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Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 02-02-2011   #1 (permalink)
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Default 1968 289 Running Rough, Decreased Power, Low Vacuum. Ideas?

Just a few days after I fixed my distributor problem and got my timing set to 10 BTDC, my pony started having problems again. The symptoms: running rough, decreased power, and low vacuum.

What I've already checked while diagnosing the problem: timing and mechanical advance (since I worked on those recently), vacuum advance, spark plugs, idle speed, fuel/air mixture.

My first thought was the ignition module, since I just put a Duraspark and module from Pull A Part. I thought maybe it was starting to give out (I know electronic issues typically fail completely or not at all, but it was worth a shot). I was given an HEI module to use down the road, so it cost me nothing to swap that in and see if it helped, but the problem persists.

My next thought was fuel/air mixture. It felt like it was either running a bit rich or a bit lean when I gave it gas; it has some hesitation along with running rough and decreased power when I hit the gas. So I played with the fuel/air mixture (it's a Holley 2bbl) while I had a vacuum gauge hooked up. At first, the vacuum was down as low as 10", and it wasn't sounding too good. Now I've read that to adjust fuel/air mixture you turn the screws all the way in, then back out 1-1/2 turns. But when I turned the screws all the way in, before backing out 1-1/2 turns, the vacuum jumped up to around 15" (see picture) and the engine sounded much better. That seems way too lean to me! And it's still not ideal vacuum.

I was running about 18" of vacuum before this. I've checked all the vacuum lines I can find, and plugging them doesn't seem to make a difference. Any other ideas of what to check?

Thanks in advance, everyone!
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Unread 02-02-2011   #2 (permalink)
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Adjust the idle mixture screws for the highest smooth idle, I usually adjust one in until the engine starts to lope, then back it out until it smooths out, then back it out 1/4 turn more, then do the other mixture screw the same way. I would say from what you are describing, the power valve has gone bad or the cork gaskets have started leaking inside, holley's are bad about this if they set too long and don't have gas in them. Good Luck.
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Unread 02-03-2011   #3 (permalink)
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Default 1968 289 Running Rough, Decreased Power, Low Vacuum. Ideas?

Hello macgeek.
U think rex may have this nailed BUt I like to check the simple things.

Try
1. -Check the 4 mounting screws for the carb see if they are tight
2. - Did you check vac with the advavne line blocked?

Where are you pulling the readings from?

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Unread 02-04-2011   #4 (permalink)
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I talked with another gearhead friend of mine and he suspected the carb as well, so I decided to rebuild the carb, replacing the power valve in the process. My carb is currently in pieces on the workbench

Sorry, Print Dad, I didn't see your post before I started rebuilding, so I didn't check the mounting screws beforehand. But I did check the vacuum with the vacuum advance line plugged when I was running tests before, and it didn't seem to affect anything.

Thanks guys! I'll post back with results once I get the carb put back together.
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Unread 02-05-2011   #5 (permalink)
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Why would fixing the distributor mess up the carb?

I would double check the work you already did.
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Unread 02-08-2011   #6 (permalink)
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Well, I'm at a loss. I got the carb rebuilt and installed, set the wet float level, idle speed, and fuel/air mixture, and... same problem

My vacuum is still much too low, and I don't know what else to check at this point. I double-checked the carb mounting nuts to make sure they were tight after reinstalling. All vacuum ports are blocked or being used.

One thing I did notice, I gained around 2", maybe more, of vacuum when I kinked the PCV hose. What could that mean? (side note: I'm pretty sure the guy at the auto parts store gave me the wrong kind of hose for the PCV line... I think he gave me a heater hose, and oil condensates on the outside).

My other thought is this: could an exhaust leak cause the vacuum to drop? I'm pretty sure I have at least one leak in my rusty pipes, and if that could cause vacuum issues, it's possible the hole got bigger recently or something.

Thanks guys! Hope I can get this figured out soon...
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Unread 02-09-2011   #7 (permalink)
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Default 1968 289 Running Rough, Decreased Power, Low Vacuum. Ideas?

Hello macgeek,
I ma certain you know that motors are like an orchestra.
All the parts need to be working in harmony.

FIRST - -get rid of the PVC hose you got. Get the right one. Take the PVC to the parts store and get one that is SNUG on the fitting.
THIS IS VERY Important. You now have a major vacuum leak if the hose is a heater hose. THIS IS A MUST

After changing the hose - -readjust the carb like you normally would. Start at 1 and 1/2 turns out from a gentle seat.

Then see what the vac reading is. We will go from there

You CAN do this. Print DAD
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Unread 02-09-2011   #8 (permalink)
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Thanks for the encouragement, Print Dad

I bought a new PCV hose today. The guy at the parts store said heater hoses will deteriorate, and got me a Weatherhead hose to replace it. He said it should last a lot longer.

So after installing that, I checked my timing again, then readjusted the carb, setting the idle speed and fuel mixture. I managed to get the vacuum up to about 18" (first picture), so I thought, this is good, I solved the problem!

Then I turned off the engine, disconnected my timing light, made sure the distributor was tight, and cleaned stuff up, since I wanted to take it for a drive to check how the power was. But I decided to do one more check before I drove her, so I started her up again with just the vacuum gauge hooked up, and the vacuum was back down to 15"! (second picture).

Still suspicious of the PCV system, I tried pulling the hose and blocked the valve with my thumb (third picture), and behold! My vacuum jumped up to around 17" (fourth picture).

