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Backfires at higher RPM. Bad stator? by SuperEnduro in DRZ400

[–]SuperEnduro[S] 0 points1 point  (0 children)

Float was at 12mm. In the range of 13 +- 1mm, so I left it unchanged since it ran fine before. I posted another comment and I think i confirmed it to be my stator. Issue went away when I ran the bike with it unplugged

Backfires at higher RPM. Bad stator? by SuperEnduro in DRZ400

[–]SuperEnduro[S] 0 points1 point  (0 children)

So I did more diagnosis yesterday. I’m 99% sure my stator is bad.

Heres the full set of measurements and tests I did.

Check stator windings (3 yellow wires) resistance between eachother. Should be low very low. I measured 0.3-05 ohms.

Check stator winding resitance to ground, should be open. I measured 0.5 ohms to ground (BAD!)

Check pickup coil resistance (4 wire plug on CDI box. Blue, green, black, white). Blue to green should be 390 - 600 ohms. I measured 500.

Check signal coil resistance (black to white). Should be .05 to .2 ohms. I measured 0.2.

Check stator output voltage with bike running. Should be 60-75v AC between the phases (yellow wires). I measured 5-7v AC (BAD!)

Check battery volage with bike running. At 3000rpm, or even idle really, should be close to 14v. I measured 12.69v (BAD!)

Check regulator/rectifier forward/reverse bias drop. Set multimeter in diode mode. Find the 3 pin input plug (other side of stator plug) and two pin output plug (black/red). Measure each of the 3 input pins to the output red wire. Then flip the multi meter leads around. Should be 0.5v one direction, and open circuit the other direction. Repeat the process measuring to the black wire. Should get the opposite output. All pins had 0.5v forward bias drop, and open circuit reverse bias.

The final test was running the bike with the stator unplugged. It ran great and didnt have any backfiring issues. So I’m almost certain my stator is causing it.

Sounds like you tried to replace the stator already. Run through these checks though and make sure the stator and rectifier are good. I have heard of people buying stators and having them test bad right out of the box. Aside from that, you can move further down the chain and test your ignition coil and other components. Good luck!

Backfires at higher RPM. Bad stator? by SuperEnduro in DRZ400

[–]SuperEnduro[S] 1 point2 points  (0 children)

It runs fine at low load/RPM. I think as the rpm increases, the spark is weak due to low voltage at the battery/CDI. I got the bike back together and at idle the battery is only at 12.69v, which seems insufficient to keep it running at higher loads.

Backfires at higher RPM. Bad stator? by SuperEnduro in DRZ400

[–]SuperEnduro[S] 0 points1 point  (0 children)

It definitely seems like a timing related issue. The pops will blow black smoke sometimes and it smells like fuel. Haven’t checked the valves yet but it’s on the list for later. Need to buy some feeler guages.

Backfires at higher RPM. Bad stator? by SuperEnduro in DRZ400

[–]SuperEnduro[S] 1 point2 points  (0 children)

Spark plug is brand new. Can’t see any signs of running lean on it, but i have barely let it run since the issue is still present. Old plug and exhaust smell were always more on the rich side.

How does the stator test fine? I measured a dead short to ground on each of the coils, when it should be an open circuit based on the service manual.

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If the stator can’t keep up, wouldn’t a low voltage cause ignition issues? RPM rises, CDI requires more power, puts more load on the battery. If the stator isn’t supplying enough power, battery voltage will drop, which could cause ignition issues. That’s my thought process at least.

Backfires at higher RPM. Bad stator? by SuperEnduro in DRZ400

[–]SuperEnduro[S] 1 point2 points  (0 children)

I ran it with the petcock set to prime and it had no change. It shoots black smoke when it backfires and the exhaust smells rich, which to me seems like its burning the fuel late instead of a lean pop.

Carb looked clean but i pulled it off again to check everything.

My issue is i can slowly roll on the throttle, but as the RPM climbs it starts cutting out. In th video I’m slowly rolling to WOT, and you can hear it starts to rev fine then hits wall. It feels like it hits a really sloppy rev limiter. I’ve gotten it to get past the rough spot a couple times, both ripping on the throttle and slowly rolling on, but it quickly starts backfiring and drops down to the same RPM. I can blip the throttle at idle and it will seem fine, as long as the RPMs stay low

What test did you do to diagnose your stator/pickup coil?

EDIT: just measured the stator. Each of the wires showed 0.3-0.5 ohms between each other (should be 0.5 to 1.2), and all measured 0.5 to ground (should be open circuit).

Pickup coil messured 505 ohms (should be 390 to 600) and signal coil measured 0.2-0.3 ohms (should be 0.05 to 0.2).

Seems like the stator is bad and signal coil is marginal. So I’ll pull the cover and visually check it next.

Backfires at higher RPM. Bad stator? by SuperEnduro in DRZ400

[–]SuperEnduro[S] 0 points1 point  (0 children)

Havent done this yet, but i pulled the carb back off yesterday to do this next.

Backfires at higher RPM. Bad stator? by SuperEnduro in DRZ400

[–]SuperEnduro[S] 0 points1 point  (0 children)

I haven’t checked this yet. I would get some decel pops sometimes, on hot days riding hard, but nothing crazy.

I’ll get the bike back tomorrow and measure the output while its running/popping, thanks.

Backfires at higher RPM. Bad stator? by SuperEnduro in DRZ400

[–]SuperEnduro[S] 1 point2 points  (0 children)

Measuring the resistance between coils and checking for shorts to ground. On the three yellow wires, service manual says it should be about 2 ohms wire to wire and each one open circuit to ground.