popping-backfire troubleshooting-help needed

[69 Polo Red]

I have a poping-backfire from my 1969 912 at low RPM utill the engine warms-up-then it only pops/backfires upon deceleration. I started by adjusting the valves, then a compression test-yielding min of 119 to a max of 125 lbs-the plugs looked OK, but old. I am is the process of replacing/adjusting all of the ignition components-including plug-wires & timing plus investigating any intake leaks.

Is there anything else I could be doing in addition? Any suggestions?

[RichardM912]

If you are talking about popping back through the carbies, I had the same problem with my 69 when I first bought it a year or so ago. Carbs are Webers but I don't think they were involved.

Problem went away (and has stayed away) once I sorted the timing. On my car there is a big difference between the static timing (engine not running, test lamp in ignition circuit and turn distributor) and the dynamic timing (strobe light). The difference is not from the centrifugal advance but (I presume) from wear in the distributor parts.

As the previous owner had it set, the timing was actually retarded at idle. I found the optimum setting and backfire gone. Popping back through the carbies is dangerous. A good way to set your car on fire so it is important to sort the problem.

[69 Polo Red]

Yes, Checking the full-advance timing was next on my list to complete. Thanks

[Christopher E]

I had the same issue. Experienced some popping when the car ran on a too lean mixture. (lean makes the engine run hotter and the heat can make the gas in the carb bowls overflow into the chamber - producing quite a backfire on startup) Check for intake and exhaust leaks. I have somewhat leaking throttle bushings on the webers which occasionally lets of a pop when enginebraking heavily or shifting down in high revs. For now that tolerable, though.

I set my timing to a total of 33 degrees (swedish gas is quite high octane, though? 95, 98) adjusted the carbs and set the mixture to a CO of 3.5-4%. Cured it.

I think it's a matter of finding the "harmony" of each car. Some engines need a richer mixture, some need to tweak the timing and so on - keep making small adjustments and let everything settle. Soon enough you'll find your own car's "sweet spot".

You're definately on the right track in keeping ignition components in good condition. I opted for an Ignitor instead of breaker points - works nicely so far.

[JT912]

There might be a slight intake/exhaust manifold leak. I had a similar problem. Timing was correct, valves correct, ignition good and ultimately replaced the clamps on my exhaust and new carb base gaskets and the problem has gone away.