Any ideas? The PCV grommet on the valve cover is pretty new, but that could be the culprit. Other than that, I'm not sure what else to check.

Thanks so much everyone!
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1968 289 Running Rough, Decreased Power, Low Vacuum. Ideas?-dsc01283.jpg  1968 289 Running Rough, Decreased Power, Low Vacuum. Ideas?-dsc01296.jpg  1968 289 Running Rough, Decreased Power, Low Vacuum. Ideas?-dsc01298.jpg  1968 289 Running Rough, Decreased Power, Low Vacuum. Ideas?-dsc01297.jpg  
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Unread 02-09-2011   #9 (permalink)
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Default 1968 289 Running Rough, Decreased Power, Low Vacuum. Ideas?

Hello macgeek,
This is ODD for sure.

I know you have set the timing with the light - -NOW I think it may be time to try
to set it by ear.

What I would try is this - -
Take a marker and mark the base of the dizzy so you know where the setting is at now. Use a fine tip marker for a cleaner mark.
Now loosen the dizzy and turn the distributor just a little clockwise. This will advance the timing say to about 12-14 BTDC. Some motors like more advance. Keep the vacuum gage hooker up and see how the vacuum does. also listen to the motor, I am thinking it should pick up the rpms.
If this doesn't have much affect on the vacuum - then I would put the dizzy back to the line you originally had which is 10 BTDC.

Next - - we gotta search to see if there is a vacuum leak. Two ways to do this.
Because you have a vac gage, I would mix some soapy water and spray it around the bas of the carb and the intake manifold. See if the vacuum rises.

The other method is to spray a carb cleaner at those areas as the motor is running.
AN increase in idle speed indicates a leak. The carb cleaner will eat paint so use just a little.

THIS IS ODD - -Let's try those few things and then we will go from there.

If you do advance the timing - -make sure she is not pinging. This would indicate she is too far advanced.

I will try to think of the next step

HAng in there - -this will get fixed I am sure.

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Unread 02-10-2011   #10 (permalink)
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Thanks, Print Dad. I adjusted my timing up to 14 BTDC, and noticed a slight increase in vacuum (~1", I believe).

I sprayed carb cleaner around the intake manifold and didn't notice any change in vacuum. I sprayed carb cleaner around the base of the carb and it seemed to have a very slight, brief effect on the vacuum. I sprayed it down the throat of the carb and the vacuum went up a good deal, and the engine sounded smoother for a second. Previously (a while ago, prior to this problem and the carb rebuild) when I did this, it seemed to bog down the engine a little bit, but now it seems to have the opposite effect. I sprayed it around the front bowl on the carb while covering the air intake, and noticed another brief increase in vacuum.

So I shut off the car and tightened everything down really tight: the nuts on the carb mounting bolts, the front bowl screws, and the accelerator pump screws. After starting the car back up, spraying some more carb cleaner down the throat, and revving the engine, the vacuum was up to 18-19" and the car sounded really good. I took it for a test drive and I had all my power back. Good news!

But then! I parked the car and let it sit for maybe 20 minutes, got some food, and then took it out again, and it was running noticeably worse again It really seems like a carb issue still, so maybe it's just time for a new carb. What do you guys think?

Thanks everyone for your help getting this figured out!
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Unread 02-20-2011   #11 (permalink)
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So I need to take another look at her, but she's still having problems. I can't afford a new carburetor and intake manifold right now, so I'm hoping there's a simpler fix I can do in the mean time. Any ideas of other things I can try?

Thanks again!
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Unread 02-20-2011   #12 (permalink)
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Default 1968 289 Running Rough, Decreased Power, Low Vacuum. Ideas?

Hello macgeek,
I have what may sound like a foolish suggestion but I think it is worth a try.

Get a bottle of stable and add it to the tank as directed on the label.

Also - -try using some high test from another gas station.

It seems as though when you spray into the carb - the vac gets better.

Perhaps you just need a better fuel.

I know it is likely a carb issue but try the gas can't hurt.

I would run the tank low and then try to add at least 1/2 tank of high test.

Gas today is just not very good. I always run a high grade gas BUt like most of us you may be getting the gas from the same station all the time.
It is possible they have an issue with their tanks.

Worth a try

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Unread 02-22-2011   #13 (permalink)
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Does your problem seem to disappear, once the motor warms up? Do you have a power brake booster?
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Unread 03-06-2011   #14 (permalink)
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Along those lines, is the choke off and engine warm when setting the timing? What is the idle rpm? Just for grins clean the pcv valve with carb cleaner, the pcv valve should close at high vacuum.
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Unread 03-27-2011   #15 (permalink)
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Alright, well I've been out of work but I finally got a job and can get back on this project!

After priming the carb, she started up just fine today after sitting the garage for a while.

Print Dad, I bought some Sea Foam and filled up with premium Shell gas. She already seems to be running a little better with that

However, I still have low vacuum, around 15-16". Still can't track that down.

Couped Up, the vacuum problem does not go away when it's warmed up, and I do not have power brakes.

bigtoed, yes, I set the timing with the choke off (open/pushed in) and the engine warm. Idle RPM is about 600. I will clean the PCV valve in the next day or so to make sure that's not the problem, thanks!

So any other ideas as to what is causing this or what I can check? I'm planning on bringing the car over to my grandpa's this week. Maybe two heads under the hood and some significantly more experience can figure this out

Thanks again everyone!
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