket and now have a steady vacuum of 18 at idle. The car is running great and now it is on the heater core. This is in another post and I think I may be OK on that as well.

 

 

Thanks to all........ Happy New year Print Dad

[danno 128]

Go for the intake manifold gasket, That sounds like the only thing left. It can be leaking from the underside and starting fluid will never get to the leak. I am not sure if this is like a Windsor, but the bottom of the intake with them is where the lifters are, the same atmosphere as the crank. Maybe pull the dipstick and see if there is a huge vacuum on it? If there is, the vacuum from the manifold could be leaking into the crank and lifters.

[foothilltom 33]

Hi Jim, I'm relatively certain I'm in the weeds here, but can you explain exactly where you are putting your Vac gauge? If the gauge is put "in front" of the throttle, you'll only read 0-5 inches on your gauge -- depending on your idle RPMs. If the gauge is "after" the throttle (on the manifold port, say, that connects to your brake power booster), you should be reading a good 20 inches or more depending on stuff.

 

In short, are you quite certain you're "after" the throttle?

 

Also, I've re-read your post and I didn't comprehend what your actual symptoms are other than running rich. Obviously a low vac reading is bad, but what physical symptom are you dealing with?

 

If you're running rich and can't stall out the engine by turning in the mixture screws, perhaps you need to decrease the jets? Have you been using the same jets with all 3 carbs?

 

Anywho, if you can clarify where you're testing vacuum (a picture perhaps?), maybe there's a simpler answer, but I'm rarely right on engine stuff.

 

Tom

[Print Dad 58]

Hello Tom and the others,

The problem is fixed. I am not sure how this thread came back to life but the problem is solved.

 

I will explain what happened in the hope it may help others.

 

The car had sat for a long time and I did a lot of prep work to get her going.

With a new fuel system - tank/lines/pump/carb etc. the car had a hesitation and

would not coast. I surged badly on downshifts.

 

I first thought I had a carb issue so I rebuilt the carb. Then after the rebuild a little different but still not good. I bought a new carb, but it looked like a return. Didn't look new in box. I eventually put on Carb #3 - (another holley) and trying to adjust the idle circuit, I couldn't get the motor to die with the idle screw in on the driver side.

 

This lead me to try a vacuum test. I bought a vac gauge and tried the vacuum. I had a steady 5 no matter where I attached the gage. I hooked it up to several ports on the intake and I used the "T" on the back of the intake.

After many attempts I realized there was a vacuum leak but it was leaking under the intake. I had sprayed the area around the base of the carb and around the intake with no signs of a problem. It was then that many people on the forum told me the problem was a leak internally.

 

So the problem was a combination of intake manifold and the carb base gasket.

 

On 4 barrel stock clevland intakes there is a cross-over channel that take warm exhaust gas from one head up into and across the intake manifold and out to the other head. The intake has a cross over channel that runs across the intake where the carb sits. I had mistakenly put on a spacer but the spacer had one large center hole instead of the required 4 holes. Because the spacer had one large hole, a small portion of the cross over channel was in the spacer opening. You had to look pretty hard to see the fact that the channel was going into the center opening of the spacer.

 

Botton line - -the car was sucking exhaust into the motor from the channel.

 

The solution was a base gasket with 4 holes instead of the one big opening. It is about 1/4 inch thick.

 

After replacing the intake and the base gasket, the car has about 18-19 inches of vacuum at idle and she is running great.

 

I hope this helps others and if anyone has a problem like this...send me a PM as I sure learned a lot from several members here.

 

 

Thanks to all. = - - - - Print Dad (Jim)