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Old 08-01-2017, 10:23 AM   #1
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Exclamation YZ 250 doesnt wanna run!!!

whats up guys, so I recently rescued a 1985 YZ 250 from the graveyard. The top end was rebuilt very recently, the piston top and skirts are shiny, cylinder walls immaculate, ports on the cylinder spotless, etc. I dont think its ever been ran yet. cranks 180 psi cold, strong blue spark, brand new reed petals that seal perfectly and new gaskets, cleaned and rebuilt carb with stock jets and settings (everything on the bike is stock), new air filter, fuel line tank and filters are clean and im running fresh fuel. this thing wont even fire once with starting fluid

I found the timing was pretty advanced so I put it back to middle setting, still no fire. not a budge. Im stumped, yamahas always give me problems
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Old 08-01-2017, 01:21 PM   #2
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Sheared woodruff key?
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Old 08-01-2017, 01:30 PM   #3
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Were the carb jets new when rebuilt or used?
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Old 08-01-2017, 01:31 PM   #4
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Actually scratch that. If it won't even try to fire on starting fluid then something is off on the timing side.
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Old 08-01-2017, 02:20 PM   #5
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Sounds like you covered all the main bases. I'd agree it has to be something to do with the timing. Process of elimination.
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Old 08-01-2017, 05:37 PM   #6
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Sheared woodruff key?
nope, woodruff key is perfectly intact
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Old 08-01-2017, 05:39 PM   #7
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Quote:
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Sounds like you covered all the main bases. I'd agree it has to be something to do with the timing. Process of elimination.
I would think starting would be more difficult with slightly retarded timing versus advanced?
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Old 08-01-2017, 07:52 PM   #8
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I'm thinking the timing it's a lot further off than just a little retarded or advanced.
It seems like with all the stuff you listed and some starter fluid, it would have to be like 180deg out to not fire a little.
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Old 08-01-2017, 09:35 PM   #9
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Quote:
Originally Posted by bumblebee View Post
I'm thinking the timing it's a lot further off than just a little retarded or advanced.
It seems like with all the stuff you listed and some starter fluid, it would have to be like 180deg out to not fire a little.
it couldn't be. ill go over basics again. I'm afraid the piston could've been installed reversed by the PO, ill check that too
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Old 08-02-2017, 09:47 PM   #10
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piston is installed correctly, tried retarding timing and it didn't work, advanced it again and still didn't work. didn't fire on starting fluid nor a good squirt of premix.

Im completely stumped..... stupid yamahahahahas

today I got my brother to help, he drove his truck while I held on and then dropped it into 2nd gear. did this multiple times and it actually fired a few times but never actually started and ran. the timing was all the way advanced while doing this, and it only fired with the throttle wide open
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Old 08-02-2017, 09:55 PM   #11
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exhaust is not blocked nor the intake, still has strong compression and good spark
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Old 08-03-2017, 06:20 AM   #12
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So to make an engine run you need fuel, air and spark.
Even if nothing else is working right, starting fluid would be fuel.
You said you have a fat blue spark so that's good.
You said you have 180psi so that's good enough.

What's left?
It seems to me that if everything is present BUT the spark is happening at the wrong time it won't fire.
Timing is my best guess at internet troubleshooting.
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Old 08-03-2017, 07:07 AM   #13
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Could be a bad CDI module.
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Old 08-03-2017, 07:40 AM   #14
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This type of dirt bike is a little before my time. But have you checked the pick up and the flywheel? Did you see if the flywheel has been installed with the key, that would be enough to throw off timing. The wrong flywheel could also, cause it to spark at the wrong time.
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Old 08-03-2017, 10:20 AM   #15
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Quote:
Originally Posted by bumblebee View Post
So to make an engine run you need fuel, air and spark.
Even if nothing else is working right, starting fluid would be fuel.
You said you have a fat blue spark so that's good.
You said you have 180psi so that's good enough.

What's left?
It seems to me that if everything is present BUT the spark is happening at the wrong time it won't fire.
Timing is my best guess at internet troubleshooting.
exactly, theres no reason why its not starting even on starting fluid, its gotta be something with the spark even though theres plenty of it. it did fire a few times though
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Old 08-03-2017, 10:26 AM   #16
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Quote:
Originally Posted by SilverBullet View Post
Could be a bad CDI module.
not sure, its got good spark and it did fire a few times, but I couldnt imagine it being anything else. with the carb and exhaust completely off, if its got good spark and compression itll still fire up with fuel in the cylinder

is there a way of testing this with a voltmeter?
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Old 08-03-2017, 10:28 AM   #17
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Quote:
Originally Posted by bob_1 View Post
This type of dirt bike is a little before my time. But have you checked the pick up and the flywheel? Did you see if the flywheel has been installed with the key, that would be enough to throw off timing. The wrong flywheel could also, cause it to spark at the wrong time.
looks like the correct flywheel to me, the timing marks match up. the woodruff key on the crank is perfectly intact
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Old 08-03-2017, 11:19 AM   #18
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Quote:
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is there a way of testing this with a voltmeter?
Not a full functional test I don't think. CDI modules are slightly different between brands and models. If completely dead you could probably ohm it out to tell. But they also control timing advance so probably cant test for that. If you're really at wits end and dont have a known good one to swap out you could try a low temperature oven bake. Been known to help cold solder joints and or heal a weak pcb trace. Google for time/temps.
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Old 08-03-2017, 01:08 PM   #19
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Quote:
Originally Posted by SilverBullet View Post
Not a full functional test I don't think. CDI modules are slightly different between brands and models. If completely dead you could probably ohm it out to tell. But they also control timing advance so probably cant test for that. If you're really at wits end and dont have a known good one to swap out you could try a low temperature oven bake. Been known to help cold solder joints and or heal a weak pcb trace. Google for time/temps.
Ive actually heard of this method, thanks! I might have to try this
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Old 08-03-2017, 02:53 PM   #20
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I would put a new pilot jet in there for and giggles.
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