Classic/Antique Car Repair: 1966 Mustang, stale gas, compression stroke

Question
I have a 66 mustang 6cyl. that has got me a bit confused, the car had been parked and started only occasionally last 6 mths, ran great a couple of days ago then suddenly developed a miss. I did the obvious and drained tank, replaced with fresh feul,new plugs, points condenser, rotor, cap, ohmed plug wires (did not replace),and even disassembled carb, cleaned and replaced. All with no changes in the miss (runs at rough idle, does not smooth out at any speed, and very weak no power. what should I check next compression? any suggestions greatly appreciatied

Answer
My guess is that you have a valve that is sticking in its guide- a common problem when a car had been run on stale gas.  I'd do a compression test to identify the bad cylinder, then remove the valve cover and let the engine idle while you spray some strong solvent on the valve stems for that cylinder as they move up and down - perhaps enough will seep down to the guide and free it up.  I'd use carburetor spray - the GUNK brand seems to be good.   

You could go further before you do this by conducting a test on each cylinder for leakage, using a fitting that allows you to put your compressor hose into a spark plug hole.  You may find that your compression tester hose has a fitting on the gauge end that you can connect to your air compressor using adaptors.  Doing this when the cylinder is at TDC on the compression stroke, with the car in gear (std trans) or the flywheel clamped (auto) to keep the engine from spinning will allow you to listen at the tail pipe and carburetor to find out which valve is stuck open.  But that's a lot of trouble, so if the compression is down on any cylinder, just try the gunk trick on both valves for that cylinder.  

I've seen the varnish on the valve stems get so bad that the valves have to be removed to be cleaned up - maybe yours won't be that bad. It is of course possible that there is some other problem, but a compression test is always a good thing to do as a first step.

You may be able to free up a valve by taking the rocker arm out of the way and disassebling the spring and keeper, then pulling the seal up off the stem, and soaking the stem with solvent while you manually move it up and down and turn it.  Be sure the piston is up when you do this so you don't lose the valve down in the cylinder!  You'll need a valve spring compressor tool to do this job.  Use your air adaptor to hold the valve closed while you re-assemble the seal, spring and keeper.

Dick