YZ 85 dies in band then big pop

My sons 03 yz 85 started to die then a big pop, so we took it to a mechanic and we found the front of the piston was in very poor shape and barrel scored, so we did a full rebuild, new conrod and piston barrel main bearings etc, while the motor was out we powder coated the frame black, new plastics, put it back together, went way better, but still died out, then does a big pop to finish.

I was told it sounded like the stator, so i grabbed another one, but it still does it.

whacked a new plug in the other day, went way better again, but still does the same thing, so im down to suspecting the CDI or coil, but before I blow another 3 or 4 hundred I have a question or two

how do you check whether the CDI is stuffed or the coil.

I thought if it was the coil, it just wouldnt go, but it starts, run and even goes into the band a couple of times, then just cuts out.

Any suggestions?

silencer packing? clogged exhasut? not allowing it to get up that high? cheaper to clean them out then buy more parts and play the guessing game

If you have the manual they give you a wiring diagram, what circuits to meter, and what values each circuit should read when functional. If you have a decent little multi meter it's pretty easy to trouble shoot these little bike ignition systems. Most electronic stores have digital multi meters starting around $20 (USD) Allot cheaper then buying the wrong part. Also Yamaha's Euro website has the manuals to down load, I'm pretty sure.

The coil should have resistance values you can test with a meter(in the manual). I think you can have a coil with some windings shorted out that will make a weak spark. Symptoms of a weak spark would be it idles OK but cuts out under load.

I don't know of tests you can perform on the CDI box short of replacing it.

I have had the coil tested and have tried another one in there , but still the same

the boy already has the exhuast off and we will give that a shot before we buy anything else.

I have this posted in the wr forum too and a few suggestions are coming through, but i'll go and clean out the exhuast and see how we go

thanks

we just took the guts out of the shorty pipe and the packing was solid with gunk, so we cleaned it out, repacked, same ?

thinkin of insuring it at the moment....

sounds like a carb problem to me. It could be running lean. You will more than likely need to disassemble the carb and clean it out really well and then inspect that there is no damage to any of the parts. Pay particular attention to the main jet to see if it has anything stuck in it. Also, check to make sure the floats are adjusted correctly and are not shutting off the needle valve too soon.

Try disconnecting the kill switch. I have seen a bad switch cause the exact problem you are having. It's quick and worth a shot.

Sounds like a very lean condition, I agree with ForMYride, most likely with the carb, but if it is with the ignition you have replaced stator and still doing it. It sounds like its running out of gas as you hit the powerband, check the float valve, and the float level. Does it run good when just putting around,? can you get it to idle? Does it do it at all throttle positions? or just hard ecceleration? Sorry will need a little more than "in band then pop" [no insult intended]. Good luck, let us know if you find the problem, if not .... get real specific on all aspects of engine dying.

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Techmc

Heyy, my bike has the same problem, it dies in the low rpms the makes a pop noise from the pipe have you found out what the problem is

My bike is a YZ85 2005 model