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  #1  
Old 08-19-2013, 07:42 PM
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Default Need some help diagnosing whats wrong with my 2002 yz125

I bought a 02 yz125 last week. Ive gotten a few rides out of it so far. On my last one it was running fine, I noticed it wasnt going fast like it usually does in 4th or 5th gear and all of the sudden the engine just stopped.

Havent had any luck figuring out whats wrong. Im also pretty newb at 2 strokes and am actually pretty new to dirt bikes in general. But heres what ive tried to so far:

New spark plug- Still didnt start

Another new spark plug: Same thing

Sprayed a little bit of ether: Wouldnt start

Tried push starting it: nothing

Took the plug out and turned the gas off and kicked it 57 times: Wouldnt start after that

I smelled the plug and it didnt really smell like fuel? more like soot i guess. Is this is a common problem that just happens and im just that much of a newb?
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Old 08-19-2013, 08:27 PM
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did u ride it wide open for a long period of time? what did the plug look like is it lockup? need more info? 2 strokes dont need to be run wide open like going down street . they should be run up and down power band.
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Old 08-20-2013, 02:16 AM
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I wasnt wide open for any extended periods of time. Just short bursts. Still getting used to the bike. When i took the plug out it was wet with oil.
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Old 08-20-2013, 09:26 AM
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You need to check the compression and spark, it should at least fire on ether or starting fluid. You probably already know, but don't use too much of that starting fluid, it doesn't have oil with it and it can wash the oil off the bearings and cylinder; then you have more problems. The coil might have broken down, so it started missing a spark (losing power), then just quit. If you have a good service manual it will tell you how and where to measure the resistance through the coil and the expected readings. Hint: you need a service manual. I found that a pebble had worked its way into my reed assembly and was holding one reed open, how it got there I don't know. But the engine had almost no power and wouldn't kick start and it finally died. I took that pebble out and made sure it was ALL clean and the engine fired right up.

1) Check reeds

2) Check spark

3) Check compression

4) Let us know what you find!
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Old 08-20-2013, 09:47 AM
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The ring (yes - the single piston ring) in a 125 only lasts for about 1.5-2 hours of run time before it starts losing its ability to seal properly. As a racer, I replaced the ring every week, replaced the piston every month. It's very likely that you have no compression, but it's also possible you siezed the motor. Are you mixing oil in the fuel? At what ratio? Are you running fresh, premium fuel? What is the condition of the air filter?

A quick compression check is to push down the kick starter by hand until it resists and hold it. If it sinks through that resistance in less than about a second or if there's no real firm resistance, then you likley need some top end maintenance. It's extremely common on a 125 2 stroke - even one that's impecably maintained and tuned to run a rich mix. You can do a top end on this bike inside of 20 minutes start to finish once you've done it a few times. You don't even have to pull the head.

These are just the first things that pop into my head. I know this bike well.

Scott
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Old 08-20-2013, 03:24 PM
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Oil mixture is 32:1, im running premium fuel, the air filter was fairly clean. Another thing to add, before it died, it wasnt hitting the powerband, kind of felt like it was topped out at around 50 mph.

As for checking Reeds and the ring, is there a nifty guide online somewhere, only thing i could find were some janky youtube videos. Is that something I should wait until I get the service manuel, or is it easy enough that I can atleast check the reeds and the ring without it?
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Old 08-20-2013, 05:21 PM
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At minimum, you're going to need a new base gasket. I wouldn't open it up to check the ring - if it has more than 2 hours run time, I would open it up with a new ring in hand, ready to replace. And do a good inspection of the piston and cylinder and crank, rod. You need a manual.

Here's a few tips to unseat the cylinder - first of, clean the hell out of the area. Remove the gas tank, drain the coolant, then scrub the entire area. Rinse out the cooling system with with the water hose and drain it well - tip the bike left and right with the drain bolt out - maybe even remove the lowest hose. Then blow it off with compressed air or a lawn blower and begin removing the carb and the pipe. Leave the head on but disconnect the top engine mount and remove the plates. Disconnect the power valve linkage, loosen the spark plug but leave it in, then loosen the base nuts and back them up until they are almost to the top of the threads on the stud. Then pop the kickstarter by hand - the cylinder will pop right off the case. Then remove the base nuts and the spark plug, lift the cylinder up to clear the studs, spin it about 45 degrees, then tilt it to the front of the bike and cycle the piston to the bottom of the stroke and the cylinder will come off without pulling the head - this will save you head o-rings which are not re-usable and are a bit pricey.

I'd wait until you have the manual in hand. Of course the manual will tell you that the head has to be removed.

The reeds are a visual inspection. Im not offering any advise on them - what some call good, I call bad and visa-versa. Look at em and make a judgement call. It's likely they are a little chipped if they're stock and they probably don't seal 100% but I've had excellent life from stock reeds. Every once in a while I would remove the pedals from the block and clean the seating surface. Some guys like to take the pedal stops off - gives those guys that extra power to pull their mullet around i guess. If it has no pedal stops then the reeds are likely toast and probably won't seal. Pedal stops are a good thing with carbon fiber (stock) reeds.

