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Xr50 no compression after inspection

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Me and my dad went to inspect my brothers xr50 piston. We took the cam cover off and I took the bolt that holds the timing gear on off. We realized I lost my 10mm wrench so we put everything back together, changed the oil and when my brother went to ride there was no competition. The timing looks good we checked that, and the from the cover I can see both valves are moving? What could cause the low compression

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If the timing is still correct. Low compression can be from worn piston rings or valves not fully seating. A simple test put a few drops of oil down the spark plug hole then take a reading with a compression tester. If the pressure goes up, rings are worn. If the pressure doesnt go up then the vavles are not fully closing/seating. A valve not closing/seating all the way could be the timing is not correct because the cam holding them open at the wrong time, bent valve(s), or worn valve seats.

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If the timing is still correct. Low compression can be from worn piston rings or valves not fully seating. A simple test put a few drops of oil down the spark plug hole then take a reading with a compression tester. If the pressure goes up, rings are worn. If the pressure doesnt go up then the vavles are not fully closing/seating. A valve not closing/seating all the way could be the timing is not correct because the cam holding them open at the wrong time, bent valve(s), or worn valve seats.

I’m thinking valves not seating, do we have to take the head off? And if it was rings wouldn’t it smoke? It ran great before we opened up the head

Also does the timing have to be like no tolerance precise or just look at it

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If the rings are bad yes it will most likely smoke. Yes the timing needs to be spot on. There is usually some sort of timing marks or dots to visually align for making sure it’s in time. It has to be on TDC on the compression stroke not the exhaust stroke when checking/setting. You may be able to find a downloadable service manual for that bike online somewhere that will guide you through the process

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If the rings are bad yes it will most likely smoke. Yes the timing needs to be spot on. There is usually some sort of timing marks or dots to visually align for making sure it’s in time. It has to be on TDC on the compression stroke not the exhaust stroke when checking/setting. You may be able to find a downloadable service manual for that bike online somewhere that will guide you through the process

We have a manual, and the timing looked to be pretty close and yes there was a mark on the flywheel and sprocket

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Did you use a compression tester?

No but I can turn it over with just the weight of my hand

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So we took the head off and put break fluid in the head with the valves closed, after about 15 minutes the exhaust port was damp and wet carbon cane out, 1 question how do I get the valve to seat all the way

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You can try to “lap” the valves to make them seat again. You will have to remove the valve keepers and springs to do that. Once the valves are out you should roll them on a flat surface to make sure the stems aren’t bent, which can also cause the valve to not fully close

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You can try to “lap” the valves to make them seat again. You will have to remove the valve keepers and springs to do that. Once the valves are out you should roll them on a flat surface to make sure the stems aren’t bent, which can also cause the valve to not fully close

They where so dirty we cleaned them and adjusted them. Now it runs again

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