Hey guys, this is my first post and I have a question for all of you. I am currently restoring a 2000 yz125 and am having a very hard time getting it to start. No matter what I do, I can't seem to get it to fire up. I fully rebuilt the motor, put new gas in it, put in a new filter, cleaned the carb, changed the oil, etc., but still cant get it to start. it has good compression as well as good spark, but still nothing. it must be fuel related, because the spark plug is dry, even after I kick the bike over multiple times. Yes, the carb does have fuel in the bowl and the jets are clean. Get this - even when I pour fuel directly into the cylinder, the bike will only blow a bit of smoke out the silencer at best. even if the problem was the carb (no fuel getting to the cylinder) shouldn't the bike start when I pour gas in the cylinder?? I also checked the reeds and they look good too. Anyone have any ideas?? there is spark and it looks good, but could it maybe be a bit too weak? Any suggestions would be greatly appreciated.
thx, Josh
Well, you said wont start no matter what... so I would assume my suggestions won't help it make it start. sorry.
Timing?
Check float height?
Have you tried pop starting it? Mine ended up flooded out pretty bad after I rebuilt it. It took about ten tries, but fired up eventually.
The Shop
This more than likely isn't the case, but I had the same issue with my 02 yz. Ran perfect and then when I got to the pits it wouldn't run no matter what. Turns out the throttle cable slipped out of the perch a tiny bit and it was tugging on the throttle a bit. That tiny little bit of throttle being applied was enough to prevent starting
Maybe there is a rag in the air box or I’ve had two strokes I’ve had to pop start after a rebuild to get them to start then we’re fine after that. Could be trying to burn off all the assembly lube.
How do you mean?
I'm guessing rag in the airbox. It's happened to us all at least once.
Did you remember to put the key in the flywheel assuming you had the flywheel off?
Ive seen that mistake before.
Ignition timing. You may have spark but if it’s not timed correctly then it won’t start. It will usually cause popping/backfiring tho.
You say you have spark. Is this with the spark plug out and connected to the coil wire and when you kick it over you see spark in the spark plug gap? I just want to make sure you see it in the gap of the spark plug.
I had an old RM not start and I couldn't figure it out. I had an older friend ask me if I tried a different spark plug and I said no, I put a new one in. He said that's your problem. Put in an old spark plug and it will start. I did and it worked. Never assume a new product works.
Josh, have you changed the powerband? That one has gotten me a time or two.
Have seen new cranks that have the key way for the flywheel like 90 deg out, would check that first , go to tdc with dial gauge or a screw driver in the spark plug hole and check your timing marks on the flywheel
yea, that was one of the first things I checked
yea I've already tried that about 20 times and no signs of it even hinting at starting lol
lol, yea I took a look, but no rag
no, I didn't
ok, I think we're on to something. Occasionally when I pour gas into the cylinder it pops and smokes, but wont start...where should I go from here?
Pit Row
I tried it out of the cylinder connected to the coil. the spark looked strong, so I don't think it could be the plug...unless maybe my spark is somehow not strong enough? It seems fine though...
Timing way off sounds like. If it has reliable spark, is getting air, and fuel, that’s the only think I can think of.
You said the plug comes out dry if you kick it over with the choke, throttle etc. on?
yea that's right...its almost completely dry unless I pour gas down the cylinder obviously lol
Umm...just realized I don’t even have a flywheel key...I guess i’ll Get one and go from there lol
Number 16 woodruff key
lostboy, you sir, are a good dude. Seriously.
Hold on, on the YZ125 there’s a common ‘long-rod’ modification for some models.
have you put a short rod in a long rod bike?
take the head off and check the piston is right at the top of the cylinder at TDC.
ive actually done this on an RM125- fitted a 2004 rod in a 2003 bike. The rod was 4mm shorter and it would not start.
ok guys, so I stupidly forgot to take the woodruff key out of my old crank 🤦🏻♂️ So I put it in and the bike started on the first kick. It shut off after like 2 seconds, but at least it started. I tried to start it again and my kick start broke 😂 so I guess i’ll Try and bump start it this morning. Thanks for all ur guys help!
This is why I love this community. Good luck with your build, Josh!
Post your journey in the Bike Builds category!
You nailed it !
Just tried her out and she’s running boys! I sincerely thank you all for your help! You guys are amazing!
Glad to hear man! Sometimes it’s the smallest/dumbest things we overlook. Guarantee you’re nowhere near the first it happened to and will definitely not be the last. Now get some videos up of her running so we can enjoy the sound of that sweet YZ125
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