[Old Henry]

Definitely want to check the spark plugs not long after reducing jet size to make sure you're not too lean for your altitude. Plug should be light tan. If too light - like white - too lean. If just dark and not tan - too rich. If black carbon deposits on them - way too rich. But I ran 51's over 12,000 feet going over Rocky Mountain National Park a year ago and my plugs were never black carbon - just dark. I had put the larger jets in a few years ago to reduce my engine temperature which it did but lowered my gas mileage. Switched them back and got 3 more highway mpg and plugs are now light tan, almost on the white side, but OK. So, it's a trade off - a delicate balance - richer is cooler but less efficient. Leaner is more efficient (better gas mileage) but hotter.