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YZ250 died because of no spark, now all the sudden I have spark again

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So Sunday I was at a muddy s****show of an MX race. (SOOO MUDDY) About  3 laps in (18-20 min lap times, yes, it was a 8.5 mile GP style track) and my bike ('04 YZ250) bogged out. It was really odd. it bogged for a second or two, then came back and ran fine for a few seconds then bogged out again to dead. It bogged as though someone was holding the kill switch button, then they let off the button, then held it again until it died. 

 

First thing I did  once I got it back to the pits was replace the spark plug because it sounded like it maybe fouled it. Plug looked fine (light mocha color), just thought I'd try it. I was lugging it and using every bit of bottom end torque a 250 has to keep it hooked up on the up hills, but I would also run the piss out of it whenever traction allowed to get some head in the cylinder and clean it out. Kicked it a few times and  then I checked for spark. No spark. Tried a few times just to make sure and another set of eyes also didnt see any spark. I disconnected my kill switch and still nothing. Since I didnt have any spare electronic parts on hand I put the bike back together and loaded it up. 

 

Then last night I washed it all off, popped the tank off and checked spark once more to start the diagnostics process.  I didnt change anything, the bike was put together the same way as it was Sunday when I showed up to the race, I figured I should double check if there is spark or not before I began the wild goose chase. THE SOB HAD SPARK!?!?! 

 

Not sure WTF is going on now? This issue terrifies me because if this were to happen again (bog out) in a whoop section, on or a face of a triple, things will not end well for me. I'm meticulous about maintenance on the bike so everything still looks new. I'm thinking its the coil because I know (at least my experience on cars, not sure about bikes) that when they start to give out, they usually have a theme. Typically, they will quit once they're hot (killing the motor or that cylinder), then cool down and start working working again Then once they get hot, they quit again until in one of these cycles they just wont work again and are cooked. On the other hand I've got a couple of my sponsors telling me its probably my CDI box that's going. But I figured that if it was the CDI box, it would just die and never come back since its a computer/circuit/brain? The only other thing it really could be is the stator, but usually if a stator goes out, dont they just completely die?

 

The bike was pretty packed with mud (took 2-3 people to load into the truck because everything was wet and it was so heavy. But the electronics seemed to be pretty clean (before I started washing). 

 

I'm trying to determine what to do before the race this weekend. I'm going to replace parts to be sure it doesnt happen again, but I just want to make sure I replace parts that are most likely to be the cause. (Oh, I should mention that I will be going through the little wiring harness tonight with multimeter and visual inspection before buying parts to make sure) 

 

Bike info: 2004 YZ250, PC pipe and shorty, ported and polished, raised compression and squish at .045", OE shaped dome and squish band width/angle, 110 leaded fuel only, at the start of the race there were about 8 hours on a completely new motor - topend, bottomend, trans bearings, etc, for this race the ignition timing was at stock or maybe a tiny bit retarded, was running 1 heat range cooler plug (BR10EG) to help pull head out of the cylinder gapped at .040" or so, fresh air filter and the reeds were inspected at tear down and were perfect (only about 30 hours on the petals).  

 

Also of note: the radiators were clear and the bike wasnt overheating when it quit. 

 

Here are my questions:

  • Has anyone had this happen before, what was the issue?
  • Am I correct about how each of these three components typically fail? 
  • What are your opinions about what the issue could be?
  • ​I'm not goin to replace the parts with new OE ones so I'm curious if people have had any experience with the following CDI, stator, and coil? (I know the coil is no better than stock, but its 1/2 the price of an OE)

CDI and Stator ---------    https://www.procomeng.com/product.php?cat_id=4&brand_id=2&model_id=103

Coil   ----------------------    http://www.amrracing.com/catalog/monster-coils/performance-monster-coil-yamaha-45.html

  • Does anyone know where I can get a 2004 YZ250 stator rebuilt at?

 

 

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your heading in the right direction, when you have the harness off check to make sure all ground points are clean, also a bit of dielectric grease on them would not hurt.

 

if the harness checks out your only option is to swap parts out until works...crappy way i know.

 

go ride a bit to get everything hot, hopefully it fails again and you can use your tester to test all the components (test procedure and values should be in the service manual, hopefully this figures out what is bad.

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your heading in the right direction, when you have the harness off check to make sure all ground points are clean, also a bit of dielectric grease on them would not hurt.

 

if the harness checks out your only option is to swap parts out until works...crappy way i know.

 

go ride a bit to get everything hot, hopefully it fails again and you can use your tester to test all the components (test procedure and values should be in the service manual, hopefully this figures out what is bad.

 

I think that's what I'm going to do. I'm just going to check out the harness and then ride it for a bit. If it doesnt act up I'm going to replace the coil and CDI but I'm not sure if I should replace the stator or not? 

 

Oh, all the grounds are clean too and I will run the tests my manual says. Hopefully even though it works something is 'out of range' and will at least give me a clue. 

 

i'm just curious about which of the 3 main components are most common to fail?

Edited by nskyline34

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I think it might be the coil.  that's exactly what happened to me and it was the coil.  I guess they don't fail completely, but intermittently or generate a weak spark and or inconsistent.  Once replaced, problem was eliminated.  However, I do think mine was due to heat--

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I think it might be the coil.  that's exactly what happened to me and it was the coil.  I guess they don't fail completely, but intermittently or generate a weak spark and or inconsistent.  Once replaced, problem was eliminated.  However, I do think mine was due to heat--

What makes you say it was due to heat? When I checked the spark it wasnt exactly the strongest I guess, but it was definitely there. 

 

The race I did wasnt hot out (about 40* F) but I def had that bike tapped in 5th a LOT... but never overheated. I could tell when it got warm so I let off to 8/10ths and would let it cool for a bit

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What makes you say it was due to heat? When I checked the spark it wasnt exactly the strongest I guess, but it was definitely there. 

 

The race I did wasnt hot out (about 40* F) but I def had that bike tapped in 5th a LOT... but never overheated. I could tell when it got warm so I let off to 8/10ths and would let it cool for a bit

 

Mine was due to heat, and if you search google or this site, I think you'll find that the coils can fail due to heat...  there must be other reasons they fail, but I do know it causes that inconsistent or weak spark. 

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I had the same issue on a trail once and it was the wiring from the kill switch. I see that you disconnected it but did you follow the wiring all of the way down to confirm that it's not pinched anywhere?

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 Well I dont know how this will relate , just a few weeks ago I was running a Trail tech with the one wire hooked into my CDI box . It came a down pour during the race and the bike started acting crazing , somewhat similar to what you described . Ive had it hooked up that way for 10 hours or so . But it cleared up until I went to the IronMan GNCC . Went through several water crossings started doing the same thing again . Had good race going so I just reached down and YANKED the wire right out of the CDI and it cleared up . You may not have anything hooked up that way . But , I thought I would share just in case .

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