keep fouling spark plugs

[bean]

Please help! I keep fouling spark plugs in the 250 in my 69. I'm wondering if the valves aren't adjusted right or the carb needs a kit... or what? Any ideas?

[ChevLoRay]

Too rich a mixture can definitely foul your plugs, as well as the wrong heat range. Too weak a spark is another issue...points, condenser, rotor, cap and wires all can affect the strength of the spark.

Later-model 6's had an HEI distributor....get one and eliminate the points and condenser as a possibility and gain a hotter spark in the process.


All too often, the carburetor gets blamed because it is a mystical device. Don't automatically assume that it is to blame. Make sure you have a good source of ignition.

On the other hand, if you see fuel leaking from the throttle shaft or other parts of the carburetor, an overhaul is certainly in the near future.

But make sure you have the best spark and are using the correct plug. Wires break down over time, due to age and from deteriorating insulators. Oil that gets on the outside of the wires can degrade them.

Worn rings allow fuel and oil to be drawn into the combustion chamber, and the oil component will eventually become baked onto the electrode and foul the plug.

What color is the deposit, and is it crusty, or just wet?

[bean]

it's just a black soot. not really wet. so would that be too rich a setting with the carb?

[ChevLoRay]

Could be. I used to set the idle air screws at about 2-turns out from the bottomed position. Of course, having a Sun machine connected to the engine made it a lot easier. You gotta be sure the PCV valve is good and that all of the vacuum hoses are, too. With the idle air screw set at an arbitrary point, and with the engine idling, and with a real good ear, you then turn the same screw SLOWLY back and forth to discern the point at which it idles best. This assumes that idle air is the culprit. Having an exhaust gas analyzer hooked up is another way to know if you have achieved the best setting.

But, it could be the engine just doesn't get warmed up properly or the choke pull-off is bad, not letting in a full wad of fresh air. The choke may be set incorrectly, too. If it has the original carb, you'll see rich and lean indicated on the black choke housing. Should be three screws holding it in place. Just loosen them and turn it achieve a change. With the engine warmed up properly, I just turned the black housing until the choke was in the upright position, and so it would close when it was cold and you had "set" the choke by pressing the accelerator pedal to the floor when the engine was cold. At best, these are guidelines and only serve as a suggestion.....they depend on trial and error.....like my attorney when I got divorced. Got the right one the second time.

[bean]

Lmao! I'll try what you said about it being too rich. You said you start with yours two turns from bottom? Then adjust by ear right?

[71big block]

I like to use a vacume guace and adjust the idle mixture screws till I get the highest reading

[Wrenchbender Ret]

Sounds like the carb needs some work. Often on that carb the link on the throttle shaft falls out of the hole in the metering rod letting it run on full rich. Look at the shaft on the side opposite of the throttle lever. there are 2 links one to the accel. pump & 1 to the metering rod. Those arnt too hard to o-haul if you are good with mechanics.

[68gmsee]

After you adjust the carb and checked the easy parts...

Check the compression on the cylinders. As ChevLoRay mentioned, bad rings can cause this problem and also worn valve guides. These old engines don't last as long as the newer ones that can go for 100k or 200k without any major engine problems. These old engines would do good to go 60k or better without some major rework.

My 69 got so bad from worn valve guides that I was changing two spark plugs a week! It finally backfired on me and blew my muffler. That's when I decided it was time for a rebuild...