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Post by Fox on Sept 3, 2015 14:06:38 GMT -5
So many people post that they are having hard starting and rough idle conditions that they can't seem solve. The issue may be leaky valves. You can check them with liquid. You have to remove the cylinder head and fill the intake/exhaust ports with water or WD40 and watch to see if any runs out of the valve on the combustion side. If it does then you are losing compression which causes the hard starting, non-starting, stumbling/missing and erratic idle issues.
This video shows the procedure on an automotive cylinder head. The test is the same for a scooter. Fill the port with water or spray some WD40 in there and watch. Flip the head over to test the other valve. If any leaks out of either valve then you have found a problem that needs to be fixed by either replacing the valves and lapping them or replacing the entire cylinder head.
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Post by humanshield on Sept 3, 2015 22:00:01 GMT -5
Been doing this for years. Works well.
One thing I do differently is blow compressed air up the port behind the valve.
Any air bubbles means it's still leaking. Keep grinding.
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Post by scooterpimp on Sept 5, 2015 11:34:03 GMT -5
Good old compression tester & oil usually tell the tale without head removal. Motor oil in cyl. Will boost comp. If rings are worn , but wont change if valves are leaky. (Usually)  |
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Post by Fox on Sept 5, 2015 17:50:50 GMT -5
Good old compression tester & oil usually tell the tale without head removal. Motor oil in cyl. Will boost comp. If rings are worn , but wont change if valves are leaky. (Usually) Compression testers don't tell you which valve and exactly where on that valve your leak is either. |
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Post by scooterpimp on Sept 5, 2015 18:08:17 GMT -5
No , but it tells you if its valve or ring related.. that way you know if rings are any good before pulling head. So you can know kinda what parts you may need prior to yanking apart & letting sit. In pieces. On a 139qmb why even pull the valve to lap it? By the time you get a valve & seal you could get a loaded head for the same cost.
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Post by Fox on Sept 5, 2015 18:41:41 GMT -5
Whatever...
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Post by dude on Sept 5, 2015 20:39:08 GMT -5
I have tested vavle leaks with compressed air. My compression tester I can remove the gauge which was a air chuck. Then remove the air steam from the other end. Close both vavles. Shot compressed air into the cylinder. Then listen for air leaking from carb or exhaust
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Post by 90GTVert on Sept 5, 2015 21:30:04 GMT -5
I've deleted the off-topic and worst of the inappropriate remarks. There is no need to turn a technical discussion (or any discussion on this forum) into anything personal. I know there are forums where arguments, flaming, and general rudeness are fair game... but it's not this forum. Anyone that can't be polite or have disagreements in a non-insulting manner, feel free to play somewhere else if you can't play nice.
No one has to be right or have the 1 and only solution all of the time. This and compression tests are both useful. There's the option of a leak down tester as well. I've got one, but honestly I've never used it on a scooter.
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