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My 1968 289 is running like total crap in idle, shaking, low rpm, struggling to run, etc... but ONLY when it is warmed up and the choke is open. I hooked up a vacuum gauge and it was hardly pulling anything; so i have almost no vacuum. and when i manually close the choke while it is running, it picks up rpm and smooths out a bunch, meaning lean mixture (vacuum leak). the only thing is that i cant hear or detect the source of the leak because there aren't any obvious hissing/sucking sounds at all. does anyone know where the leak could be coming from, keeping in mind that i recently installed a new carb/baseplate (stock 2 barrel equipment) and sealed the heck out of them? any help is much appreciated.
 

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Not all vacuum leaks can be heard. You're going to have to check all your vacuum lines, make sure that they're all connected and not brittle or cracked. Take some carb cleaner, and spray it around the base of the carb also the edges all the way around the intake. Make sure you do that first, reason being it could be something as simple as a loose bolt on the carb or intake. If you don't find it there, then check your vacuum lines, Mike. :bigthumbsup:bigthumbsup
 

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Many a person has missed plugging a vacuum port when installing a new carb, including me. Double check that you installed both gaskets.
 

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'68 289- Massive vacuum leak, but i cant hear it...

Hello themustangkid,
As advised spray some carb cleaner around the base of the carb and see if the rpm picks up. Do the same for the intake manifold.

It is most likely the result of changing the carb

Start simple -
1. - Check the 4 nuts holding the carb to the intake
2. - Check for open vac port

I am curious - what did you seal the carb base with????

You should only need gaskets and no sealer there.

Did the base-plate have a tube to run the PVC? Perhaps you have a port on the carb itself for the PVC and you may have an extra on the base plate. If that is the case - - I could cap the on eon the carb.

Just so you know - - - the reason you probably can't hear the sucking or whistling is the hole is probably quite large like an open port.
When you hear the sucking and such - - it is most often caused by a tiny leak. You hear the suction because the hole is smaller and working hard to suck. If it is large - there will be no sounds most of the time

Let us know about the "sealer"

Print Dad
 

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I had a practically microscopic leak in my 351's carb base gasket once. It was so tiny that it really made no noticeable difference in performance, but at a certain engine speed and temperature, it would whistle very loudly. Very embarrassing!
 

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Discussion Starter #6
print dad,

I just used some generic gasket sealer from my local auto parts store. i just ran a bead around where metal touched gasket (intake to gasket, gasket to base plate, base plate to gasket, gasket to carb) the base plate has a hole for pcv which i hooked up, as far as i can tell, the only opening on the carb that isnt plugged is the 'fresh air' tube which i cant figure out how to install. its about a 1/8 hole, would that be enough to do it? also, i should probably replace a couple of my vacuum hoses because they've seen better days.
 

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'68 289- Massive vacuum leak, but i cant hear it...

Hello themustangkid,
Do you know if this is a "smog" Holley carb? I am thinking the tube you refer to is on the front of the bowl and just ahead of the choke housing.

To be honest - -I don't think that is the issue. You said you had practically no vacuum.
This would indicate a major leak.

If possible could you post a picture of the carb?

You should be pulling about 18 on a vac gauge at idle.

You must have a major leak there. Double and triple check that all hoses are good
and that all ports are capped.

After spraying with carb cleaner - - if you still have the issue, I think you may have to pull the spacer and look at it.

Something is not right there.

Be sure to check the "tree" on the back of the intake manifold. It feeds a tree on the firewall and could also feed power brake booster.

You have a major leak and we gotta figure where.

I bet it will turn out to be something easy to fix once we figure this out.

Let us know if you can post a few pictures.

Print Dad

Added note - I do not use any sealer under the carb. It could cause the carb to not fully seat.
I think your better off without it. Just use a good gasket
 

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plug off every vacuum line and see what happens...

Here is an idea, I once had a massive vacuum leak, and found that I had installed the base plate for the carb upside down which caused a massive vacuum leak, but I never heard a thing.
 

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68 289- Massive vacuum leak, but i cant hear it...

HelloJoes72/65 and others,
I just had to post this link to a vac leak I had last year.

