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yz250 carb problems

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it only stays running when you give it gas, if you let off it cuts out. i tried adjusting air screw and choke screw and they dont do anything at all the only one that does anything is the throttle screw but i have to loosen it all the way out to keep it running. None of this was happening untill i did a top end rebuild with wiseco. and its fouling plugs in a day of riding if that. please help cant figure this out. 

Edited by jason0827

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Does the bike overflow fuel from the vent line when tilted over a bit ?

Check your fuel inlet valve and adjust the float height, also inspect the reeds for damage while you have the carb removed.

 

Edited by mlatour

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yes and whats the fuel inlet valve? and i adjusted the float to 5 mm and reeds are fine i inspected them during rebuild

 

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Confirm the reeds seal all the way around each petal and aren't chipped/cracked,

they do wear out with usage, despite looking fine they loose their tension.

 

Don't guess, adjust the carburetor to the service manual's specs: jetting, air screw, float height etc. 

The inlet 'valve' is the inlet needle and seat, also wear items.

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you mean seal where the gasket is? and i will check them. 5mm is the spec, jetting idk about but it ran fine before the rebuild. air screw i did 1 1/2 turns out and like i said it dosent change any thing and what should i look for in the needle seat? i put the needle clip on the 3 position according to PC specs.

you mean seal where the gasket is? and i will check them. 5mm is the spec, jetting idk about but it ran fine before the rebuild. air screw i did 1 1/2 turns out and like i said it dosent change any thing and what should i look for in the needle seat? i put the needle clip on the 3 position according to PC specs.

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Where the fuel enter the fuel bowl,

from the fuel line, past the inlet seat and needle which is attached to the float.

 

If it ran fine before the top end rebuild, nothing should have changed carb wise.

I'd set everything to stock specs and maybe start troubleshooting elsewhere.

 

Out of curiosity, did you measure the cylinder before ordering a new piston?

did you check piston to cyl. clearances, ring end gap etc.

Has the gearbox oil been mysteriously getting low these past few rides,

are you cleaning / re-using the same plugs after they foul etc. ?

 

Edited by mlatour
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The rubber tip on it is fine and it fit perfectly. Gear box is fine no leaks.

And the plugs keep fouling like every 2 hours if that I just keep putting a new plug in one theory I had was the CDI or the ignition coil gone bad and not enough spark or something. This has stumped me the only thing that will keep it idleing is the keep your hand on 5he throttle giving it a lil gas that's why the throttle adjustment did that

 

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If you turn your air screw all the way out do the rpm rise? You may have way more signal from your fresh top end pulling fuel through your pilot and need a smaller pilot. You didn't mention if you'd tried a smaller pilot or if adjusting your air screw made the proper change in fueling to determine if you're in the right range of adjustment. Its most likely that you have to give it throttle to keep it idling because its not getting enough air to mix whereas your comment suggests you're thinking you need to "give it gas".

 

 

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Think I found it I'm going to buy new reeds and see what happens

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1491653665647-1778750280.jpg

Edited by jason0827
Mis spelling

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