06 YZ450F Random Low RPM Stalls, Advice And Input Welcome

OK, the weekend before Easter I took my 450 out for the first time of the season, (I'm just now remembering additional symptoms which is why I'm doubting the route I took to fix the issue). The bike was running strong on the first ride, second ride I had a low RPM stall, I was probably just 1st gear putting along waiting on buddies to catch up when it happened. I think I had 3 or 4 more stalls almost identical to that one the, bike always fired right up after. Then the last ride I took, I was 3rd gear pinned hitting a pretty big dune and I had what felt like the kill switch was hit and the bike died. I looked things over and noticed my radiators were cold, so I removed the cap didn't see any coolant at that point I figured I had blown motor. I didn't want to make anything worse so I didn't try to start it just pulled it back to camp then got it home. In the garage I kicked it over and it and would fire for 1 or 2 seconds then quickly die. I still figured ether loss of compression or bad crank, I tore the motor down and the crank looked good with zero up down and barley any side to side play, piston and cylinder looked pretty worn but not blown at all. So I rebuilt the motor and sprung for a Cylinder Works 478 Big Bore and a set of Hot Cams Stage 1 got the bike back together and I'm still have a stalling issue. It will idle fine and just randomly stall, the stalls seem to become more frequent as the motor worms up 


And I'm just remembering the bike died 4 or 5 times on my first trip. Before I had what I thought was a overheating related blown motor, maybe it was just another stall? I'v rode with people who have rode along way with no coolant in there YZ/YFZ's so Im starting to think the coolant was just a coincidence.


At this point I'm starting to think that the stator may be my problem and the stalls are electrical related especially since they are more frequent as the bike worms up. 


Any input would or any experiences with intermittent problems with the 450 stator would be appreciated. 



Get a service manual and test the stator as directed. Also confirm that your fuel tank is venting properly.

One more thing...check all wire connections for corrosion and/or looseness.

Your motor wore out


So has your carb


You need to replace the wear items


slide plate and seal

needle jet

apump diaphragm

fuel screw oring (us an R&D fuel screw)


Next you need to clean your stator pick up of debris, clean the corrosion off the stator harness plug

My next step is to inspect and test the stator, I'v tested bad banshee stators in the past and they have had the correct ohms because the failure only occurred when the bike was hot enough to short out, hopefully if there is a problem with this one ill be able to tell, I have heard of people hitting the stator with a blow dryer/heat gun to simulate operating temps.

That's a place to look.  What's your typical operating altitude when this problem occurs, and how are you currently jetted? 

My elevation is 5,000 above

The Jets iv tried since the big bore rebuild

Main jet, 170, 175

Pilot jet, 45, 48

Needle, stock, jd blue #5

Leak down: boyesen quick shot adjustable

Runs best with 170, 48, jd blue

The stalling occurs with all the listed jetting combos

Well, your pilot is too big.  In fact, the 45 is probably too big at 5000'.  If your stalling is occurring when you chop the throttle suddenly after a brief moderate load, that's probably the cause.

Yeah I can throw a 42 in there and see if it helps, I've had the bike for 7 years and it always liked to run fat on the jets without issue, and although I cant see the bike getting leaner with the increased displacement from the big bore I guess anything is possible and worth a shot

The bike fires up great and runs fine then as soon as it warms up then it dies, I'll start it back up then it quckly dies again and so on, the throttle doesn't have much influence unless I keep it in the high rpms to keep it running as soon as they drop it stalls.... I can through up a YouTube vid if needed?

Edited by goat089

I meant to say I cant see it getting richer with a big bore

I meant to say I cant see it getting richer with a big bore


It can happen.  Bigger engine means more air through the same carb, means more air velocity, which means a stronger signal to the jets, which means they flow more fuel. 


You might also have a look at your slide vacuum release plate.  If it's quite worn at the corners, or chipped or cracked, you can start running into trouble.

Bigger engine means more air through the same carb, means more air velocity, which means a stronger signal to the jets, which means they flow more fuel. 



That makes sense.


Well tested my stator and the resistance is all within spec. I wanted to test it after running the bike and waiting for it to die but its late and I don't wanna piss off the neighbors. But I found a problem, and I hope its THE problem, the manual specs .08-.10 ohms on the coil and mine tested .02 and then I decided to throw it on BBQ until it felt close to 100 degrees and i got .00 then put it in the freezer for 10mins and it went back to .02 so I think I might try to run down to Rocky Mountain ATV tomorrow and grab a new coil, cross my fingers and hope ill be ripping up dunes this weekend.

Edited by goat089

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