Triumph Repair: Electrical and tach, starter solenoid, reverse polarity

Question
I have a 1976 Spitfire.  When I bought recently, I drove it the 240 miles home without a hitch.  Then shortly there after it would start sputtering and die. The fuel filter that was installed (next to Weber carb) would barely have fuel in it so I replaced the filter(replaced the one next to the gas tank too.  replace the manual fuel pump..plenty of fuel, but float would not stay closed, so I rebuilt the carb.  ran excellent, unless under load, then it would sputter and die underload.  I noticed at that point the coil was getting extremely hot.  If I let cool down, then I could get it started enough to get home(less than a mile).  replaced the coil.  ran excellent for about 6 or seven miles and started sputtering.  I managed to make it in the driveway before totally dying(killed that coil).  One of the the things I did to try and figure out what was wrong was to turn the key to the on position and start checking voltage.  12.46volts to the + side of coil(grounded to frame, but have 7.56 volts if I ground on the coil- ground wire.  If I take the coil out of the equation, I get the same results.  Turns out I have a reverse polarity of 3.5 volts coming from the tach. (positive from the tach to the negative side of the coil).  Could this negative feed back cause the coil to heat up and short out(no spark)? Also, there is just one wire I can not get out of the tach(I think it is part of the dash light system).Any tricks to that?
Last but not least, just some more background.  it has an older version of the Allison xr 700 electronic ignition.  all that wiring seems to check out.

JJ  

Answer
Hi Jeff,
If you grounded the negative side of the coil while you had a volt meter connected to the positive side of the coil and noted 6 or 7 volts when you grounded the negative side, then it is correct as the power to the coil goes through a resistor wire to get to the positive side. However in the "Start" position the starter solenoid will supply a full 12v to the positive side of the coil. It will only test 10 or 11 volts because the starter is drawing a lot of current.

However, I believe you are wasting time and need to test "End Result" at the time of failure. Meaning, does it have spark AT THE TIME of failure? And does it have FUEL at the time of failure?

If it will not restart just when it dies spray some starter fluid into the intake. If it starts for a second or two or will keep running only if you keep spraying then you have a fuel problem not an ignition problem. If it don't restart with starter fluid then you have an ignition problem. If you suspect the tach disconnect it to see if that is the problem. (I would be surprised)

One other test you need to run before all of this. Connect a long hose to intake manifold vacuum and run it out from under the hood and put a vacuum gauge under a wiper arm so you can read vacuum while driving and note the vacuum reading as the car starts to fail. Let me know the readings.

Howard