390 Vacuum Leak Or Other Issues....

[TEL73]

I have a 390 that started out with a slight miss across the full rpm range. I've not been able to isolate it to one particular cylinder though. The idle quality has progressively gotten worse as well. It used to idle OK when in park then very rough in gear. Then, it would die when put in gear but still idle extremely rough when in park (vacuum was at 4). I was going to try and find what I believe is a vacuum leak yesterday but now it won't even idle in park. You have to keep the rpms up around 2k to even keep it running. All vacuum lines have been blocked for accessories and there was no change. I pulled carb block off, checked flatness and installed new gaskets and no change. I'm thinking it's a intake gasket leaking. Your thoughts?

[1964Fastback]

More information about the engine/vehicle/situation would help.  Do you have an automatic transmission or power brakes?  Those both have vacuum lines that can leak.  Usually intake manifolds don't develop leaks after years of being fine.  Can happen I guess but not the norm.  Was the intake recently removed/reinstalled?  Any other work recently done to the engine for that matter?  Often things like this can start after working on something else (e.g. new condensers are often not of great quality).  What kind of carburetor and ignition (i.e. points, duraspark, etc.)?  You might check the spark quality by pulling the center wire from the distributor and having someone crank the engine while holding the end of the wire near the engine block.  Use gloves and insulated pliers, and it's easier to see indoors or when dark.  Should have a good blue spark.

Pat

Edit: Sorry - you mentioned in park, so it has an AT.  Did you block the line going to the vacuum modulator on the transmission in your earlier testing?

[TEL73]

Vehicle is a 65 Thunderbird. It has an Edelbrock carb. Nothing has been changed in over a year and it's gotten progressively worse. EVERY vacuum port has been blocked off. Nothing connected on intake ports or carb ports and it still does the same thing.

[1964Fastback]

I'm not familiar with those carbs but I'll assume that it's fairly new and clean, and that you've tried adjusting the mixture screws already.  I'll also assume it's still points ignition since not specified otherwise.  I would make sure the point gap/dwell is correct, then check the spark quality as mentioned, then adjust the timing to say 12 degrees BTDC and idle to 800 RPM (if able to).  See how the vacuum gauge looks then.

Pat

[My427stang]

Some troubleshooting ideas

1 - pull all 8 plugs and see if any look like they aren't firing
2 - measure resistance on any wire that feeds an ugly plug, consider check all 8
3 - Start with basic steps - set idle screws to 1 1/8 out
4 - check initial timing at idle no vacuum, readjust idle speed if it was very retarded
5 - look under the cap for any carbon tracking or heavy corrosion
6 - Make sure all the wires are pushed down into the cap
7 - Double check firing order
8 - Block the oil fill tube or vent on one valve cover, unhook PCV and see if the hole the PCV was in builds vacuum (not the PCV the valve cover) - There should be mild pressure not vacuum, if huffing pressure, may have ring seal issue, if vacuum, can be sucking from bottom of intake.  (keep in mind though, some intake leaks aren't bad enough to build vacuum)

[TJ]

Sometimes those ebrock carbs need their floats adjusted.

[GerryP]

Most carburetion problems are actually in the ignition.