Spark plug is cylinder 4 breaks?

[ricky_rich]

I have 1300 -70 where the spark plug on cylinder 4, have broken four times over the years. The other plugs are running fine and have done so for many years. I recently bought a 1967 1300 beetle, put in new plugs, and after just a week the one in cylinder four got shorted out. Have run both NGK and Bosch plugs. What’s the thing with this cylinder that eats plugs??

[Multi69s]

When you say shorted out, do you mean fouled, like covered with oil? What does the plug look like? Can you show us a picture? There is nothing in the VWs engine construction that would make the #4 cylinder a weak point.
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[ricky_rich]

The center electrode has a short connection to the shell. So with plug removed and engine running, I have the spark from plug shell to engine block. Plug looks black and wet since it has been running without a spark for a while.

[enjoyther1de]

That's usually carbon or bad gas that causes plugs to "weld" themselves shut.
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[ashman40]

Post your specific spark plug make/model#

I'm not clear what you are describing when you say "center electrode has a short connection to the shell"??
When you look at the spark plug, is there a gap between the center electrode and the bent electrode from the metal body of the plug? Stock gap should be 0.025". If there is no gap between the two electrodes then your electrodes are shorted.
If you are removing you spark plug while it is still connected to the end of the spark plug wire and holding the metal body of the plug NEAR a ground you should expect a spark to jump from the body to ground. The spark is actually jumping from the center electrode to the side electrode (body) and then from the spark plug body to ground. Nothing wrong here as long as the spark is jumping between the two electrodes first. The spark wants to jump from the center electrode to ground. Normally the body of the spark plug is grounded so it jumps to the curved electrode. If you remove the spark plug (body no longer grounded) and hold a grounded wire close to the center electrode I'd expect the spark to jump from the center electrode to the wire instead of the body.
A better test would be to ground the body of the spark plug (use battery jumper cables so you can hold the plug in the clamps and confirm there is a spark jumping the gap between the electrodes. If you cannot see the spark (low light may be needed) jumping the gap THEN you may have a shorting problem, or a bad spark plug.

On the problem with your #4 cylinder.... are you running the correct spark plugs for your head? VW Germany heads all used 1/2" long thread spark plugs. Other head makers upgraded to 3/4" long spark plug threads to allow thicker heads. If you accidentally used the longer spark plugs in the 1/2" threaded heads the end of the plug would extend into the chamber. This could possibly allow the piston to come in contact with the end of the plug. Could this be what is happening in your case? This would be pretty obvious because the electrodes would be in contact.

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AshMan40
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'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[ricky_rich]

Here is a pic of the plugs I have been running. From what I can find out both should be the right model. So maybe I just had bad luck, and put in a plug that didn't last as long as it should have?
Bosch W8AC
NGK B5HS

[GArBa]

IMHO, by just looking at the pic, the only way those plugs could be shorted is a crack in the ceramic insulation, which is not a very likely event unless you happened to drop both plugs on the floor hard enough to break them in the same way.

The oily dirt may be enough to foul the electrodes and weaken/stop the spark though, so give them a clean and test them as ashman indicated in his post. then you'll know whether they really are shorted or not.
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[ashman40]

Those are the stock 1/2" long spark plugs. I have used both and they are both good brands.

If you take an ohm meter and place one lead on the tip of the plug where you spark plug wires attach (top in your pic) and the other lead to the metal body of the spark plug where the threads are cut and the model # is stamped in... you are saying there will be zero resistance between these two points? And you have measured this? I've never heard of this happening except when the piston slams into the exposed electrode and pushes the two electrodes together. From your pic that has not happened.
Or are you basing your statement only on some empirical evidence? eg. spark jumping from the metal body to ground? Please provide more details.

I have heard of the opposite... where the electrode in the center of the spark plug breaks and you have no continuity between the center electrode and the spark plug wire.
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AshMan40
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'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[Starbucket]

Those plugs look like your motor has a big carbon build-up. You need to clean or replace the bad plug and take the car for a long high speed run (at least 50 miles. Be sure you are revving the engine up when shifting. Remember this is a modified airplane motor so it likes to run at 3000 - 3500 RPMS all day long. Lugging it around town will carbon it up. Take your plug wrench and spare plug as you will probably fowl a few before the cyl. and head are free of excess carbon.

[slalombuggy]

With that much carbon build up I'd say you have either a bad intake valve guide or bad rings. If the other cylinders are firing fine it won't be a tune up issue unless you have a bad plug wire. but then it would run poorly all the time.

brad