Dirtbike won't start backfires also

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Oct 7, 2019
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#1
I have a 2012 Yamaha yz250f someone plz help me i recently took apart my cylinder head down to the piston but didn't mess with the piston what so ever i put everything back together the way i was supposed to but when i tried to start the bike it wouldn't start and back fires occasionally

new spark plug gets spark
timing is on
carb is getting fuel
although there is no compression
 

truespode

Wheelie King
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Jun 30, 1999
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#2
Check the valve clearances and make sure you put it at TDC when you put everything back together. Having the cams not aligned or valves that are out of clearance can cause these type of issues.
 
Likes: Ryan Matz
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#3
Check the valve clearances and make sure you put it at TDC when you put everything back together. Having the cams not aligned or valves that are out of clearance can cause these type of issues.
i have made sure about the tdc so maybe it's the valve clearance i will check up on that
 
Likes: The Redrider
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#5
I have a 2012 Yamaha yz250f someone plz help me i recently took apart my cylinder head down to the piston but didn't mess with the piston what so ever i put everything back together the way i was supposed to but when i tried to start the bike it wouldn't start and back fires occasionally

new spark plug gets spark
timing is on
carb is getting fuel
although there is no compression
Grab a shop manual and re check everything.
During rebuilds, you must always install fresh gaskets on everything you rebuild...
 
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#8
yup did exactly that ordered full top end gaskets and use me the manual the whole time
Compression release is or should be on your handle bar, it opens up a port in the cylinder head to release the compression on thumper for easy kick starting. If its stuck open like during a rebuild or not set up correctly then it will act like it has no compression. Check to be sure it is installed correctly. Make sure your timing chain and sprockets are set up correctly. Then you should be ok. manual is very important here...
 
Likes: Ryan Matz

truespode

Wheelie King
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#9
The 2012 should have an auto compression release and not a cable. Can't remember the specifics but it will be in the shop manual in the area of re-assembly of the top end.
 
Likes: Ryan Matz
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#12
The 2012 should have an auto compression release and not a cable. Can't remember the specifics but it will be in the shop manual in the area of re-assembly of the top end.
Well hell man, old xr's, xt's, dr's and klx had em, these new fangled bikes dont have em? I'll be dipped in sheep s___, Lol! tumblr_mr5j0mUk0F1squi78o5_250.gif