Strange 2003 YZ450F Hot bog/stall

Been lurking for a while here but just finally signed up. Great forum.

So I got this 2003 YZ450F. Bike runs awesome for the first 4 miles on the trail, then in a short time bogs down, and becomes unresponsive, then finally stalls. It seems to be heat related. When it happens, I tried pulling in the hot start, and then trying the choke to see if it was fuel related - no change either way. It seems to idle normal-ish, but when the throttle is cracked, and it is in gear, only the tone changes. Totally no power, and there is nothing I can do to keep it from dying.

Once it cools down for a bit (20 minutesit starts and runs great again for another couple miles until it get good and hot.

For what it's worth it was slow trail riding so I'm s the bike got pretty hot, but it never boiled over.

Main jet is a 175, riding pretty close to sea level with a full length exhaust and header.

Supposedly has a brand new CDI box replaced by previous owner (now I know why he put it in).

Does anyone know if they have a temp sensor that kills the ignition partially if it gets too hot?

I would love to check it all out, but riding 4 miles then ripping the seat, shrouds, and gas tank off in the middle of nowhere (before it cools off) to check things out is a little inconvenient.

I am leaning away from thinking it is a fuel/plug problem, because it comes on suddenly, not like a fouled plug where is gets sluggish at first. Response is very snappy, up until it hits, then all of a sudden it seems to be firing only every-other and will build no RPM what so ever, then another 100 yards or so and it stalls out completely.

Would love to hear from anyone with something similar or maybe point me in a direction here. Feel free to be technical - I am an ASE Master Tech and own my own repair shop for my day job.

Thanks in advance.

When it starts to act up, loosen the gas cap. See if that clears it up.

If your gas cap vent line has the OEM one way check valve in it, be sure that it's oriented with the arrow pointing toward the tank.

Good idea - its an aftermarket one. Just checked it - air comes through the tube no problem. Back to the drawing board... Any other ideas?

You blew from the hose down through the cap? Might not hurt to try the test I suggested anyway.

Another thing to try is to remove the fuel line and see if you get a continuous flow of fuel at an adequate rate. If that works, reconnect it and remove the bowl plug from the bottom of the carb and turn the gas on again. Should be nearly as fast coming from the carb as from the tank.

Obviously, do this outside, away from sources of ignition. Gasoline in the open is dangerous.

Thanks Grayracer513

Yes, I actually pulled the hose off at the cap and sucked on it. I can suck air through the valve, but can't blow air back through it. The tank would not have negative pressure, as the bleeder would let air in, but will not let it escape.

Gas flows completely freely through the line and carb. One of the things I did was pull the bowl plug and main jet out on the trail one time it stalled - just to make sure nothing was blocking the tube and no foreign material in the bowl. All looked good.

Someone I talked to today mentioned a stator "breaking down" when it got hot? Heard f anything like that?

I wish there was something I could test with a meter or visually, but needing 4 miles worth of run time to make it fail, I'm guessing that is asking a lot.

I'm gonna chenge the plug and look at the color, maybe jet the main down to a 170 while I'm there (from reading jetting posts I'm guessing 175 is a bit high). I kinda doubt that will fix anything, but its better than just staring at it. Anyone know how to test a stator or a coil on one of these? Alongside a trail?

I checked it out and it does in fact have a brand new looking procom CDI box on it. The wiring looks suspicious, so I plan on swapping out the harness, and probably the coil and stator as well.

UPDATE: Has new Cdi box, replaced harness, coil, plug, and stator. Exact same issue. Now I have a coil n plug to check it, it has no spark when it's hot. Since I replaced the majority of electrical stuff on the bike now (except TPS and neutral switch), I am at a loss as to where to look next. Ideas???

Just a thought when I had my 1994 CR250 I had problems with the pick up coil once take a look at that.

It pickes up the pulse signal off the Stator fly wheel I hope thats it for you I dont know if this bikes have a clearance between the flywheel and pick up check that also.

If you have a buddy with the same bike as you start swapping out parts My OLD KTM did the same when it got hot then die after it cooled down it run fine it was the CDI I know you swapped it out but you're a mechanic new dont mean it's good been there done that my self.

Edited by marv02

I am guessing a stator issue

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