Why would a 69 VW Beetle still backfire even after replacing plugs wires cap and rotor and even adjusting the valves?

Answer 1

Your timing is off--WAY too retarded. How to fix: You already did the tuneup thing. You got the points set to 0.016" and the plugs--always Bosch Supers please--to 0.028" (they come out of the factory set to that, but check them before screwing them in) so now dig out your dwell/tach and your timing light, and a 10mm socket on a long extension. Find the timing mark--a notch filed into the rim of the crankshaft pulley--and paint it white so it's easy to see. White-Out works. Warm your car up, and then... set the idle to about 1000rpm and turn the engine off. hook up the timing light...red clip to a source of power, black clip to a known ground, spark sensor clip around #1 spark plug wire. start the engine loosen the distributor clamp, but BE CAREFUL!!! The fan belt is right there! pull the trigger on the timing light and point it at the crankshaft pulley. slowly rotate the distributor until the timing mark on the pulley aligns with the seam in the engine case. If it goes the wrong way, turn the distributor the other way until it lines up. Then tighten the distributor clamp, turn off the engine, unhook and stow everything, and you're done. The actual timing is 5 degrees before top dead center, but VW was nice and offset the mark for you so you wouldn't have to estimate anything. If you've got a pulley with a degree wheel on it, the engine turns clockwise so the 5 degree mark you want is the one to the right of zero. Plug wires hooked up incorrectly.

The Beetle exhaust system is prone to leaks.....any air entering the exhaust will cause back-fire pop on the over run...ie: coming off the throttle at highway speeds.

About the timing...do NOT use a 009 Bosch distributor. Use the factory distributor with vacuum advance that takes the Bosch 044 points set. If you have dual carbs just leave vacuum unhooked on the distributor..

Set timing for 30 to 33 degrees BTDC at 4000 rpm with no vacuum hooked up if this is a stock motor. If it has had the compression ratio reduced or hemi cut heads, use 36 BTDC as timing.. On a stock pulley, the DENT in the edge rolled lip next to the block is TDC and the sharp edged notches are 7.5 and 5 btdc. Case center parting line is TDC reference. If you have dual carbs with no vacuum ports you can't use a vacuum-advance distributor. The spark has to advance as you accelerate, and if you unhook the only advance mechanism your distributor has, your car will run horribly. If you've got a single carb, you can use the "single vacuum dual advance" distributor aircooled.net sells; this is a Bosch 009 with an added vacuum advance can, and it works well. If you have a carb that doesn't have a vacuum port, you need something that's centrifugal-only. Either a 009, a Mallory or an MSD.