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2003 Honda CRF150F won't start

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So, I picked up a '03 CRF150F a few weeks back. Found out later that it had been rebuilt but UTI, supposedly. It has a BBR exhaust and has been re-jetted with a BBR jetting kit. Ran good until it started having difficulties starting. Changed the oil and it made it easier to start for about a week. Put in a new Rev Box because that's what it had in it, and that has fixed the hard to start issue. But even more lately, there's been a knocking or clunking sound from the bottom end of the engine. You can only hear this knocking sound though when the engine is returning to idle after revving it in neutral. And it has this knock or clunk whether its cold or warmed up, although it does get louder and more frequent when it's warmed. Don't know what it is and haven't figured that out, but when I was test driving it, I got up to fifth gear and all of a sudden it sounded like a valve or something from the top end broke. It was if the engine lost all compression because you could hear the air just blowing in and out of the cylinder. Pushed the bike forward and I could feel that the engine wasn't seized up, but trying to start it with the kick start wouldn't work. Wasn't able to start the bike so I removed the engine and disassembled the top end expecting to see a broken valve or one that wasn't seating right. However, the cylinder head is black and has quite a bit of carbon buildup, so I don't know if that would cause it. Or maybe a valve is out of adjustment or the timing? The connecting rod seems to be tight enough because when you pull the piston up and push it down, there's no slack, except for side to side, which is normal I believe. Valves? Timing? Carbon buildup? Something in the lower end? Any ideas?

Edited by suzi_drz400s

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No disrespect but...
another poorly running engine after a "rebuild" 
When we buy a bike that's been "rebuilt" we know all we can do it part it out on eBay.

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Use your genuine Honda Factory Shop Manual after you completely disassemble the engine, clean all parts and inspect each and every part. Step by step. Then order all parts that are bad, worn a lot, or questionable. Measure everything and check that all pieces are within specs.

Reassemble engine using your manual and a torque wrench.

Re-install engine and fill it with oil.

Change oil after 1/2 hour, then ride it like you stole it. It will run well for a long time if you pay attention to detail.

 

Hopefully you will learn that one should never purchase a used bike without the intention of going over every single maintenance item and adjustment, front to rear, top to bottom, FIRST Thing, before you go for one more ride after the initial test drive that you took before you laid down the cash.

 

Good luck and enjoy.

 

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