Hello, I just traded a dirt bike for a 05 YFZ450. It has a HMF full exhaust system with K&N filter. With lid off breaks up around mid range. Guy said he put jet kit in. I am going to check jets this weekend to see what is there. what should be there and what settings should I try ?
I have a 2008 yfz450, I currently have a ncvq needle I ordered for the carb. I’m at 900ft above sea level and have an hmf pipe with air box lid on, can anyone enlighten me on the jet/needle settings? And what leak jet should be on these bikes mine has a #90 I believe?
jetting is all about air flow. the most restrictive part of the system is the air box. the fmf may sound better but the improvements in airflow will be minimal at best unless you open that air box. the needle it self does not change the jetting it moves the fuel curve, to match the yz fuel curve if memory serves. moving the needle up and down does fine tune the main jet/fuel mix by bringing more fuel in at a different rpm. given the information you gave i would run factory jets and lift the needle/lower the clip one notch. run it and then check the plug. if the plug looks lean go back to the middle and one up one the main. (i cant remember the factory jetting off the top of my head but rocky mountain atv has a data sheet for every machine). the pilot is probably ok but if its hard to start up one on the pilot. i have never had to change a leak jet on any of the fcr flat slide carbs. jetting is always trial and error and most of what you read on the internet is wrong and will have you over jetted. it really sucks to jet the modern 4 strokes because you have to remove the tank to check the plug but that is the only way to get it right. if you want to get anything out of your current mods you will need to open that air box and put a k&n filter on. there are several options to do that. i cant remember for sure ( i have 6 bikes with that carb) but my yfz with a pipe, jd jet needle and the air box cut 50/170 or 175 is what i run at sea level
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