Do spark plugs usually foul up in city driving?

1966 420, 2x HD8s 4200cc. Champion N11YCs

Hi all, how long should a set of spark plugs last before fouling in heavy city driving? Should they foul up at all?

I ask because my tune feels excellent. Perfect idle, cruises beautifully, power is plentiful. But in a few hundred KMs I’m removing/cleaning the plugs due to dry fouling and rough running. No black smoke, no smelly exhaust. Any leaner and the car runs poorly. I’ve just upped heat range to the NY11s but still having issues.

I’m very confident the engine is in good condition, no oil burning, no vacuum leaks ECT. The plugs clear up somewhat after 30mins on the highway.

This car is daily driven in stop start city traffic. I don’t mind cleaning the plugs, I’m just concerned that there may be other issues I’m not seeing. Maybe my AED is set too rich, I will look into this.

I’d appreciate your thoughts.

I went from UM to UB needles and adjusted it slightly lean (too lean for the test in the manual) at idle - still idles smoothly, not very smelly and runs fine. If your plugs soot up in the city try a hotter plug: you did that already.

Do you mean it runs poorly or it idles poorly?
Why do you have an AED on a 420? You should have an ASC. This can be killed by very quickly turning the engine off and on again, I do that sometimes. The ASC setup is described in the manual. An ASC should never cause plug fouling.

What’s Your city mpg ?

In normal city use, they should never foul up at all. Something is majorly wrong with your mixture settings.

I concur. My dead stock 3.8S (HD6 carbs) does not foul plugs and hasn’t in the 56 years I’ve had it. Plug life is about 10K-12K miles though, but maybe better with unleaded gasoline now.

Perhaps installing a manually controlled choke mechanism would solve the problem

The guys have tackled most of this, I expect. The few times the plugs have fouled in our 4.2 XJ6, it’s been my fault. Are the front and rear three equally fouled? Also, what fuel grade are you using? In Australia we get 98RON, thats what I run.

The grade of fuel should make very little difference: properly set up, you can use the lowest grade or the highest grade, and the plugs won’t foul.

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Thanks for the inputs, I will continue to look for a problem. I use an auxiliary starting carb original to the 420 with a manual switch. 98 unleaded and an electronic ignition. Around 15, 16 mpg with a manual gearbox.

I have noticed the rear carb soots up more but I struggle to explain why. The float height is set correctly, seals are good, butterflies set how they should be, air flow balanced. Any leaner and it runs poorly.

I will look into using different needles again. I have UEs on hand. Thanks