Help Vacuum Leak Possibly carb Spacer

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M

Machman

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My Car
1971 Mach 1
Hi Guys ;

This is really driving me krazy , I just had a friend over who knows engines well and this had him scratching his head.

We both indepenently think its a possible vacuum leak but not obvious where if this proves correct.

Car is a 1971 w/ a 302 Motor & Auto transmission Carb is a motocraft 2100

When I got the car it was sitting for a while it had a filthy carb w/ lots of sludge in it a couple of tablespoons of rust chunks in the carb from the gas tank , rotton and mis routed vacuum lines , crappy gaskets , and lots of other stuff.

In that state with all those problems it ran after I put a new gas tank and fuel pump and sender in it. When it ran I got a steady idle with the only problem was it was too fast due to the carb high speed choke setting being mis adjusted.

I said to myself great it runs well so I can do all the other work and it should run great.

I rebuilt the carb (the only differences in the carb parts in the rebuild kit was the power valve on the bottom originally had 4 small holes in it and the new one had two large oval holes on the side instead of the 4 smaller ones. The accelerator pump gasket was different as well.

I put new gaskets in the carb and spacer

I cleaned the spacer

I put a new fuel pump in it

I put new vacuum lines in it and properly routed them

I cleaned and gaped all the spark plugs which seem to be incorrect plugs for the car (motorcraft ASF 32C)

Results of all of this

A Very uneven idle with engine wanting to stall if idle is set too low. when warm if I close the choke by hand it revs higher which is opposite what it should.

Spark is strong and present on all cylinders

When I do rev it the engine sounds very strong and revs like it can win any race but idle and stalling totally sucks and car can not be driven.

I also noticed when the car is running that the fuel in the glass filter

gets all sucked down untill the filter is empty then in a second it fills up again and this cycle repeats itself as long as the car is running

I suspect whern I cleaned the carb spacer and freed up long closed off areas this allowed the vacuum leak to occur but am not sure.

Question any ideas what this can be???

Is there a spacer that I can get that will fit the motorcraft 2100 carb ??

 
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It sounds like a fuel delivery problem. The filter should never empty out. A vacuum leak may also be present, but I'd remove the line from the pump to the filter and blow it out and then test the fuel pump pressure with a gauge teed into the line

 
4 main areas of vacuum leak:

Carb itself

Carb to manifold gasket

Throttle plates

Manifold at heads.

Minor areas would be your vacuum appliances- brake booster, distributor, ram air flaps, etc.

Quick test here...if you block the top of the carburetor so it does not get air through the top, does your car still run? (Warning- don't use paper, foam, cardboard, or anything else that could get sucked into the carb. I usually use my bare hand(ring removed), but maybe someone here has a better idea...choke plus the hand might be the ticket for a 2V).

If YES, then I would suspect a vacuum leak. You should be able to block off the top of the carb and stall the car.

Second, do you have a vacuum gage? You can measure manifold vacuum at idle, which for your car should be a good 20 inches(assuming you have good compression), certainly no less then 15. I am not sure how to measure carb vacuum, it varies by carburetor.

With the vacuum gage on a manifold tee, as you block off the carb you should have a significant increase in the vacuum. (You can also do this on a good running car for comparison)

To understand what you are checking for, consider a shop vac. The shop vac has basically 3 areas where it can lose vacuum. The vacuum hose(your carb), the hose to vacuum cleaner connection(gasket), or the seal on the shop vac lid(the intake manifold).

With the vacuum on, if you leave the end of the vacuum hose open(the carb), the hose itself is at low vacuum(you know this because the hose doesn't get squished up). As you block the end of the hose(block off the carb), the vacuum in the hose increases and the hose starts to collapse.

If you have a hole in your vacuum hose or lid, blocking off the end would have little or no effect on the hose collapsing. If you have a vacuum leak on your car, blocking off the top of the carb will have little or no effect.

Finding the vacuum leak should also be easier with the top of the carb blocked off because as you fiddle around, if you do find plug the leak, the vacuum will go up(which you should see on the gage) and the car will stall.

You can also check your throttle plates. Did you make sure they were set to close to the proper specification?

If you have vacuum hoses, are the items receiving vacuum sealed? You can plug the vacuum lines one at a time and see if that increases manifold vacuum.

