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86 xr80 no compression after rebuild

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Just installled new piston/rings and cylinder on my XR. When i was doing the cam timing I read that the piston should be at the top of the cylinder and the timing mark pointed at "T" , and the cam lobes facing down. No matter what way I turn the motor it never gets the piston at the top of the cylinder when on the "T", its always on the bottom.  Motor feels like it has compression when i kick it over but get zero on compression gauge.  What did I do wrong here??? Thanks!!

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What year is the motor?  Did the flywheel slip? Pre-1992 flywheels tend to slip.

 

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Tried spinning flywheel 360 the timing mark "t" never lines up with the piston being at top of cylinder. The cam shaft and sprocket are the same that was on bike when it was running. Motor is a 1986. Should i get the piston to top of the cylinder and then loosen flywheel and get it to the "t" mark?

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Either your cam chain jumped or your flywheel slipped.  I would pull the flywheel and check it.  It will mark up the crank a bit if it slipped.

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i ordered fly wheel tool so will check next week. That has to be problem . Even with cam chain/sprocket off could never get the "t" lined up with the piston on top. Will update thanks!

 

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If the bike ran before you disassembled it, it’s unlikely the flywheel is out of position unless you removed it. 

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On 6/28/2020 at 4:13 PM, Gizmolvr said:

Tried spinning flywheel 360 the timing mark "t" never lines up with the piston being at top of cylinder. The cam shaft and sprocket are the same that was on bike when it was running. Motor is a 1986. Should i get the piston to top of the cylinder and then loosen flywheel and get it to the "t" mark?

No. 
 

Re-synchronize your cam and crank so everything lines up properly. Loosen the cam chain so you can align the cam and crank. Don’t take anything apart that you already haven’t disassembled. 

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With your flywheel T mark aligned with the index, and your cam sprocket bolts parallel to the cylinder head surface, the “O” mark should be up when the cam lobes face down. I‘ve provided images. 

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My comments assume the bike was running and Gizmolvr disassembled it to rebuild the top end. If this engine was assembled in pieces from a non running state, then yes it’s possible the flywheel was installed incorrectly. 

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thanks for all the help! The woodruff key holding flywheel in correct postion was broken off. I ordered a new one and will update once i reinstall. 

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Just to clarify....

The key doesn't hold the flywheel in position.... It aligns the flywheel correctly on the crank...

It's the force of the interference fit of the taper of the crank that holds the flywheel in position... If it could be aligned without the key, the key wouldn't be needed at all.... 

If that key shears, it's because of insufficient torque on the nut holding the flywheel in place....

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Got it back together. Bike now starts but will only idle with throttle wide open. Getting air blowback into air filter where it should be sucking in air. Is this a valve clearance adjustment? thanks! 

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The first thing anyone should do after purchasing any machine is buy an original factory service manual for it.... I have a full library for every machine I have owned...

Your cam timing is way off... You are just a step away from breaking the bank ruining the machine... Buy and read the OEM Honda service manual... The hard copy, not the stupid download....

Best possible advice...

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And if the key sheared there is most likely damage to the taper on the crank and the rotor. Those tapers need to be clean (no oil film) and smooth so they can seize when the bolt is tightened.  As previously explained the key is only to index the rotor/crank, it is the surface of the taper and the force from the bolt the that prevents slippage.  Tapers are very common, and reliably used,  in machinery and automotive applications. 

The bigger XRs are not "spinner" engines and mis timing the cam can cause bent valves, I don't know if the 80s are "spinners" or if you are lucky so far.  Get the manual and read it. Good luck.

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