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XR500 cam timing help


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Just rebuilt the top end on the 82 XR500R and the thing wont run hardly at all. Basically won't even run. So I decided to pull the engine back out of the frame and take a look at anything I might have overlooked. I started to think how do you know which way the cam goes. Even when it is at TDC the cam lobes can either be facing upward or 180 degrees off and face downward. Does the ignition spark on both the compression stroke and exhaust stroke? I did not see anything in the manual regarding how to place the cam in the head. The sprocket is lined up correctly, but like I said the cam can either have lobes facing up or down... Maybe I'm looking at this too complicated so let me know.

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It's fairly simple..You get the T mark on the Flywheel lined up in the inspection hole. You then place the Cam into position with the lobes facing down..You then make sure the Cam chains on the bottom sprocket in that T position and then place the Cam chain onto the top Sprocket preferably with the holes more or less lined up with the bolt holes on the Cam all the time not really moving off the T mark on the Flywheel. Wiggle/prise the Sprocket up onto the Cam and fit the bolts that hold it there. You put one bolt in and partially tighten it then rotate the cam, flywheel, chain all at once and fit the other bolt into its hole if it's partially hidden from view..If not just bolt it up.

Edited by Horri
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Since Honda singles generally fire on the exhaust stroke too, you can time the cam either way. If you put it in lobes up and turn the crank one turn it will be lobes down. Since the spark doesn't have to be timed to the compression stroke alone both ways are fine.

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I'll just add to my first post and say the top Cam sprocket should have two grooves on the backside of the Sprocket..they are supposed to line up like this _ _ with the top surface of the cylinder..I have to say it's a while since I've done one and while that sounds a bit odd to me it's whats stated in the Clymer manual I have which covers the 82 500..Leave it in your hands..

Edited by Horri
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Ah yes, after a day yesterday studying the bike and pulling the ignition cover the spark does go off at both the exhaust and compression strokes. makes it simple. It is almost impossible to get the cam sprocket on the chain, you have to play with it and rotate the flywheel about 90degrees to get some more clearance. then rotate it back and adjust your sprocket where the T mark is ligned up and also where the - - marks line up with the top of the head. The cam essentially is what is determining your exhaust and compression strokes. So hopefully it should run now, I had sheared off the kickstarter when trying to start it this last time and put that in yesterday. A definate hint if you ever do that is keep the engine in tact and turn it up side down, since these cases split horizontally rather than vertically you can just pull the bottom case off. Made this much simpler to do. Ordering some more minor stuff and hopefully she fires up. Thanks for the help.

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  • 10 years later...

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