914-6 Popping and Missing

[cdn914]

I have a completely stock 914-6 with a persistent problem - popping from the tailpipe and missing intermittently. I have changed every ignition component including the ignition switch and distributor, and even the carburetors but the exact same problem persists. I have substituted four CDS units and the problem is exactly the same with all four. However, none of the four CD units is new. All emit a hum with the ignition on and the car does run. How would I know if the problem is the CD unit? Can they be tested and if so, how? Do they completely fail (car will not run at all), or can an intermittent problem be caused by a bad CD (or in this case four bad CD units)?
Is there a better alternative to the CD ignition? Any and all suggestions very welcome.

[Early_S_Man]

There is nothing wrong with your CDI-units ... the quality of your points and distributor condition is suspect, though! And the carbs contribute to popping and backfiring, too!

Read this thread about distributors ... and consider removing your dist. at TDC #1 ... and doing a partial disassembly (the drive gear doesn't need to come off unless play and wear of bushings or fiber washers (endplay shouldn't be more than 0.1 mm) is evident!

distributor lube...

[jtf914]

When did you start having the problem? Does it happen all the time? Have you checked the gas tank for rust?

I was having a similar problem, and went through the entire ignition. Then I realized that it occurred consistently after hard driving (sharp fast corners), and after refueling. I pulled the tank, and it was filled with rust. I sealed the tank, rebuilt the carbs, and with the help of this board, learned how to tune the webers. My -6 has run smooth ever since. Check out the post by T Bird in this link for weber tuning instructions:

Weber rebuild

Justin

[cdn914]

I have replaced with new parts: the points, the plugs (with NGKs), the cap and rotor, the coil, the ignition wires with OEM Bosch wires, the heat exchangers (new - expensive!), the muffler(another used unit), the ignition switch, the relay board (with a used unit), the complete distributor (with a low mileage unit), the alternator and regulator, the battery, the carbs (with rebuilt identical Weber 40 IDTs), the fuel filter, all fuel lines, the gas tank, the fuel pump (new)(fuel flow and pressure are good), the fuel tank cap, and air filter. I even have removed the screen filters in the Banjo fittings on the carbs. There is nothing left to replace. I have had the carbs adjusted by a local expert twice in the last couple of years. The car will idle perfectly and accelerate but the popping from the tailpipe persists and eventually it starts to miss again. The ignition timing has been set multiple times. The only constant seems to be that no matter what I do or change, the exact same problem persists.

[cdn914]

One more thing - no matter how the carbs are adjusted, I cannot get the car to run rich.

[si2t3m]

Any idea if it's missing randomly (any cylinder) or always on the same ones? Trying to rule out the dizzy or the trigger wire going to the CDI or the wire going to the coil.

I'd check every spark plug wires with a stobe light to figure if they are missing randomly or not.

If it's only one or a couple of them with the intermittent problem, you could reindex the dizzy 180 deg and plug the spark plug cables accordingly in the cap. Same one missing? then it's related somewhere in the spark plug wires or the plugs.

You could also test by bypassing the CDI by using a condenser?

To bench test a CDI, check www.permatune.com

HTH

Marc-Andre

[Early_S_Man]

The stock set-up and jetting of the 914-6 carbs was far from perfect ... and hot weather drivability problems were common when they were new! Are these problems worse in hot weather? The 914-6 typically ran 20° F hotter than a 911T with the same engine! Do you have any idea what temps your oil is running?

If it was me, I would try larger main and idle jets ... 125 - 135 on the mains and 60 - 65 idle jets.

Which NGK spark plugs are you running?

[T Bird]

CDN914
You don't state under what circumstances you experience "popping from the tailpipe." Does this occur under load? At idle? Above/below 3000RPM? It makes it hard to diagnose whether you have a carburetor problem without knowing a bit more. Popping or banging in the muffler can indicate a lean mixture. However missing is another matter and suggests to me an ignition related problem. If the mixture is lean enough, you will get some really nasty spitting through the carb barrels. Is this what you mean when you say that your engine is "missing?"

I would not start replacing jets unless I was first certain that the carbs were set up properly (float levels, accelerator pump volumes, balance and idle mixture adjusted correctly). Assuming that you were able to eliminate these adjustments as possible sources of the problem, I would take a look at those idle jets. If I'm remembering correctly, the factory idle jet for the 2 liter was a 45. That's a wee bit small for today's gas. 50 or 55 should solve lean popping.

I don't understand your statement that you can't get the engine to "run rich." How do you know? Are you using an exhaust gas analyzer? If so, that further suggests the need for larger idle jets.

jft914- I'm glad that my Weber set up instructions have helped you.

[Wayne 962]

Run a compresson check and make sure that your valves are properly adjusted. If the engine is leaking past the guides and the rings, it will cause similar backfires...

-Wayne

[cdn914]

What is the collective wisdom on installing a 1967 911 coil and condensor in place of the CD ignition, at least as a test?

[Early_S_Man]

Sure, you can do that ... but it wasn't an entirely acceptable ignition system from '65 thru '68, so don't be surprised if drivability problems persist!

Have you looked at the spark plugs, lately?

An alternative would be to hook the ignition up to an analyzer ... ignition scope of the Sun type ... and see if there are any abnormalities present! Guessing with so little information isn't getting anywhere ...