Caution - -keep the volume down - -it hurts the ears

YouTube - Car Vids

v8only, I think you are on the right idea.
This must be a major leak, such as the base plate or the line feeding the power brakes if applicable. Something major is left open I think.

I just had to post the noise my Mach was making. Funny now - -but not at the time

Print Dad
 

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Yeah, that sounds a lot like that leak I was talking about on my '72! I definitely share your sentiment about the humor of it.
 

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Discussion Starter #11
YouTube - Vacuum leak- video for forum post.
like i said, it sounds is fine at cold idle but the rpms are pretty high. when it warms up, the rpms are too low and it is shaking like crazy and i have to feather the gas to keep it alive when i come to a stop. also, the kickdown to first gear when i come to a stop seems like its late (automatic tranny)
 

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'68 289- Massive vacuum leak, but i cant hear it...

Hello themustangkid,
FIRST - may I say what great video. Very clean and well spoken.
You could have a future in video mechanics.

NEXT - -I am a bit confused here. PLease bear with me for a day or two.

When you hooked up the vacuum gauge where did you hook it up and what where the numbers?

The reason I ask - -is I see perhaps no real source of a leak from the video.

MAny thoughts - -
The part you couldn't think of the name attached to the distributor is called the vacuum advance. You motor has what is known as" dual" vacuum advance.
The idea was that the outer port (furthest from distributor would advance the timing and the inner port would work to retard the timing.

Just a quick little thought on that set up.
Can't tell from the video but the hoses look pretty fat for vacuum lines. PLease be sure that you are using vacuum lines.
Also on that subject - - the outer hose (away from distributor) should run the bottom of the carb.

The inner hose - -should run to a thermal port switch. It will look something like the "Tree" on the back of the intake. Not the same but it will have several hoses and ports. Make certain everything there is hooked up as well

Another comment -- the hose leading to the PVC - should be just one piece.
The center section (brownish) looks too small.

I know I may seem fussy here but you have a GREAT looking motor.

NEXT - can you do me a favor - -please count how many bolts are in the intake manifold? It looks like a see a bolt hole early in the video at about 8 seconds.
This hole should have a bolt if I am looking at it right.

LASTLY - - I am thinking perhaps the carb needs to be adjusted to a better idle.

Would you mind making a video of the car running?

I am not 100% certain you have a vacuum issue. PLease don't take any of my post the wrong way. I am trying to help get this running right.

ANother thought - I doubt it - but check to see if the 4 screws on the top of the carb are tight.

I am starting to think that you may have to tune the carb to get a better idle and this will help the vacuum.

I am thinking you may not know how to adjust the carb idle mixture screws and the idle setting. I am trying to not be wise or fresh.

I think looking at the video - this motor will run well. We gotta just diagnose a few things.

Print Dad:worship
 

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Discussion Starter #13
Print Dad,
First, thank you very much for the nice compliment, i did the video on the fly and didnt have high hopes for it.

okay, on to the diagnostics. Ever since i tightened the bolts on the carb down, it ran noticibly better, but still some shaking and slow/ fast idle disparity between drive and park.

the numbers i got on the vacuum gauge were between 5 and 8. i havent checked it since tightening down the bolts on the carb though. but its almost no vacuum at all. I hooked the vac gauge to an open port on the vacuum "tree" thing just above the thermostat housing. if you are unfamiliar with this i can take pictures.

next, the outer hose on the advance does run to the base of the carb however the inner hose runs to the tree that i mentioned above and not the one on the back of the intake you are talking about. i dont know if this makes any difference or not though.

the brownish piece you mentioned on the pcv is the previos owner's jerry rigged copper tube with the original fittings on the ends. from what i can tell it seems to be doing its job just fine. ill replace it with a factory tube eventually.

i do know how to mess with idle speed and mixture, but i have no idea what adjustments need to be made yet.

also, i counted, and there are a total of 12 bolts on the intake manifold that bolt it to the block. 6 on each side of the engine.

when i get a chance, ill do a video of the car running, ive just been really busy lately. ill shoot for tomorrow afternoon, or possibly this afternoon if you happen to be on the east coast. :)
 

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'68 289- Massive vacuum leak, but i cant hear it...

