by JimDirt » Mon Jun 06, 2016 8:24 am
Make sure the AP pump is squirting , and that it just misses the slide (you have to remove/lift the subframe to look into the back of the carb to see the squirt) , it is possible that there is a internal blockage , also , if you have one of those fuel cap vent caps (instead of just the regular vent hose) , try removing it and see if it runs at lower RPM then , (if it does run better then just replace the vent cap)
If replacing the jet does not fix the issue , then its either a Internal Blockage or the AP Pump or the Vent Cap (if installed) And/OR , also make sure your Hot Start is working properly , if the plunger in the carb is stuck it can cause a lean condition , and/or the O-Ring on the Fuel Screw can cause a lean condition as well , those are the area's to check , since you have already confirmed the valves are in spec , and the Pilot Jet is clean/new , though the "middle of last season" is a long time ago , and valves can go out in a few rides and be in spec , then out a ride or 2 later , so don't rule them out unless you "just" checked them , same goes for the pilot
And Most Important .... ALWAYS put FRESH gas in when trying to start/ride the bike after sitting for more than 1 month (Do This FIRST)
Your best bet is when the bike is going to sit for more than 1 month , is to drain the gas out of the carb (things like Sta-Bil don't really work in our bikes , in your Lawn Mower , yes , but these bikes require higher octane and FRESH gas to run) , you can drain the gas from your bike that wont start , and put it in your lawn mower or car/truck and it will run just fine , pour it back in the bike and no go , but put FRESH gas in and it will start right up, also, you should be able to find some Non Ethanol gas somewhere , most every state has it (Phillips 66 , and places like that) , i actually run a mixture of 110 Octane Sunoco Race Gas (or sometimes Av gas from the local small airport) , mixed with Non Ethanol Premium (1 gallon of Race/Av Gas mixed with 5 Gallons of Non Ethanol Premium , the bike will run and idle better and run cooler) , it also stopped the clogged jets every spring
2020 CRF450R
2006 CRF450X
Weiser , Idaho