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diagnose this...
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Posted by: slazzies---------------------
so i bought a brand new YZ 450F about 10 days ago. needless to say i was pretty excited, 1st new bike i ever bought, and to this point i've been riding a '96 KX125. i got it home and took it straight out and rode it for about an hour, not real hard, just trying to break it in, get a feel for the powerband and simply riding a bigger bike. it felt a little weird, but thought nothing of it at the time.
this past weekend, i went out again, and rode for a few hours, and i think there definitely is something wrong with it. it backfires, and doesn't seem to have top end power, when you get to about the top 2/3 of the power band it seems to want to stall out and not want to catch. i drained the carb, thinking that might help, it didn't. i had originally thought maybe it was the jetting and that it was running lean (its running really hot), but if it were i would think that it would run better not worse.
anyone have any ideas to what it might be, or have the same bike (or YZ 400.426) and experience any similar problems?
thanks for all the help,
S
Posted by: marcusgunby---------------------
If it is running lean it will lack top end power and run hot.
Posted by: Ty---------------------
Make sure your hot start is not in a bind ( held open)
Posted by: Ty---------------------
..also, make sure that the head to carb manifold is secured on both ends..
Posted by: slazzies---------------------
thanks for the help. i'd checked the hot start and it appears to be fine, i'll check the carb manifold, tomorrow, hopefully thats it so that i can ride this weekend.
-s
Posted by: CanadianRidr---------------------
Slazzies, read up on the jetting of this bike. I doubt it's your manifold, it sounds like the bike is defiantely lean.
Posted by: MxKid512---------------------
Looks like you'll have plenty of time to figure it out this weekend. We need all this rain, but it certainly stinks for riding. 
Posted by: BillyWho---------------------
If it back fires when you let off the gas and it's running hot, and with top end loss, I would say it is for sure way lean. I would get the next two sizes up on mains, try the first step up and if that helps, jump right to the next size and start moveing your needle positions to fine tune it after you get the main squared away. My CRF450 had the same simptoms when I went to OK., it took two sizes up on the main to fix it. Better to be a tad rich than lean.
Posted by: 380EXCman---------------------
Take it back to the dealer. They should atleast deliver a bike to you that is ball-park in the jetting dept.
Posted by: Camstyn---------------------
If back firing is attributed to being lean, what signs are there when your 4-stroke is too rich?
Posted by: Ty---------------------
I agree with everyone that thinks it is a lean condition, I just find it strange that a manuf. would send out a bike that far off. That's why I suggested looking for air leaks
Posted by: biglou---------------------
I would check jetting, air leaks and clean the carb thoroughly. Look for obstructions in the jets and carb passages.
Cam-It is my experience that a rich condition (in the pilot circuits) leads to an off-idle stumble/stall. Not sure about up top, I rarely spend extended time in the upper rpm range.
Posted by: Vic---------------------
Might wanna check the timing and replace the plug. Bad plugs are pretty rare, but they happen. Or, someone may have dropped it or hit it with a wrench or something and cracked the insulator. Always try the easy stuff, first.
Other thoughts: (unlikely, but possible)
valves misadjusted/bad
cam timing off
Posted by: slazzies---------------------
thanks for all the help.
i took it back to the dealer and they rejetted, one size up. as MxKid512 said, the weather here sucked this weekend. late in the afternoon i rode it around my neighborhood for about 15 minutes, let it get nice and warm and then opened up, no problem, no backfiring, no loss of power.
i won't get to really ride until next week, but hopefully this corrected the problem (cross's fingers)
for those look at '03 YZ 450F and YZ 250F, quite a few seem to come jetted lean, once this is fixed your golden.
thanks again,
slazzies
Posted by: cmott---------------------
I got this info from BBR's Web site
Thumper Troubleshooting
Bike won’t start after a crash
Pilot too lean
Idle set too high
Improper starting procedure
Bike wants hot start button
Bike runs on or won’t idle down when throttle is chopped
Idle set too high
Air leak intake or engine
Pilot too rich (when bike is hot)
Bike won’t start when cold outside
Pilot jet too lean
Air filter is over-oiled
Motor oil too thick for temperature
Bike sputters/won’t clean out at high RPM
Main jet too rich
Air filter is over-oiled
Spark plug has debris on electrode
Bike coughs and stalls in slow turns
Pilot too lean
Idle set too low
Valves set too tight
Decompressor is set too tight, so turning the bars engages release slightly
Bike hesitates or bogs over deep whoops or G-outs
Float level too low
Carb vent tubes blocked
Main jet splash shield not installed
Float level too high, gas is trapped in vent tubes (install T-vents)
Bike starts but won’t take throttle without sputtering
Pilot jet too rich
Water in fuel
Debris in main jet
Bike suddenly starts sputtering/gas flows from vent tubes
Stuck float check valve
Debris in gas or carb
Bike runs hot/feels slow and flat on straights
Main jet is too lean
Fuel octane low, causing detonation
Bike coughs and stalls when you wick open throttle
Needle too lean
Slide cutaway too lean
Pumper circuit blocked or too lean
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