98 YZ 125 won't start

Fuel, spark. Two out of three ain't bad. Does it have any compression?

Spark, fuel, compression. You have spark so take some of your premix and put a thimble full into the top end by way of the spark plug hole. If it pops more than likely the carb is gummed up, if it still doesn't try to start its probably loss of compression, another way to check compression without a compression tester is remove the spark plug (reinsert it into the plug cap and push it away from the plug hole) put you thumb over the plug hole and have someone turn the motor over with the kickstarter, if it has compression it will push your thumb off the hole.

Fuel down the spark plug hole...not the carb. Just a few drops, then reinstall the plug and cap. If it bangs and fires for a second, you'll need to go at the carb, particularly the pilot circuit. If it still doesn't kick, you might want to make absolutely sure it's not the kill switch. You didn't mention if you washed it before you put it away. (confirm the kill switch anyway)

Whoops...just realized you said you had spark, so if it's constant and consistant, you can forget that easy diagnostic. Just make sure it's not intermittent.

I had a 250 once with similar issues. It started fine after I bought it used as well, and ran decently. It wasn't a great starter, but then it stopped starting. I found out the previous owner had cold-seized it, started it cold and didn't let it properly warm up before getting on it. The piston expanding quicker than the cylinder wall caused a mild seizure, scoring the walls of the nikasil cylinder . Needed a new cylinder/top-end. The thing with this is that compression was decent and like I said it ran "okay" if it started. Obviously it wasn't okay though. (probably why the guy was selling it for the deal he was) I had never heard of cold seizure before this happened. This could be a possible cause if the other issues mentioned pan out okay. Definitely exhaust the other listed options first though.

I would say take the top end down and see if the piston or rings are seized. What pre-mix oil are you using? If you have a compression guage check it with the throttle wide open and it should be around 130-150 PSI. If its less than 100 pounds it usually won't start at all.

Like I said in my post earlier, my bike was getting fairly decent compression....enough to run strong when it started. The only real issue I was having with it was starting cold. In fact now that I recall, the main issue I was having was getting it to idle. It was not a good cold starter in that condition, but now I remember that idling was the issue. I knew something was wrong as it just would not hold an idle. After lots of investigating, a savvy dealer turned me in the right direction.

The reed valve shouldn't have anything to do with compression. What your describing sounds like a plugged pilot jet or possibly a cracked reed (hard starting). Compression on a 2 stroke is in the compression chamber (above the piston), one other thing to check is make sure the exhaust power-valve is not gummed up or binding.

When you check the compression, kick it through quickly 5 or 6 times to get an accurate reading. One kick will not do it.

The reed may cause this problem.I once had a yz 80 blow a piston and was able to fix it at the track and run the rest of the races. After getting home for practice later,it wouldn't start. After disassembling the motor I found a piece of the piston holding the reed valve open. You have an old bike,there is no easy fix. You need to carefully take the motor apart and see the problem.If you can kick it over,the bottom would appear to be good,and the top also.Nothing is seized. You need to determine if you have spark first.If you don't then you don't have to tear down the motor. If you have spark,and the carb is cleaned,then you are in for a top end job.Hopefully the cylinder is in good shape,but being a "98,it may be a steel jug,which is cheap to overbore and refit with a new piston and ring. Gas is easy, air is easy. Be sure you have good spark before ripping into it. Good luck.

Compression is the pressure measured when the cylinder contents are compressed between the piston crown and the cylinder head. The reed cage is below the rings and the head with the piston at TDC.

On a two-stroke, four or five good kicks with the THROTTLE WIDE OPEN, even with the cheapo rubber-tipped hand-held compression gauge should give you a representative reading.

Even with the reed valves and carb removed, the amount of air (even under atmospheric pressure) entering the combustion chamber and being compressed should bring the gauge to over 100 psi. It's physics.

I agree race guy. I'm looking at as why it won't start.He doesn't know if he has compression or not. Best diagnostic tool is to check the easy things first.If you have experience,you can tell when you step on the starter if you have compression or not.I was trying to have him investigate the problem from the aspect of checking everything that he needs to remove prior to removing the jug,to save work and or money. He put fuel in the cylinder and didn't get a put. That would indicate no spark,as doing that even with low compression would at least make it fire. But as long as the motor kicks over,I would suspect worn ring and piston.Hopefully the cylinder isn't damaged. With the age of the bike,it's probably due for some work. You can do it all yourself for not a lot of money. Be glad you didn't buy a 4 stroke. You would have to get a 2nd job to fix it. YZMAVEN,if you need help,I can give you step by step directions. I have rebuilt YZ 125's many times. Go slow and observe if you take it apart. There are many clues that apprise you of what happened. I'm here to help. Good luck with it.

If it has not had a fairly recent crank rebuild, I would be hesitant to just throw a new top end in it. Would suck to have it spit a needle bearing or thrust washer and waste a new piston / ring and cylinder.

98? It is probably in need of a crank, plating, top end ,and mains for a reliable rebuild.

I would have to say check the reed valve as well. May not be the problem, but easy enough to check. If you are blowing as much back through the carb as you are up through the engine, things aren't going to go as planned. I had one eat part of a reed and it wouldn't start period.

yes.......but I would still pull the cylinder off and look it over.

If you have worked on all those things you mentioned then dont be scared, if it all looks good then throw a set of rings on it anyway.

Saw you post and do feel an F1 iBOOSTER can improve the situation. A much faster, hotter spark that does improve both cold and hot starting, improves piston/ring/valve life and slightly better snap off bottom.

Careful of return comments to this post from ding-dongs saying 'it won't add horsepower'. We know! We don't ever sell it as 'increased horsepower' item.

Its an $89 retail product that improves starting, cleans hesitation on low-end roll on and certainly (especially on four-strokes) improves hot-starting. So listen to MXA (5 stars) and TWMX ("Under $100, better bottom end, better snap, I guarantee it") and VITAL MX's ("Before installing the iBooster, I had experienced an occasional bogging issue on the YZ125 at high RPMs that was jetting related but I had yet to figure out. After adding the simple device, this problem seems to been eliminated. For around $90, you receive a whole lot of bang for your buck".)

www.boostermotorsports.com

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