I have a 57 312 Thunderbird with Fordomatic A/T that idles roughly at anything less than ~900 RPM. It will idle at ~500 RPM but is quite rough. Everything is stock on the engine. New plugs, points, condenser, cap, rotor, 26 degrees dwell, 8 degrees timing advance with the vacuum advance disconnected. Idle set as well as I can get it on the carb.



145 to 150 psi compression on all cylinders except one is 125 psi.



When I start the car the manifold vacuum is steady at about 17 inches Hg at idle. As it warms up it drops to a steady 14 inches Hg. The idle gets worse as it warms up.



Any suggestions? My guess is a burned valve on the cylinder with 125 psi compression. I would not think that would be bad enough to give a crummy idle though.



thanks

Dave Christensen

Dave Christensen

Falllbrook CA

57 Tbird

Has it ever idled smoothly while in your ownership?

Have you checked your valve adjustment?


Lawrenceville, GA

Thats a good suggestion from Charlie.Pay special attention to that lower compression cylinder.I was thinking vacuum leak.Check the adjustment first and then check around manifold or carb for possible leak.A burnt valve would cause the vacuum gauge to change each time that cylinder fired.

  

It has never idled smoothly. It is getting worse. It is bad enough for me to try doing something about it now. I had problems getting the intake manifold sealed when I had it off awhile ago. I tried the propane torch (unlit) next to all the runners and could not find a leak.



I have not checked the valve clearance since it has gotten worse. A couple thousand miles ago it was OK.



thanks

Dave Christensen

Falllbrook CA

57 Tbird

Do you know the condition of the crankshaft damper? The damper ring can slip and falsely indicate the timing. Does it run better if you set the timing by ear?

Is the firing order correct?


Lawrenceville, GA

Dave

   You write that you have a rough idle and "When I start the car the manifold vacuum is steady at about 17 inches Hg at idle. As it warms up it drops to a steady 14 inches Hg. The idle gets worse as it warms up."

   Start there. Try to fix the vacuum leak first. Until you do eveything else will only be a waste of time. You may or may not have other issues but until this is fixed you do not know. Pete

Hello Pete! If the valves are too tight to start with.As it warms up the clearance closes even more.Would that not cause the vacuum reading to drop?What was the problem with "getting the intake to seal" when you had it apart?Timing can also be set with a vacuum gauge.Slowly advance the timing until the highest steady vacuum reading.Back it off until vacuum starts to drop.Set timing halfway between high and drop point.Old time mechanics sometimes still set timing this way.

  

Charlie:

The timing marks on a T-Bird are not on the damper ring, rather they are on the pulley itself.  Cannot slip.

Dave:

Perhaps the power valve diaphragm or the gasket is leaking.  A little richer mixture can be tolerated when cold, but warm it becomes a problem.

John - "The Hoosier Hurricane"

You can check power valve by turning mixture screws all the way in.Motor should quit.If not fuel is still comming in from somewhere besides the idle circuit.Power valve maybe.

  

I know the problem is related to the low vacuum. I have disconnected and plugged all the vacuum lines and tried the propane torch trick all around the intake manifold and carb.



When I turn the idle mixture screws all the way in the engine dies. If I had a vacuum leak I would think that opening the screws would smooth it out, but it does not.



I will check the valves this weekend and see if that does anything.



thanks

Dave Christensen

Dave Christensen

Falllbrook CA

57 Tbird