Pardon any typos - I am a tad short on time. Will proof read tomorrow maybe.

Scott
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Old 08-20-2013, 05:32 PM
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I would do as suggested and have the ring in hand when you start. Honest, if you have to ask how to check it, you are not going to be able to. Slip on a new ring. Wipe the cylinder dry. If there is no aluminum deposits I would rub it down good with a scotchbrite pad. Clean it well with soap and water, and go back together.
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Old 08-20-2013, 06:01 PM
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Yea I think Ive decided to just take it in to a shop. I have 2 gasket kits though so hopefully that will cut down on some cost. Thanks guys, I hope to learn how to do all of this in eventually, I think this is a little too much to start off with though.
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Old 08-20-2013, 06:21 PM
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Yes I'm sure a shop will get you going however, you will buy the bike many times over if you have everything that is needed serviced by the pros.
Good luck, and eventually you will take on task and wonder what you were afraid of

Quote:
Originally Posted by MXtras View Post
Of course the manual will tell you that the head has to be removed.
The manual is just the manufacturers opinion!
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Old 08-20-2013, 06:39 PM
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I figure its just something minor that I just don't know how to fix, so hopefully they don't charge me too much.
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Old 08-20-2013, 08:40 PM
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Quote:
Originally Posted by Drew1609 View Post
I figure its just something minor that I just don't know how to fix, so hopefully they don't charge me too much.
Most shops in my area charge a minimum of $65 per hour with a 1 hour minimum for any thing. A service manual will cost a lot less and then we can help you too.

FYI: I would suggest that the first thing you should do is pull the exhaust pipe and look up through the exhaust port of the engine and look at the piston. If it has scratches it is shot.
You did not tell us what the spark plug looked like when it died. Was it Black , Gray or White on the tip?

Paw Paw
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Old 08-21-2013, 09:44 AM
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If you ride a 125, you need to learn how to work on it. It's not hard to learn - it's just a lot to learn at first.

125s need a lot of maintenance. They are still cheaper to maintain the a 4 stroke.

You can do the work yourself. Like Paw said - we will help! I know this bike like the back of my hand and I think there are several others here that do too. We can walk you through things, it just takes a while longer. Don't give up!

Scott
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  #14  
Old 08-21-2013, 03:36 PM
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I had my uncle look at it, he used to work at the powersports shop in town, and he says I need a new CDI unit. There was no spark to the spark plug he said. Where should I go about getting one of those? And should I stick to an oem part, or is there a longer lasting aftermarket one I can get?

Another thing Im noticing is, and worrying about, How do i know if its the coil or the actual CDI that I need to replace?
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Old 08-21-2013, 03:52 PM
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Ok, first off "no spark to the plug" does not mean the CDI is bad.
Have you checked the coil, plug wire and plug cap?
Have you tested the stator?

No aftermarket units that I know of for your bike.

I would not buy one until all testing has been exhausted first!

Paw Paw
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Old 08-21-2013, 03:55 PM
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How do I check the coil? The plug cap looked perfectly fine.
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Old 08-21-2013, 04:39 PM
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Ok, you have to do an ohms test on the coil on both the primary and secondary sides. Your service manual will give you the desired readings and the service limits.

There is no way to test the CDI.

The Stator can also be ohms tested as well.

Remember...purchasing a part you don't need is not a good way to get it running again. Get a service manual and do the testing and then only purchase what is needed.

Paw Paw
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Old 08-21-2013, 04:48 PM
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Anytime someone "looks" at a cdi and says it is bad makes me think of three things:

1. the unit shows physical damage.
2. he is confused.
3. he is "devine".
again, dont buy anything until you have done some real diagnosis. if you arent going to learn it now, when are you going to learn it?
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Old 08-22-2013, 12:58 AM
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Check that the kill switch isnt playing up as well
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  #20  
Old 08-22-2013, 12:15 PM
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I doubt the CDI is bad. They are pretty damned robust.

Make sure your plug is good - plugs do burn up.

It's likely a bad ground, a bad connection - possibly where the plug boot connects to the wire, or as mentioned - the kill switch. Disconnect the kill switch and check again.

The plug boot unthreads from the plug wire. Unthread it, trim about 3/8" off the wire cleanly, then thread the cap back onto the wire. Use a clear silicone adhesive to seal and lock it in place, but it's easily twisted off in the future if needed.

Open up your stator cover and make sure the flywheel is still attached to the crankshaft. Some folks don't know how to tighten things properly - the flywheel could be spinning on the crankshaft (seen it before). Check the ground wire. Check the ground wire at the ignition coil, check all the electrical connections to the CDI.

As I said - I seriously doubt you need to replace the CDI. I havent seen a bad CDI on a Yamaha since about 96-97 and I have raced YZs almost every one of those years all the way up until '08. The 99's had an issue with the CDI overheating - they would break up at high RPM when they got hot but they re-located the CDI in 00 and eliminated the problem if I recall.

Scott
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