Help Me.......

Ok, I have an 07 WR450, riding between 5000 and 8000 feet. All the free mods and FMF 4.1 exhaust.

I've been working on the jetting and definitely it's running way better (Much thanks to William1) however at WOT the bike misses something terrible, it's cutting out sporadically and it only happens at very high RPM, hight than you would usually get to trail riding. It's not consistent like a rev limiter more random and choppy. If I'm in 4th or 5th on a steep hill then I can be at WOT for 5-10 sec as the RPMs rise and so the accelerator pump is not at play and then when the revs get high enough it happens. If I'm 1st or 2nd is always happens but it is so quick to get the revs up that high it's hard to know if the AP is involved. I've tried changing the main, going all the way up to a 168 and down to a 158 and no luck, it's always there. In general the bike seems to like the 160. It seems to be much more correlated to RPM than throttle position which has me thinking that maybe it's is in the ignition?


could I really need to go down to a 155, I don't have one so have not been able to try that but would be worth getting one to try.

if it's in the ignition somewhere how do I diagnose: plug, coil, CDI, crank sensor?

Any help would be greatly appreciated.


Lean the main jet. Main jet comes into play at about 7/8's throttle to WOT. 160 is kinda big, even at sea level. You should probalby be running a 150 or so,

Well that's worth a try, will need to round up a few more jets, but looking at the 450 jetting database almost no one is running anything smaller than a 160 even at my altitude so I'm skeptical.

My understanding is that jetting is more throttle position dependent, not RPM. This only happens at really high revs?


If you are going up a steep hill, under high load, you could be at WOT and only 7,000 RPM. Going back down the hill, you could be at only 1/3 throttle and at 10,000 rpms. It is the uphill run that will get you on the main and tell you if it is rich or lean.

Ok, so I think the jetting is good, 155 main, 42 pilot, NFLR needle 3rd clip.

Something else is a miss...

When the bike is cold or warm it runs great, if I go for a hard trail ride, 1st-2nd gear on steep slow rocky and rough trails the bike starts to run terrible as if the plug is fouled or something. It misses really bad when I rev it up or try to ride. Always starts OK though? It only happens on hard rides.

If I get the out on the pavement and run it WOT for several minutes it goes away.

Two things seem to correlate, rough slow speed riding and getting very hot. I don't know if running it fast for a bit is helping to clean off the plug or cooling it's down, or both?

I've checked the float level, everything seems right according to the manual, but I have very little experience. I'm going to try switch out the coil as well. What else could cause this?

I've seen a small bit of oil in the air box but doesn't seem like there is a enough to cause this, would oil spay get past the air filter, foul the plug?


You probably are still a tad rich. Slow riding makes the bike very hot which inturn requires a leaner mixture. Try the clip on the second from the top position.

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