Triumph Repair: GT6 quits after driving 20-30 minutes, vacuum line, vacuum advance

Question

Question: My 67 GT6 runs well for a while then quits on me until I leave it for a while to 'rest'.

Background: It started after a drive about 60 miles. This was the first long drive for the car after I bought it and got it running . It had sat for 20 years. I drove 30 miles on mostly highway at highway speeds (non-OD/original gearing, 55 mph) where it performed well but every once in a while it would 'miss'. It was like the ignition was turned off for a stroke, but very brief. This got progressively more frequent but still just a single miss now and again. Car did not reach upper temp ranges (according to the gauge).

On the return trip it was a different story. I stopped at for 30 minutes or so before starting the return trip. On the return trip it began misfiring again. This time it would misfire for several seconds and recover. But it got progressively worse to the point where it would miss for 3-4 seconds after about half way into the return trip. It finally stopped after I left a traffic light and would not cross the intersection about 26 miles into the return trip. I had to get towed home (4 miles from home). After it cooled down for 15-20 minutes I was able to start it and drive it onto the flatbed.


Car info:
Delco distributor:
     New cap, rotor, wires, plugs
     Since problem occurred I replaced the pts/cnd (gap .016), dynamic time 13 BTDC
     Replaced bad vacuum advance with NOS (exact correct one), had capped vacuum line before

Carbs:
     CD150 pair with no pollution control as original (ie no by-pass valve)
     Original carb
     Has PCV valve from valve cover to intake manifold (not to carbs like later engines)
     PCV valve has good diaphragm, hoses are not original but don't leak, could be a little crimped as the original was formed (s-shaped) and the replacements are strait hose stock.
     Float height set and new rubber o-rings and diaphragm installed, engine oil toped up in dashpot.
     When warmed up the carbs appear to be set to the correct mixture (lift piston test)
     Original type air filters and housing installed (was missing when bought, found used one at a local shop)
     Have not sprayed around carbs looking for vacuum leaks

Fuel:
     New generic electric fuel pump rated at 1.5-3 lbs pressure
     Car sat for 20 years, pulled tank and swished out
     New fuel filter after tank swish looks clean (between tank/pump)

Motor:
    Compression test, all cylinders at 160 or 161 (dry test)
    Valve gap reset at .010 (head not yet re-torqued)
    All heater and radiator hoses replaced (just throwing that in there)

Drove the car around last night including a few full throttle blasts. It does miss a bit under full throttle at above 3K but if you accelerate steady it will run between 3-4K without problems. Then the symptom happened again after 30 minutes of running around the neighborhood. Once again after the car cooled off it would start and I drove it back home (2 miles or so).

I am thinking the coil is getting 'tired' after driving around. It may be getting worse since it ran well at first in the car's initial long drive but failed sooner on my running around last night.

Any suggested troubleshooting steps?

Thanks,Mike


Answer
Hi Mike,
Your symptoms do not tell if it is fuel or fire related so you need to be able to see both at the time of failure. To do that just connect your timing light to the coil wire (not a plug wire) and tape the trigger down and run the wires out from under the hood and put the light under a wiper arm. If this is going to be done in bright sun light you may need to tape a piece of cardboard over the light end on the windshield so you can see the flash of the light while driving. Any flicker in the steady looking flash is an indication of a primary ignition failure. (the coil is a possible)

You should check fuel too the same way by putting a "T" in the fuel line between the pump and the carbs and run a long hose out from under the hood and put a gauge under the other wiper arm so you can see fuel pressure at the time of failure. It is important to see the pressure at the time of failure. It is no use checking it when it is running ok. This will check fuel supply as a possible fault.
Howard