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Jeremyparkerau

YZ125 won’t start, new reeds,plugs,with spark

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Posted (edited)

2002 YZ125, bought non running, previous owner could not get it to start. He replaced the top end and new carb with no luck. I took it home and immediately took the top end off, you know how everyone has a new top end with 2 hrs on rebuild right? So I checked it out, sure enough has a new top end, new piston, new carb. I checked the ring gap just to make sure and it’s all in spec, zero play in crank. Piston is a cheap eBay piston, Namura, I plan to swap it for a wiseco but for now it will serve the purpose until I get it to crank. Compression checked good at 150. Carb I am skeptical on as it’s a cheap eBay carb but has good reviews. 55 and 450 are the jets. They could be adjusted as well but certainly good enough to at least get it started. I am getting spark, but I really don’t know if it’s enough. I replaced the reeds as well, and made sure they were seated correctly. Also replaced the boot to the carb as the old one had a crack, and just trying to eliminate problem areas. New air filter. Yes it is getting gas because the end of the spark plug is wet. I don’t think it is flooding. It’s not even acting like it wants to fire. So I am wondering if I need to replace the coil or cdi box. I premixed the gas at 32:1 and double checked it. Any suggestions? I think I am out of options other than the coil and cdi? Would you guys agree? FYI I just ordered another boot and coil.

Edited by Jeremyparkerau

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Posted (edited)

Since you're getting compression, fuel and spark,

-make sure the flywheel's woodruff key is installed / not sheared.

-check the ignition timing marks / make sure the stator plate isn't loose.

 

Double check the number as that #55 pilot jet sounds extremely rich.

(usually in the #30 to #40 range)

Edited by mlatour
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I added a little gas to the cylinder and nothing. I am still getting spark on new plug. I have checked the timing and everything is where it’s suppose to be. No loose stator plate. Just changed spark plug boot as well. I am on my 2nd pair of shoes kicking this thing. I hate to pay someone to fix it but I am all out of ideas. Any suggestions? I am more than willing to take any pics if it would help. 

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1996-2001

Measure source coil #1 between the Green/White and Black/Red wires.The resistance should measure between 720 to 1080 ohms.

My #1 coil metered at 856 ohms.

My 02 was .852

 

Measure source coil #2 between the Black and Green/Blue wires.

The resistance should measure between 44 to 66 ohms.

My #2 coil metered at 55 ohms

Mine was 54.3

 

Measure the Pickup Coil between the White/Blue and White/Red wires.

The resistance should measure between 248 and 372 ohms.

My pickup coil metered at 284 ohms

Mine is .293

 

 

The primary coil is metered between the CDI connection and the mounting flange on the same end.

1994-1995 models should meter between 0.45-0.61 ohms

1996-2001 models should meter between 0.14-0.20 ohms

Mine is .9

 

The secondary winding is metered between the same mounting flange as the primary winding and the unterminated spark plug wire.

1994-1995 models should meter between 10,700-14,500 ohms

1996-2001 models should meter between 6,700-10,100 ohms

Mine is 13.29

 

This would indicate I need a new coil correct?

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Have you checked compression with another tester?  maybe your 150psi result is way off reality.

CDI's can fail in mysterious ways, mine despite always showing a faint spark would die after 10 minutes of running and only re-start after a long cool off.

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I have only used the one compression tester a few times and got the same results. It is smoking out the pipe as I kick it but refuses to start. I tried holding throttle open but nothing. I am wondering if it is the carb, it is a cheap $40 carb. But it should still fire. Maybe crank seals are leaking? I am out of ideas. I think I may have to take it to a shop.

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Possibly  flooded ,    remove ex. Pipe ,  light your little propane torch and have someone hold torch close to outlet while you kick it ..      Be careful .......   It may surprise you ,     keep extinguisher handy .......

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Update: I have installed a new OEM carb with factory jetting, also new coil and plug boot. New plugs.Unplugged kill switch. New shorty silencer ( the one that came with the bike was for a different year bike so didnt bolt up). Checked timing again plus double checked woodruf key. Still getting spark with a little smoke coming out of pipe but not starting. My question is this, I went ahead and ordered a new CDI box even though everywhere i read says i dont need one if i am getting spark, could this bike possible have a cdi box from a different year bike and that is what is not letting it fire? What am i missing? I am getting spark so that eliminates kill switch, i am getting slight smoke and plug is wet, and its a new OEM carb with factory jetting, that eliminates fuel? I have tested the compression with a cold motor at 150, that eliminates compression. New air filter and tried with air filter off so that eliminates air? I have checked my timing with a indicator and its spot on so that eliminates timing other than the timing also coming from CDI box so that tells me CDI box failure regardless of spark? I have read where weak spark could be getting blown out during compression stroke, but i have a new OEM coil, but i know those can still arrive dead. Any suggestions or did i make the right move by buying the CDI box?

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Air, fuel and spark. If you have spark and it won’t start, you are lean on the pilot circuit. Go up 2 sizes. Is spark pure blue? Sometimes a spark isn’t a good spark.

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1 hour ago, hondaman331 said:

Air, fuel and spark. If you have spark and it won’t start, you are lean on the pilot circuit. Go up 2 sizes. Is spark pure blue? Sometimes a spark isn’t a good spark.

The pilot screw that came with the OEM carb was really low, 20 or 25, i have a 37.5 and a 40 that i ordered but havent put it in yet. I thought what was in it would be good enough to see it fire, but I will put the 37.5 in it and see if that helps. The spark is blue, but hard to say if it is a good spark or not. Thats why i ordered a new coil and checked my stator, which checked out good.

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On 5/14/2021 at 9:22 PM, mlatour said:

CDI's can fail in mysterious ways, mine despite always showing a faint spark would die after 10 minutes of running and only re-start after a long cool off.

Interesting..... sounds a lot like whats going on with my bike right now

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Put your propane torch in the spark plug hole and let er eat, maybe it will dry it out and let it fire. Do the same to the spark plug.  I would also swap that piston out.

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5 hours ago, TheRockyMoutainKing said:

Put your propane torch in the spark plug hole and let er eat, maybe it will dry it out and let it fire. Do the same to the spark plug.  I would also swap that piston out.

What will this do? Is this because you think it is flooded and that will clear everything out? I though pulling the plug out and kicking it would clear out any fuel, along with leaning bike over to clear the carb? But at this point I am willing to try anything lol

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14 minutes ago, Jeremyparkerau said:

What will this do? Is this because you think it is flooded and that will clear everything out? I though pulling the plug out and kicking it would clear out any fuel, along with leaning bike over to clear the carb? But at this point I am willing to try anything lol

Just cleans it out, we do this to our snowmobiles whenever they wont start, helps a lot.

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On 4/15/2021 at 12:29 AM, mlatour said:

.Double check the number as that #55 pilot jet sounds extremely rich.

(usually in the #30 to #40 range)

Agreed on the jetting, that bike should have a 400 main and a 30 pilot ...some even use the 390/27.5 combo

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1 hour ago, ss-racing66 said:

Agreed on the jetting, that bike should have a 400 main and a 30 pilot ...some even use the 390/27.5 combo

I went with a 410 main and 40 pilot. That seemed to be what most recommended, it seems to run great right now.

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On 6/3/2021 at 7:04 PM, hondaman331 said:

Air, fuel and spark. If you have spark and it won’t start, you are lean on the pilot circuit. Go up 2 sizes. Is spark pure blue? Sometimes a spark isn’t a good spark.

Color of spark depends on Ion's in the atmosphere .  Not voltage .... get a proper spark tester....

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