Yamaha Dirt Bikes Forum

Ok so the other day it was cutting out in the higher gears. It would cut out for a second then kick back in. Then I went riding today and it was doing it, so i let my friend ride it, we stopped and talked about it. When i went to start it up it just wouldnt. SO i brought it home and put a new plug in it and it wont start up. Dosent even sound like it wants to. What could be the problem ?  

im gonna say top-end
because that is simular to what my bike was doing before it imploded  
=====================i eat the desert and the dessert

you think the float in the carb is sticking. THe top end should be good. When i bought it the guy had just put a new top end in it. IT ran super good for the first two rides.  

I don't think this is a float issue.  One way to check is to turn the gas off, take the bowl off the carb, hold the float up, and turn the gas back in.  If fuel flows, the float needle is toast.

It sounds suspiciosly like a worn topend, but it could also be a clogged jet ot fuel filter, or a faulty CDI.  
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Here we go again!

DO a compression check and that will tell you if its top-end
and you cant believe what previous owners tell you they will always say it has a new or fairly new top end  
=====================i eat the desert and the dessert

Hmm. How do i do a compression test. ANd what should the numbers come out to if its good or not ?

I kno i cant belive what the previous owner says, but he looked legit, had a nice shop with a bike stand and a few other bikes sitting around.  

To do a compression test, you need a compression guage.  They sell them at Napa, AUtuzone, Pep Boys, Sears, etc.. it has a hose that you screw into the hole the spark plug goes in.  You then hold the throttle wide open and kick it until the needle stops rising.

For an 03 YZ125 the engine should have 185PSI for a fresh topend, and would be considered worn at 165PSI.  
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Here we go again!

Top end is burnt out. The piston had bad scratches in one place and looked burnt. The radiator, when we drained it looked like pure antifreeze no water. Some seals are warped. Theres a slight rough spot in the cylinder, very slight(honing ?) .

So, pure antifreeze = little or no cooling, engine runs hot, burns out top end. Sound right ?  

can you take a pic of the scratch on the cylnder and post it

how does the crank look?  
=====================i eat the desert and the dessert

and you are right a hot running bike wears out quickly
but sometimes it is hard to tell if the coolant has been mixed or not i use Engine ice and that works good i havnt had a over heat since i started running it another up side of the engine ice is if your animals get into it it wont kill them (non toxic)  
=====================i eat the desert and the dessert

Ill try to get some pics tomower. The crank looks pretty good, Im guessing, it looks like its in good condition to me.  

rscottkdesigns:
Top end is burnt out. The piston had bad scratches in one place and looked burnt. The radiator, when we drained it looked like pure antifreeze no water. Some seals are warped. Theres a slight rough spot in the cylinder, very slight(honing ?) .

So, pure antifreeze = little or no cooling, engine runs hot, burns out top end. Sound right ?

Bad scratches were probably caused by dirt getting in past a dirty air filter. The top of a piston on a properly jetted bike SHOULD look burnt black with just a tiny bit of aluminum still showing at the edges - that's normal.

The coolant Yamaha puts in looks like straight antifreeze to me too, but it isn't.  Even if it were, pure antifreeze still provides the cooling effect.

I'd sure like to see a pic of that piston.  Either way though, it will need to be replaced.  While you're at it - it's better to check the crank by measuring the side clearance with a feeler guage - that way you'll know for sure if it's ok.  
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im with skip  
=====================i eat the desert and the dessert

I'm with Quazymoto...  
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Here we go again!

meaning check the crank where ? sorry, new to engines ?