antijam
CCCUK Member
For more than a year now I’ve been experiencing an infrequent and so far undiagnosed, misfire. The engine is a 1971 stock (as far as I know) base 350/270 sbc of unknown mileage. It has points ignition, professionally rebuilt stock Quadrajet carb and headers. The car always fires up on the button and with everything on song, performs very well. Just occasionally it starts up with at least one cylinder misfiring or not firing at all. The first time this happened #5 was the culprit and after fiddling around with changed plug, HT lead and anything else I could think of, it seemed cured. A few weeks later it recurred, this time with #7 joining as a non-combatant. This immediately implied an electrical problem; not only are cylinders 5 and 7 adjacent on the engine, they’re adjacent in firing order. As well as verifying the resistance of the HT leads and plugs, I checked the obvious possibilities of cross firing between leads and tracking in the distributor cap but found nothing conclusive and sure enough the next time it was started, all cylinders were firing again. The misfire recurred infrequently over the next few months so in desperation I changed all the HT leads, the spark plugs, the coil, the distributor cap, points (though not the condenser) and rotor; one change at a time to see which, if any, had a curative effect – none did.
So, after the most recent recurrence, with my timing lamp showing both #5 and #7 not firing at all. I removed one plug at a time, rested it on the block with the HT lead connected and after fitting a spare plug in the vacated cylinder, fired it up. All plugs fired perfectly in fresh air. Even though they were firing in the open, perhaps I didn’t have a strong enough spark to always fire under compression, so I disconnected the plugs and connected an adjustable spark tester between the HT lead and earth. All leads produced a healthy spark at a 10mm gap, so the HT voltage should be more than adequate.
After this last testing, on firing up again with #5 and #7 still not firing, they were joined by an intermittent misfire on #3. Aha! 3,5 and 7 are all on the LH bank; perhaps I don’t have an electrical problem, perhaps I have a head gasket problem? I’ve done satisfactory compression tests on the engine in the past but I repeated it again today – results below. Compression still seems pretty much on point.
I’m running out of ideas now, with the only obvious possible problem left being fuel. If no fuel is getting to a cylinder, the spark plug would still fire so that’s not the problem. If too much fuel is getting there, it probably wouldn’t. The only thing I can think of that would cause unregulated fuel to enter cylinders on the same bank is if the intake manifold gasket was failing on that side, although why this would be intermittent I don’t know.
There’s no way to check this without removing the manifold, so before I start draining coolant and removing the distributor, carb and vacuum lines, does anyone have any other logical suggestions for causes of my problem?
So, after the most recent recurrence, with my timing lamp showing both #5 and #7 not firing at all. I removed one plug at a time, rested it on the block with the HT lead connected and after fitting a spare plug in the vacated cylinder, fired it up. All plugs fired perfectly in fresh air. Even though they were firing in the open, perhaps I didn’t have a strong enough spark to always fire under compression, so I disconnected the plugs and connected an adjustable spark tester between the HT lead and earth. All leads produced a healthy spark at a 10mm gap, so the HT voltage should be more than adequate.
After this last testing, on firing up again with #5 and #7 still not firing, they were joined by an intermittent misfire on #3. Aha! 3,5 and 7 are all on the LH bank; perhaps I don’t have an electrical problem, perhaps I have a head gasket problem? I’ve done satisfactory compression tests on the engine in the past but I repeated it again today – results below. Compression still seems pretty much on point.
I’m running out of ideas now, with the only obvious possible problem left being fuel. If no fuel is getting to a cylinder, the spark plug would still fire so that’s not the problem. If too much fuel is getting there, it probably wouldn’t. The only thing I can think of that would cause unregulated fuel to enter cylinders on the same bank is if the intake manifold gasket was failing on that side, although why this would be intermittent I don’t know.
There’s no way to check this without removing the manifold, so before I start draining coolant and removing the distributor, carb and vacuum lines, does anyone have any other logical suggestions for causes of my problem?
Hopefully by the time I come to put it all back together it'll be a bit cooler.