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Yz250 impossible to kickstart-carb issues

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Hi, i picked up a 87 yz250 yesterday and she started right up but bogged hard on full throttle. I picked it up however and tore into it. Found the main jet was just sitting in the float bowl, and the power valve linkage was not even connected. Got the carb cleaned, put the linkage back on and threw it back together. Could not for the life of me get it to start. So i pull started it and it rips way hard on full throttle however on 0-3/4 throttle it doesnt respond at all. I know that the pilot circuit is responsible for 0-1/4 throttle and the jet needle is responsible for 1/4-3/4 throttle. The jets are stock and the carb is totally clean. One thing i noticed was the jet needle was totally loose in the slide and so i took it apart and found nothing wrong there.. Im wondering if the slide is wore out where the jet needle rides through.

 

i literally have to guide the needle into the holder through the carb to get the slide to fall into place.. Never had to do that on any of the other mikuni carbs ive worked with..quite odd.

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Funny you say that, every Mikuni I've cleaned/rebuilt has needed me to guide the needle into its slot, I don't think that's the issue. Anyways, did you check your plug? That may have some signs as to how rich/lean the bike is. Pull it out and look at the electrode, it should be a chocolate brown. If it's black, it's rich. If it's white or grey it's lean. And if it's wet, it may be flooded

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Yeah...similar experience...you have to set the rotation of the slide to align with the pins with one hand and stick a finger in the end of the carb to help align the needle to drop in place...nothing should be forced...

 

What altitude are you at? and what premix ratio are you running? Is the stock premix ratio 20:1 or 32:1 ? Your pilot circuit may be too rich to kick-start easily with stock jets if you are at an altitude significantly higher than sea level and running less oil than stock in your premix.

 

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Im running 32:1. stock jets near sea level. Got another 87 yz250 with the same settings and it runs perfect. Like i said the bike literally has no power until youre wide open throttle then its gone lol. It wants to die if your not 3/4-full throttle

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Air filter is new, reeds appear new as well, didnt see any breaks in them. Look to seal just fine.. At a loss haha

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No i have not, i was actually just gonna try swapping the slide and carb off my other yz just to see. Clip is in the number 3 slot. Dont know if i mentioned but compression is 145 psi.

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Going back to your first symptom, the needle seemed too loose...

Also, the fact that the main jet was loose in the carb...

I don't know the details on your carb, but on my old Mikuni ('78 YZ250E), the main jet screwed into the carb body which retains the round fuel baffle and the hollow cylindrical part the needle slides into...pull that out and see if it is missing or worn..? You can see the part in the upper right of the second pic.

20170422_120256.jpg

20170422_133901.jpg

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Come to think about it...doesn't the main jet actually screw into that cylindrical piece and traps the fuel baffle. That cylindrical piece comes in from below and is retained against the carb body when you screw the main jet in. So, if the main jet was sitting in the float bowl, what was holding it in place? Some gunk, I guess...so did you take it out and clean it?

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I don't know how helpful this will be but here's my .02.

I picked up a really clean DS80 for my kid.  It had been sitting for a while.  Same symptoms as you describe - EXACTLY.  Except for the loose main jet that is.

I was certain it was a carb/plugged jet issue.  I took the carb apart and cleaned it about 3-4 times with no change.  Then I went further.  The pilot jet was clean, but the passage from the pilot to the venturi ABOVE where the pilot screws in was plugged.  I soaked it in carb cleaner and used compressed air with a rubber-tip blow gun to blow it out.  It made a very audible "pop" sound when the blockage finally cleared.  Then I could hear the air passing freely through the passage.  I re-assembled the carb and all was good.

So even though a carb looks clean, it may not be...

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You say you have another '87 250? Try running its carb on this bike, if it doesn't work then it ain't the carb.

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Well, picked up another new plug on my way home from work. Threw it in and it fired up and ran fine after 4 kicks... Wtf makes no sense but oh well got her going thanks for the insite guys. Well see if she runs tomorrow hahaha

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