My son has an '04 CRF100F and we ride at 5,300 ft elevation. Stock it was OK, but in riding it myself, it felt quite weak. It wouldn't even pull up the front tire on 1-2 power shifts. We put over 350 miles on it before beginning any mods.
I decided to apply some standard hop ups so I drilled a 1" diameter hole in the air filter cover (located behind the number plate), installed a Uni filter and removed the exhaust silencer. (BTW, I have a ThrottleUp on order so we can have a free flowing spark arrestor.) As expected, with no other changes, the bike ran like a sewer pickle. I had to go out from about 1 turn on the fuel screw to about 3.5+ turns to get an OK idle. Then, there was a major off idle bog and the bike had a bad lean miss at WOT and high rpm. All of this is reasonable if mods caused a significant increase in air flow. Stock jets were 35/98.
I installed a 102 main jet and all is MUCH improved. There is no lean miss at WOT and high rpm and the bike feels much stronger it will easily lift the front tire on 1-2 shifts, almost on 2-3 shifts. With the fuel screw set at 3+ turns out, the off-idle bog is nearly completely gone. I have a 38 slow jet on order and the bike feels like it can use an even larger main. Haven't played with the needle setting, but that may help the part throttle some. For reference, a 102 main jet corresponds to approximately a 107 at sea level and a 38 slow jet corresponds to roughly a 43 at sea level. If I go to a 105 main jet, that would correspond to roughly a 111 at sea level.
What jet combos have others settled on with similar mods? And at what elevation? Come on, some one must have been down this road. Thanks in advance!
Follow up: I tried the 105 main jet with the stock 35 slow jet. It appeared to be too rich. Not enough to generate black smoke, but it began to slightly "burble" at WOT. Went back to 102 MJ and it's feeling pretty good. BTW, fuel screw is still approx. 3+ turns out for best response and idle. New 38 slow jet is in at the dealer and I'll update when installed. Throttleup should arrive today for this weekend. Later.
Put in the 38 slow jet and the bike runs very well now with about 1.5 turns out on the mixture screw. Equivalent jetting for sea level would be 42/108. We ran it on the dyno and at both idle and WOT, the jetting was spot on. At part throttle cruise, it was rich so I need to drop the needle from the stock position. Total mods:
Uni filter 1" hole in filter cover Denso Iridium plug ThrottleUp exhaust tip 38/102 jetting