914-6 engine popping

I need any and all advice on this one guys.I have a stock 914-6 (1991 cc "T" engine). A couple of years ago on a long trip into Missouri, the engine started popping and backfiring substantially. I managed to nurse the car back home to Canada with the help of Octane booster and replacement plugs and points. Since then, I have replaced everything I can think of which may cause the problem: carburetors(still exactly the same problem), all carb gaskets, needle valves, complete exhaust system, relays, CD box (I have tried three different units), ignition wires, points, rotor and cap, coil, distributor, plugs (many times with Bosch WR5DPs and NGKs),alternator and regulator. I have removed all of the carb jets and adjusters and ensured they are clean. I have run every type of carb cleaner through the carbs I can think of and run Dextron III transmission fluid through the cylinders to try to clean out any excess carbon. I have set the timing numerous times and adjusted the valves at least twice, once with the engine out of the car. Nothing I have done has made a very significant difference. I did a compression check (not sure I did it correctly) and got readings on all of the cylinders between 100 and 118 PSI. I suspect that I should have held the throttle open while cranking the engine (any feedback is welcome). I am completely at a loss as to what to do next. Help!

In all of your work over the past two years, have you done any thorough cleaning of all of the ground connections throughout the electrical system? I seem to recall that 914's are known for some faulty grounds due to corrosion. I would think a cleaning campaign regarding your grounding might quite productive!

And, yes, your compression should have been done with the throttle held wide open, so I would recommend trying another test.

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Warren Hall
1973 911S Targa

Perhaps there's more about this problem than you've told us. I still am curious. You had to replace points to get home? Well, if the car is eating points then the fuels system is not doing that, right? maybe I'm missing something.

What do the spark plugs look like when they come out? Smell?

You see, popping suggests a lean mixture which would make spark plugs white. Ignition systems that fail to ignite the gas at times will cause backfiring (louder) when the ignition comes back on with unburned fuel in the exhaust.

Of course, bad gound connections are devlish gremlins.

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'83 SC

I have thoroghly cleaned all ground connections many times. The new points made little difference and the old ones were still in decent condition. Changing the points in a 914-6 is quite a job due to their location, so it is not worthwhile generally to put the old ones back in. The plugs used to be quite black (rich mixture) but now generally come out a copper/brown. I would prefer to run the car very rich but none of my adjustments seem to have this effect.

Thanks to all who responded to my inquiry. I have now enjoyed three days of trouble-free driving - no popping. The problem turned out to be the ignition switch. It seemed to work fine but must have been cutting out almost imperceptibly, but enough to throw out the ignition system. I replaced the electrical portion and it now runs very well. Live and learn!

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