I hope this doesn't turn into an oil thread? I'm putting it here because I don't always have time to respond the all of them very well. In the last 2 years I have been getting a WHOLE bunch of the same inquiry about "tuning" carbs. It goes something like this:
"I [added or subtracted] exhaust pipes of (some kind) to my bike and wonder how I should increase the jetting?
Here is the proper answer: use the stock size jets, unless the pipes are bona fide, open megaphone, RACING pipes. I don't mean Kerker, or [fill-in-the-blank] 4-1 or 4-2 pipes, but REAL racing pipes like the Yoshimura or Dunstall systems. Then, increasing the jetting 5%-10% may be appropriate.
Or another one:
"I big-bored my SOHC4 and need to know what jetting I should install."
Answer: the same as it was before. You will likely find that if it ran fine before, it may be a little on the rich side now, especially if it is the 750 or 350F and you went oversquare on the bore, because now the mainjet will be sending fuel a little bit longer during each stroke due to the vacuum being deeper-per-RPM by a little bit. Mostly, though, if you suffer ethanol-laced fuels that run about 2%-3% lean by design, it may fall spot-on instead.
The 3rd big one is:
"I installed a [long-duration, hi-lift] cam and now it starts poorly and the plugs are dark. How do I rejet?"
Answer for this one: the idle circuit needs to be leaned out because the airflow during start and less-than-2500 RPM speeds is lower than it used to be. This is due to spitback during the overlap cycle, which is always increased in long-duration cams that open early. When the air moves slower, the carb acts as if the engine is running slower: these carbs mix much richer at low speeds than at medium or high speeds, because of the short pulse-feed duration. So, since most of these bikes before 1976 used $40 pilot jets, try a #38 with the very same air screw setting, or if still rich (rare) try a #35 pilot jet, still using stock-like air screw turns. On the post-1976 engines, the pilots are usually between #38 and #35 already: on those bikes, this works out fine all by itself in most cases, and the starting is better than it was with the stock setup.
The 4th most common one is:
"I have to set my air screws to [either way inward past spec, or way outward past spec] to make it idle, but then it [acts wrong, somehow]." Here's the #1 clue for this one: set the air screws to the factory setting first, THEN find out why it won't work with that setting. In other words, use the built-in design to help you find out what ELSE is broken. Otherwise you will chase your tail (and probably someone else's too!) trying to find out what the trouble really is: the carbs can be used to point you in the right direction, more quickly. Don't try to use the carb's adjustments to cover over some other failure, unless you're on the road and need to limp home.