Hello themustangkid,

I like to make lists - it keep smy thoughts organized so bear with me.

1. - The inner hose of the vacuum advance runs to a "thermal switch" that is located at the front of the intake like you said. So I assume things are great there

2. - The fresh air tube is not an issue just yet. It will not affect vacuum.

3. - I look forward to seeing some more video's or pictures.


Looking at things - -I am now feeling like the issue could be the vacuum modulator on the transmission. Follow the hose down and unplug it from the "pot" that it attaches to on the pass side of the tranmission. If you pull the hose - - see if there is liquid there. If there is the modulator is shot.

4. - Another idea is to unplug the transmission line at the intake. Cap the little port and check the vacuum. You should be pulling at least 18.

I think this is going to be a sinple fix, once we diagnose a little more.

But looking at things on the motor - it sure looks like everything is GREAT.

I look forward to ehlping more.

Print DAd
 

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Discussion Starter #15
Print Dad,

So i spent a fair amount of time today messing around under the hood. i taped all of the important stuff, hopefully some of it will help. i found the results to be very surprising. the videos are just meshed together in chronological order.

also, i would just like to thank you so much for the awesome help on this problem so far. Its rare to come by helpful and genuine people any more, except for on this forum of course :bigthumbsup

YouTube - ‪Vacuum Leak Diagnosis-1968 Mustang‬‎
 

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I can answer one of your questions. You won't read any vacuum off an Autolite 2100 carburetor base at idle because it is ported vacuum. This means the vacuum orifice inside the carburetor is physically located above the throttle plates at idle, so there is no vacuum advance at idle. This was done for emissions purposes. However, the fact that you found such improved response while driving with the distributor connected directly to manifold vacuum leads me to believe yo may have blockage some where.
Try this: Hook up the vacuum gauge to the vacuum hose where it connects to the distributor (advance side). At idle, you should still have nothing. Then advance the throttle past idle (1000-1500 should be plenty) and see if you get a reading. (You should now.) If not, your vacuum advance may be restricted at some point.
 

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Discussion Starter #17
great, when i get a chance ill test that, but i may not need to since i found this vacuum diagram. the reason i am not getting any vacuum at the thing above the thermostat is because it isnt a source of vacuum, it is a switch. i didnt have any hoses connecting to it, therefore i wasnt getting any advance (or retardation?) because of no vacuum :smartass:.
here is the pic i was talking about: vacuumhose.jpg
 

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Update: I just got new vacuum hose and plugs and set up the system exactly the way it should be set up, and im reading a hard earned 18 on my vacuum gauge. after tuning the idle speed and the mixture, it is running like a million bucks. thanks so much everyone for helping me work through this. hopefully ill be able to return the favor sometime!
 

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68 289- Massive vacuum leak, but i cant hear it...

Hello themustangkid,
You are the MAN - -great work. You are right this was a hard earned 18.

Well done, I can't tell you how glad I am that you got things running better.

I am curious - - was the vacuum line to the distributor a little too fat?

This is just great news. Once you get things going a bit more, perhaps we can focus
on getting a little better idle.

Have uoi checked the plugs and the gap etc? I am also wondering about the point dwell or gap.

The car sounded pretty darn good. I think we may be able to fix things up even more.

You have a great looking motor and you sure make GREAT videos.

it was a pleasure to have helped if only in a small way.

Can't wait to hear what the next project is.

Just take things slow and easy and try to investigate issues like you have done.

Great news - - keep us updated please

Print DAd
 

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Print Dad,

The vacuum line to the advance was particularly thick, but the inner diameter was just fine actually.

the idle did improve significantly with the new vacuum lines, but it could always use some fine tuning, ill try to put up another video soon with what ive done.

I checked and gapped the plugs about 3 months ago so they should be fine. and there arent any points to gap because i recently switched over to a pertronix ignitor, which was leaps and bounds better than my rusty old set of points. I also have new pertronix coil and plug wires, so thankfully, i dont have to worry about any ignition components for a while :rollgrin:.

Thanks again for all the help, you definitely helped me in more than just a small way; you just helped improve my entire driving experience with this little fix (and im sure power and mileage are going to see improvements as well).

I'll keep you posted!

-themustangkid
 
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