Slant Six Forum

Fouled Spark Plugs
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Author:  Mr. Mopar [ Sun May 19, 2019 10:34 am ]
Post subject:  Fouled Spark Plugs

Hello all,

I know there are a million threads about this topic already, but before I go through the laundry list of solutions they offer, I want to see if I'm missing something simple and specific to my situation (instead of messing things up that are already fine).

My car, an original-engine 1971 225-cube Six with the Holley 1920, is fouling up its spark plugs and, as a result, misfiring and running really rough on idle.

A similar rough idle issue was, a few weeks ago, linked to a massive vacuum leak: my valve cover bolts had backed off, and oil got in a bunch of places it shouldn't, clogging the air filter and wreaking havoc. I figured that out during a tune-up in which I also replaced the plugs with new NGK platinum-tip UR5s; and the choke thermostat, which I had been told a few years back by a carb expert was in need of replacement. (It looks like the linkage rod was shortened and re-welded together.)

After the tune-up, the car ran great for about a half-hour, absolutely perfectly; the engine was already warm from being on and off during our troubleshooting, so not sure what the choke thermostat was up to at that point. Two days later, I gave it a start from cold and let it idle: smooth for the first minute, and then gradually rougher for the next minute. After a twenty-minute drive, it was missing badly at idle, and almost stalling at stoplights. Pulling a plug (pictured) shows dry fouling, I'm assuming from running rich.

I have a few theories I want to mull over before giving it and taking it to a shop:

1) Is it possible the plugs themselves could be the issue, and replacing them with a more-correct set would solve the whole thing? I got my hands on some NGK ZFR5Ns, removed the crush washers and gapped them to between 0.035 and 0.040, but don't want to try this guess out just to foul them up, too.

2) Could the new choke thermostat be feeding it too much fuel at idle? If so, is it better to switch back to the old, original one or to tune the carb with this new one in place?

3) The car just came out of storage so the gas is stale, and, as noted above, there's probably still oil in a bunch of wrong places. Is it possible these things could contribute to the plug fouling? Is it maybe wet-fouling and not a mixture issue at all, and I still have a leak somewhere?

Sorry for clogging up the forum with an age-old question. With the really rough idle, I don't want to take it farther than the Crappy Tire (er, Canadian Tire) up the street, but I really hate going to a shop where they do who-knows-what to it.

Thanks for any help! Much appreciated!

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spark plugs May 2019 2 resize.jpg [ 178.23 KiB | Viewed 3055 times ]

Author:  Dart270 [ Sun May 19, 2019 11:14 am ]
Post subject:  Re: Fouled Spark Plugs

Have you adjusted the valve lash? If not, time to do that. That can cause these symptoms, as can other things, but it is best to do the valve adjustment and make sure that is not the problem before you work on other things.


Author:  slantzilla [ Sun May 19, 2019 11:45 am ]
Post subject:  Re: Fouled Spark Plugs

Just my experience, platinum ugs are not the best choice, especially for cars that get started and shut off frequently. They need to be started, warmed up, and run a good amount of time before being shut off or they foul.

Hyster used GM V-6s that had platinum plugs in them. We would get trucks in that had already fouled some just loading and unloading them, and that was with fuel injected engines. The factory fix was to have us pull them and install regular plugs.

I'm guess that with a choke you're not running it long enough at a time.

Author:  Joshie225 [ Sun May 19, 2019 1:35 pm ]
Post subject:  Re: Fouled Spark Plugs

Those are the wrong spark plugs for a 1971 engine with spark plug tubes. Taper seat plugs are for later engines without spark plug tubes. You'll need to install the new plugs regardless.

As a slant requires more frequent maintenance than newer engines I suggest standard spark plugs so you get a more frequent view into the combustion chamber.

If the valve cover leaked enough oil into the spark plug tubes to soak the spark plug boots you'll need to change the spark plug wires.

I second the call for a valve lash check. .010" on the intake valves and .020" on the exhaust. If this gets reversed the engine will idle well cold and then get rough when warm.

Take the air cleaner off with the engine cold. Operate the throttle linkage to see that the choke closes. Start the engine and observe the position of the choke plate. The choke pull-off should immediately open the choke a set amount. If it doesn't check the choke pull-off operation with a vacuum pump. Warm up the engine and again observe the choke position. If it isn't fully open then you need to troubleshoot and/or adjust the choke thermostat. Also look down the carburetor with a light and make sure no fuel is dripping from the main venturi at idle. If fuel is dripping then there is a problem with the float system.

Author:  Dart270 [ Sun May 19, 2019 3:32 pm ]
Post subject:  Re: Fouled Spark Plugs

Great catch on the plugs, Josh. Plugs that are 3/8" too short will probably foul pretty fast! Still should do/check the valve lash...


Author:  SlantSixDan [ Sun May 19, 2019 4:58 pm ]
Post subject:  Re: Fouled Spark Plugs

Others have already ID'd your incorrect plugs.

Tune-up parts and technique suggestions in this post. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.

Author:  Mr. Mopar [ Sat May 25, 2019 6:43 pm ]
Post subject:  Re: Fouled Spark Plugs

Thanks for the advice, guys! I've heard valve lash adjustment from a few different sources.

I may have to get around to doing that sooner than later, but I found out the more immediate issue: I still have a massive vacuum leak that gets worse as the engine warms up, now.

The really poor running was caused in part, too, but the vacuum advance line falling off on the distributor side at some point, but I rectified that and am still seeing major trouble.

It goes from not-great-but-decent at idle when I first start and run it for five minutes; to a little bit worse if I stop it, let it cool and re-start it; to even worse on the next re-start; to absolute garbage by the fourth try. (Same exact behaviour driving it around the block and stopping at stop signs—worse and worse as I go, to the point of stalling every time I come down to idle, now.)

I got around to getting a vacuum gauge and hooked it up to the line off the base off the carb that usually feeds the air cleaner (I was doing this diagnostic with the air cleaner assembly off). On that second semi-crappy start, I was pulling a steady 14 inches of vacuum until after a minute the engine smoothed out and kicked up to a steady 16. On the third start it went from a dancing-needle 9 inches to a steady 15; and on the garbage-running fourth start it was a dancing 6 inches, "smoothing" out to a dancing 10. So, yeah, massive vacuum leak somewhere I can't readily spot.

1) Does it make sense that a vacuum leak would make the engine run lean? I'm still getting a heavy raw gasoline smell, and if it's running rich, I think that would explain my fouled carbs.

2) Where should I be checking for a leak that gets worse as the engine gets warmer? I know the diaphragm in the carb is one suspect, but I also know there was a once-small crack in a runner in my exhaust manifold that I patched with cement.

Oh, one other symptom: I'm getting a few wisps of some sort of vapor coming out of the valve cover oil breather hose – I just replaced the breather with a new one – after shut-off, if that's relevant.

Joshie225, I think you're on to something with the choke thermostat. As I mentioned, I recently replaced the looks-like-it's-been-shortened old choke thermostat with a new one, but am going to try switching it back first thing.

Thanks again, guys, and apologies for any errors or incorrect assumptions in the above. I'm still learning a whole lot and am feeling at the end of my rope.

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