1974 VW Beetle

When warm. Spark plug in cylinder #1 doesn’t seem to fire. When attempting to set the timing, the timing light works fine when the engine is cold, but once it warms up, there is no longer a light to set timing. If the probe is moved to #2, #3, or #4 spark plug wire, the light works fine. It just won’t work on #1 spark plug wire. When warm the engine runs very poorly. I just replaced points, rotor, distributor cap, plugs, and plug wires. Any idea what could be causing the problem?

That is a most curious situation. Since 2 3 and 4 fire it narrows it down to distributor cap or plug wire, or poor connection of the wire at the cap. I think another distributor cap or cleaning and inspection of the current one is in order, followed by #1 plug wire replacement, followed by #1 plug replacement, I usually do not like throwing parts at a problem but given the cost of the parts if you can do the repairs that is my fix.

Thanks for your thoughts. I came to the same conclusions. I replaced the plug wires after discovering the problem. Since the distributor cap was new, I put the old one on (everything worked until I tried tuning it up). Then I swapped Plug #2 and #1. Result - no fix, #1 still doesn’t fire. Is it possible Plug #1 got cross threaded, and isn’t making a good connection with the block? As you might gather, I’m grasping at straws!

Test that theory. Get an old plug, plug the spark plug wire into it, and have someone turn the engine over as you hold the plug to a metal part of the engine. If that plug fires without problem, the trouble could very well be at the plug and head.

I’m leaning to a problem back at the distributor. What does the points cam on the distributor shaft look like? Does one lobe look worn or bad? Do the points open correctly on all lobes, checked with a feeler gauge?

Could be just a dud spark plug, if the plug isn’t sparking then it won’t trigger the timing light. You could pull the plug and check it to be sure, but testing a plug out of the cylinder isn’t a 100% test due to the spark plugs dielectric qualities (or lack of them) - it can spark out of the engine on the end of a HT lead but fail to spark when screwed into the cylinder where it is subjected to fuel vapor and compression.

Your cheapest route is to just fit another spark plug.

Crossthreads make for excellent electrical contact, one of the few things the are good for. Rather than depend on the timing light,does you idle change if you remove #1 plug wire when warm?

Something else that changes when warm, valve adjustment,hows yours? if a valve is getting tight when engine is warm,you will have a misfire

Bad plug or the distributor is junk…

If there’s a miss fire when hot adjust the valves. In fact, adjust the valves every time you change the oil. Those engines are notorious for valves losing their cleaarence and burning.

Low compression, bad spark plug, or possibly a bad plug wire resistor IF you got a generic set of wires that involved changing the screw-on plastic or Bakelite resistors over to the new wires.

If the plug wire job involved changing the resistors over then the resistor should be checked with an ohmmeter. A normal reading is about 1000 ohms.