is jetting neccesary?

I have a 2003 xr100 with an fmf powercore 4 system and a uni air filter I have naticed som power increase but not as much as I had expected is this because I need to rejet it :D

I can't tell you what sizes but you probably really need to rejet :D

Check your spark plug. Most likely you are running lean. Correct me if I am wrong but it is white or gray stuff on the plug when it is lean right? Also if there is any bogging, slow revs, or a not very smooth accel (compared to the properly jetted stock bike) it could need to be adjusted.

no it isnt running lean if any thing I propably need to open up the mixture a turn it feels starved at high rpm :D

Im thinking about getting some iridium plugs has anyone tried them do they make a difference?

also, try, i think a 102 main...

no it isnt running lean if any thing I propably need to open up the mixture a turn it feels starved at high rpm :D

If your bike was jetted correctly before the mods it should be running lean. When you put a high flow filter on you are adding more air, which means you need more gas (lean). Also I believe aftermarket exhaust will make them run lean as well. Just try larger mains. Also when you say it feels starved, it probably is starved...of fuel!

Yep, starved means lean. Get a 102 or 105 main.

I've tried the FMF and spoke to the folks at the factory... "They say" the pipe was designed to be a bolt on application with no jetting needed... I was not happy with the performance of the pipe...

The factory sets these bikes up on the cusp of being lean anyway... Emissions, EPA or whatever... Try playing with the jets... A good way to tune for power is find a hill that is gradual... try running the hill is second or third gear at around 3/4 to full throttle and as you pull the hill if the bike starts to pop or starve for fuel go up one size on the main. Every time you change the jet check your plug...

Also since you just put the pipe on... what color is the inside of the can... Black and sooty or white to grey...?

more white to gray

that says it's lean... not enough fuel... too much air... good way to burn up an air cooled motor... go up two sizes on the main... ( always better to be rich ) and then try the hill trick... grey to tan is where I like it...

They go in strange increments... depending on manufacturer..

typically 3 2 3 2 3 2... or 102, 105, 107, 110, 112, 115 and so on...

what gets strange is I've seen 107.5 and 97.5 .... grab a handfull where ever you get them... they are always good to have around...

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