Basic jetting help

[lisnup65]

My neighbor has a 1999 RM 125. He just picked the machine a few weeks ago for his son. The bike has a FMF Fatty with FMF silencer. The air cleaner is a Twin air. The machine seems to load up quite easy at idle and during low rpm's. I asked him if the jetting was stock or if the previous owner rejetted like they should have with the new pipe and air filter. We checked and its stock jets, so its time to make the change. I know that jetting is kinda of trial and error, but we are looking for a basic step in the right direction. I checked the FMF website and they give jetting for the Fatty pipe with stick filter. I beleive the Twin air is equivalant to stock with better particale removal. He thinks it allows the machine to breather better like a K&N so he is leary about using the FMF recommended jetting. Anyone ever run a simple setup like this that can help determine if the FMF recommendation would be the best starting point?

[TimSr]

The best way to jet anything is take plug readings. You first need a wide open reading, running at full throttle under load for at least 15 seconds, (doesnt have to be in high gear) then pull the clutch and kill the bike while its coasting to a stop. Check the plug which will tell you okay, too rich, or too lean. Adjust main jet if necessary. Rerun test if adjustents are made. Only after you have the main jet setup right, repeat the test at half throttle, and adjust needle accordingly. Then you blip the throttle off of idle, and adjust the idle air screw at 1/4 turn increments in both directions, until you get the best throttle response off of idle.

A common mistake people make is in trying to follow some formula of this jet for that pipe or this air filter. The problem is you can by two brand new machines of the same model from the same dealer, and they may require different carburetion settings when you get them home. I have also found that if your carburetion is setup right to begin with, changing filters and even pipes requires jet changes less than half the time.