I solved my rough idle/fluctuating vacuum problem but I am embarrassed about what turned out to be the cause.  This 292 ran great on an engine stand until both valves on cylinder # 6 on the left bank started sticking and I bent a total of 7 pushrods and used a whole can of WD-40 before the problem resolved.  I had compression tested the engine before the sticking valves occurred and all cylinders were in the 120-135 range.  I retested the compression today and all cylinders in the left bank tested again at 120-135.  However, cylinder #2 in the right bank had ZERO compression.  I pulled the valve cover and cranked the engine over and the rocker for the exhaust valve of #2 cylinder was not even moving.  I pulled that pushrod and it was bent like a pretzel.  Inserted a new one and adjusted the lash to .18 thousands.  I retested the compression on #2 and it is a bit low at 90 PSI.  I am hoping that the low reading is caused by some crud which may have accumulated around the valve seat by running the engine with it not firing????   I started the engine with the valve cover off and ran it for about an hour with no sign of the valve sticking again.  It now idles smoothly.  Exhaust now sounds normal out of both tailpipes with a light purring pattern.  Checked the timing with a light and it seems to run best at about 10 degrees BTDC.  Retested the vacuum and it is steady at 20 inches with no bobbing needle.  I will run it for awhile and retest the compression in #2 to see if it has improved to normal.  Am I being too optimistic?  Thanks everyone for your help.  Alan

Yes.  A cranking compression test with the throttle opened up would also help to confirm this.  There's also the chance that a leak down test would also help to confirm exactly which valve is not sealing but from your description, it would be an exhaust valve.

Lorena, Texas (South of Waco)

The vacuum readings & the sound the engine is making sounds like a valve problem. With the vacuum gauge hooked up & engine running, pull the plug wires off one at a time while watching the gauge. The vacuum gauge should drop as you pull each plug wire off. When you pull a wire off & the gauge doesn't drop that's your bad cylinder. Next step would be to check compression on the bad cylinder. You could also pull the valve cover & check for something out of place (stuck valve, flat cam lobe, push rod etc.).

Dan      Kingman Az.      86409

Check compression.A leaky exh valve will give the symptoms you describe..As rpm goes up the valves will seal better letting the engine run OK under load.

I just thought I would mention that this engine had sat for about 10 years without having been started before I bought the car that it was in.  I found the compression to be good and the valve train inside the valve covers was real clean with zero sludge.  It looked like the heads had just been done.  When I first got the engine running, I had some problems with sticking valves (an intake and an exhaust on one cylinder) which bent some pushrods..  I freed up the valves by soaking them and there have been no bent pushrods in a long time.  Alan 

Thanks for all of the help, especially that article on interpreting the vacuum gauge readings.  I checked the engine vacuum today and it doesn't look good.  It is acting close to "Scenerio # 8" in the article.  The vacuum at idle rapidly fluctuates between 9 and 13 inches.  Increasing the engine speed results in slightly higher vacuum of 15 inches but the needle is steady.  According to the article the diagnosis is worn valve guides.  Is there any other test to confirm this as the problem short of pulling the heads and taking them to the machine shop?  Would worn valve guides make the engine run rough at idle with that distinct "put...put...put" exhaust sound?  Thanks again, Alan

all 6volt coils are not the same,some wher neg ground all fords positive ground, the coils must be wired depending on the ground,ford coils usualy say bat that wire comes from ign switch,after market say      - and + i cant remember for sure but think - goes to dist,some  one hear will remember,older motors manuals tell how it should be hooked up depending on ground. I do know it will efect how the engin runs if in corect. should have a resister as well ,6 0r 12 volt

Coil wired backwards?

Thanks Slick.  I only wish that there were 12 normal scenarios and 3 bad ones.  I don't like the odds.


Mark

1956 Mercury M100
1955 Ford Fairlane Club Sedan
Delta, British Columbia

If there is irregular vacuum, here is a great article on pinpointing problems

South Australia