Vacuum leaks are difficult. But in my opinion, you need to begin by getting a good manifold vacuum reading and eliminate the carb as a source. I understand you rebuilt the carb, so it is probably tight. But if the carb is not pulling in air, it should also not be supplying fuel.

 
Right. What I would do is block off all accessories (brake booster, etc) with new plugs. This eliminates any outside sources of leaks.

Verify you have plugged all of the ports on the carb too.

Before you go too far with the carb... have you checked your timing and verified the firing order?

 
Right. What I would do is block off all accessories (brake booster, etc) with new plugs. This eliminates any outside sources of leaks.

Verify you have plugged all of the ports on the carb too.

Before you go too far with the carb... have you checked your timing and verified the firing order?
Click to expand...
I had originally plugged up everything to test it but noticed no change , my buddy is coming over next week w/ a propane cylinder and percision hose that he says can pinpoint any leak.

I put my hand over the choke and closed the choke plate when engine was warm and instead of stalling the revs went up pointing to a vacuum leak somewhere

I never checked the firing order but assume it is correct as the revs and acc seem good

What is the firing order for a 302 motor and which cylinder is #1 so I can check to confirm its good?



4 main areas of vacuum leak:

Carb itself

Carb to manifold gasket

Throttle plates

Manifold at heads.

Minor areas would be your vacuum appliances- brake booster, distributor, ram air flaps, etc.

Quick test here...if you block the top of the carburetor so it does not get air through the top, does your car still run? (Warning- don't use paper, foam, cardboard, or anything else that could get sucked into the carb. I usually use my bare hand(ring removed), but maybe someone here has a better idea...choke plus the hand might be the ticket for a 2V).

If YES, then I would suspect a vacuum leak. You should be able to block off the top of the carb and stall the car.

Second, do you have a vacuum gage? You can measure manifold vacuum at idle, which for your car should be a good 20 inches(assuming you have good compression), certainly no less then 15. I am not sure how to measure carb vacuum, it varies by carburetor.

With the vacuum gage on a manifold tee, as you block off the carb you should have a significant increase in the vacuum. (You can also do this on a good running car for comparison)

To understand what you are checking for, consider a shop vac. The shop vac has basically 3 areas where it can lose vacuum. The vacuum hose(your carb), the hose to vacuum cleaner connection(gasket), or the seal on the shop vac lid(the intake manifold).

With the vacuum on, if you leave the end of the vacuum hose open(the carb), the hose itself is at low vacuum(you know this because the hose doesn't get squished up). As you block the end of the hose(block off the carb), the vacuum in the hose increases and the hose starts to collapse.

If you have a hole in your vacuum hose or lid, blocking off the end would have little or no effect on the hose collapsing. If you have a vacuum leak on your car, blocking off the top of the carb will have little or no effect.

Finding the vacuum leak should also be easier with the top of the carb blocked off because as you fiddle around, if you do find plug the leak, the vacuum will go up(which you should see on the gage) and the car will stall.

You can also check your throttle plates. Did you make sure they were set to close to the proper specification?

If you have vacuum hoses, are the items receiving vacuum sealed? You can plug the vacuum lines one at a time and see if that increases manifold vacuum.

Vacuum leaks are difficult. But in my opinion, you need to begin by getting a good manifold vacuum reading and eliminate the carb as a source. I understand you rebuilt the carb, so it is probably tight. But if the carb is not pulling in air, it should also not be supplying fuel.
Click to expand...
Thank you for your help and suggestions

When I had first tested for leaks by putting my hand over the opening and closed the choke plate when engine was warm and instead of stalling the revs went up pointing to a vacuum leak somewhere

I also tried pluging up everything to test it but noticed no change ,

A friend is coming over next week w/ a vacuum gauge and a propane cylinder and percision hose that he says can pinpoint any leak.

 
Last edited by a moderator:
Before 1982, 15426378. After 1982 13726548. Number 1 is front passenger side cylinder. Passenger side, front to back is 1234. Driver side, front to back is 5678.

What happens when you adjust the two AF mixture volume screws? Does engine idle speed change?

The difference in the Power valve "windows" is not a problem. Chuck

 
When you are at idle, 800 RPM, your car is pulling in 2 cu ft per second, which is about 16 gallons of air. That is not a small amount of air.

