ConcoursMustang Forums
Restoring - General discussions that span across many different groups of years and models => Drivetrain => Topic started by: Hipo giddyup on October 17, 2016, 12:23:21 PM
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As you see in my signature , this is in regards to my 67' coupe that still has it's original 289, 3 spd, 2V carb, and distributor all of which have been rebuilt. The distributor has a NOS vacuum advance unit and was rebuilt and tuned by a gentlemen in CA years ago on his SUN distributor machine. (He was well known for these rebuilds but has since retired from that years ago, darn I cannot remember his name!!). Anyway, I have noticed that the car sounds as if there is a miss every so often but if I disconnect the vacuum advance line it goes away and runs smoothly. Could this be a failed vacuum advance?? The carb was rebuilt years ago as well by Jon at Pony carbs and other than this miss, everything runs like a top. Maybe someone has experienced this before and could lead me into a fix or diagnosis before I go tearing down the distibutor or carb. :D
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Jack Koger
Distributor Dynamics
California
Now retired. Did a great job on distributors.
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I've got a phone number for them so send me a P/M if you're interested. No guarantees as I too, have heard they were "tapering off".
Jim
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The miss is idling in the driveway, going down the road at speed, or somewhere in between ? Vacuum adv. does not usually play a role in the spark plug firing. Before getting into the mechanical's I think I would be checking the vacuum line and then move onto the plugs, wires,cap and points. Brian
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That was him!!! Jack Koger, excellent guy and had rebuilt several of my distributors. Thanks Scott302!
The miss is at idle but also when i hit the "spot" between getting off the gas and stepping on the gas, you gently feel a "bump" ever so slight. I actually drove the car to work today (it is supposed to be in the 80's in Maryland today), and i was thinking of disconnecting the vacuum line for the drive home to see if it makes any difference. You can't always reproduce it while driving. I was also wondering if this early 67' would have had a metal line or rubber vacuum hose? I just replaced and put back what i took off during the restoration and now fixing the incorrectness with the help of this site. ;)
I guess I'll ask that question back on the 67-68 section. It currently has a rubber hose I have yet to inspect it for a leak . Thanks for everyone's input.
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When you disconnect the vacuum line I assume you are plugging the end to prevent a vacuum leak. ;)
TOB
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It is pretty easy to check the vacuum advance diaphragm by applying vacuum. You can also watch the movement of the breaker plate if you remove dist. cap while applying vacuum. Attaching a hose and sucking on it should be sufficient if vacuum pump not available.
As far as I know, no vacuum advance is in play at idle if the vacuum source is at the vacuum port of the carb. Vacuum only appears there off idle.
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The miss is at idle but also when i hit the "spot" between getting off the gas and stepping on the gas,
I was asking just as a gentle reminder. ;)
TOB
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It is pretty easy to check the vacuum advance diaphragm by applying vacuum. You can also watch the movement of the breaker plate if you remove dist. cap while applying vacuum. Attaching a hose and sucking on it should be sufficient if vacuum pump not available.
As far as I know, no vacuum advance is in play at idle if the vacuum source is at the vacuum port of the carb. Vacuum only appears there off idle.
If you don't have a vacuum pump you can still check to see if the diaphragm is functioning with it still mounted on the distributor. With the cap off and rotor removed, remove the tube (metal or rubber) from the port to the carburetor, and disconnect the diaphragm arm inside the distributor. Put your finger over the port and move the arm outward. You should have a small amount of pressure at the port. With the arm outward, put a finger over the port and pull the arm inward. You should have some suction indicated by the arm wanting to go back outward. It's a quick test. If no pressure or suction, bad diaphragm. This test does not indicate if any of the internal washer spacers or the stop are correct.
Jim
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Good pick up on the vaccum advance not being used at idle, but i believe I was giving it gas when i noticed the miss disappeared. :o Its been a week since i was playing with this and have been quite busy to get back to tinkering. I do have a vacuum pump and will test that.
It sounds like this car should have a steel line as factory original so i have an NOS line I will be installing tongiht and will perform the test at that point. If the diapram is good, the steel line replacing the rubber hose (to rule out a crack or other issue), then what part of the carb could be creating the miss? Perhaps a pasage is fouled or blocked?? The car was fully restored in 2000 and since then, only has about 3,000 miles on it. Alot of time idling in and out of the garage when I couldn't take it out for a spin, such as during winter, so maybe there is carbon build up? just guessing at this point. I can always do a quick carb rebuild with a new kit. That's easy and cheap enough.
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I am following up on this thread as the "last" thing on my punch list is this very same issue. When the vacuum line is removed from the advance, there is no miss or rough running when the engine speed is increased from idle. With the vacuum line installed, the engine goes into a noticeable shudder as I bring it off idle. Points, plugs, wires, condenser, rotor, cap are all new (not concours!). Problem existed before all of these components were replaced.
The vacuum advance mounting holes are oversized and the attaching bolts are undersized (not concours!). I noticed that the vacuum advance physically moves (with vacuum line installed) when I bring the engine up off idle. So the advance is working.
I am going to check the advance for its ability to hold a vacuum.
In addition to the items above, any other ideas?
Thanks,
Mark
PS. Application is NJ July 67 289 automatic, 2bbl, PS
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Do you close off the vacuum leak when you pull the line off or leave the line open?
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Yep...I close off vacuum source during timing adjustment and during the testing described.
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Sounds like a vacuum leak in the advance. You should be able to disconnect arm in dist and using a clean vacuum hose suck on it and see if it holds suction.
TOB
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The vacuum advance mounting holes are oversized and the attaching bolts are undersized (not concours!). I noticed that the vacuum advance physically moves (with vacuum line installed)
Correcting this should be the 1st step. When the advance moves it can (will) change timing. That being said if the vacuum line is connected to the correct port and the throttle plates are closed as they should be there should be no vacuum on the hose at idle
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TOB
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Thank you. I addressed the oversized holes/fasteners, verified vacuum source to be ported, and replaced the vacuum advance. Problem solved. However, the original vacuum advance passed the "suction" test. I did notice the coil lead and the condenser lead were slightly angled. Maybe with the oversized hole condition, the advance linkage was momentarily shorting out on these wires....anyway it is resolved.
Thank you for providing guidance and advice.
Mark