TOPIC:

I have questions about air flow and jetting:
If I installed a K&N air filter in an otherwise completely 'stock' as-it-came-from-the-factory OEM bike (79 KZ650-D2), will I positively need to bump up the size of the 'main jet' by one size? If I don't what can be expected?
I have learned from the forum members that there can be many causes for running rich and there are ways of solving each of the causes. However, If I wanted to lean-out a rich running condition (and) didn't want to change the jetting -generally speaking- wouldn't the increased air flow from the K&N tend to help?

This link is in the KZrider references links page . It is an excellent article that describes how to read spark plugs to determine proper jetting, ignition timing, and spark plug selection. I suggest that you read the article before making any changes.

How you can read spark plugs and select them - by Gordon Jennings

Ron

By changing to a K&N filter from an OEM one, you will flow a bit more air. This means you will need to flow a bit more gas to keep the air/fuel mixture the same. Before deciding to "lean" your mixture to prevent sooty plugs, first determine the cause of the sooty plugs. If the problem is not carburetor related, then all you will be doing is introducing another complication. I doubt your sooty plug issue is main jet related. To check to see if you have a main jet issue, do a "plug chop" test and use the fine article recommended by Ron to check the condition of your plugs while burning ONLY ON THE MAIN JET. You sooty plug issue is more likely electrical or from the pilot system (idle circuit) rather than main jet related. In fact, I would suspect that the burn on your plugs is influenced soley by the main jet less than 5 percent of the time. Most riding is done on the jet needle/needle jet metering of fuel.