You can take your Mustang to your local liquor store and purchase a couple of swisher sweets. Go home, and light up one of those stogies. Open the hood, block the top of the carb and blow some smoke around your carb/intake. If you have too much wind, disconnect your fan/alternator belts(won't hurt for a couple of minutes) and the air flow should be pretty calm...except where the leak is.

 
I checked ther firing order it's dead on 1 5 4 2 6 3 7 8

I took the carb off again and noticed the spacer to manifold gasket was incorrect and was too narrow to accommodate the exhaust to intake exchange opening on the manifold side which was used to exchange gases through the EGR valve. The orig gasket which was wider had this opening (for the exhaust share) and fit over the manifold opening completely while the new one ended at the manifold exhaust opening edge.

As both intake and exhaust are communal on the spacer this may explain the bad vacuum leak, I am not sure yet but hopeful and I ordered a new gasket and it should be in the store Monday.

I believe the fuel pump may be a 1977 and if so this may also explain the odd behaviour of the fuel pump mentioned above in my first post.

I got the following number off the bottom of the carb cast into the metal --- H. 26.70

The manifold had the following number ---------- 15426378

 
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From your earlier post: "I also noticed when the car is running that the fuel in the glass filter gets all sucked down untill the filter is empty then in a second it fills up again and this cycle repeats itself as long as the car is running"

I think it may be the cycle of the float bowl filling then the needle and seat closing off fuel flow until the float drops again.

H 26 70 decodes as August 26, 1970. Would be appropriate for an early production 1971 carb

NAPA lists the same pump for 76,77,and 78. I didn't check any other years.

 
The manifold had the following number ---------- 15426378

Do these numbers sound like they are from a 1977 engine??
Click to expand...
that's the firing order
Click to expand...
::tease:: Doy... I can't believe I didn't notice that , too much co on the brain::laugh::

 
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I put the new spacer to manifold gasket in and the engine is better but not perfect and still rough and un-drivable . This time when I cover the carb opening w/ my hand the idle raises slightly then the engine dies as it should.

 
Does anything change when you turn the two AF mixture screws??????

What is the timing set at???? Chuck

 
Have you vacuum gauged the car yet?

If you don't have enough vacuum, or if you have an inconsistent vacuum, that may be:

1. A sign a cylinder or two or three are not pulling enough air through the carb to properly fuel your system. Need a cylinder compression gauge. Some guys have suggested and had luck with an IR detector to measure the temperature of each cylinder. Low temp cylinders can be isolated.

2. I don't remember your first post, perhaps you have a hi performance cam that is intended to be undriveable. If you cannot confirm the cam type, you would pull off a valve cover and degree a lobe on the cam to get the lift and duration.

3. Have you confirmed a good ignition?

 
Does anything change when you turn the two AF mixture screws??????

What is the timing set at???? Chuck
Click to expand...
Hi Chuck;

When I turn the screws the engine idle change slightly but its still rough , it's better than it was from my first post but not really drivable yet. I can start it from cold w/ no problem now and also start it after it stalls which I was not able to do before.



Have you vacuum gauged the car yet?

If you don't have enough vacuum, or if you have an inconsistent vacuum, that may be:

1. A sign a cylinder or two or three are not pulling enough air through the carb to properly fuel your system. Need a cylinder compression gauge. Some guys have suggested and had luck with an IR detector to measure the temperature of each cylinder. Low temp cylinders can be isolated.

2. I don't remember your first post, perhaps you have a hi performance cam that is intended to be undriveable. If you cannot confirm the cam type, you would pull off a valve cover and degree a lobe on the cam to get the lift and duration.

3. Have you confirmed a good ignition?
Click to expand...
A buddy of mine is coming over next week w/ some misc tools , vacuum gauge , etc.

I have a laser infra red temp gun I use for my pet reptiles, what is the temp I should look for in a normal and abnormal cylinder?

Each cylinder had good spark when I grounded the wires

 
Last edited by a moderator:
verify top dead center mechanically to be sure that the outer ring on your harmonic balancer hasn't shifted and that you haven't timed the car way off. Better yet put a vacuum gauge on the engine and tell us what the needle does as you open the throttle and at idle.

 

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