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Post by scooter on Jul 5, 2015 18:36:49 GMT -5
I am getting cold compression readings on a 139QMB of only 90psi. Another test showed 70 psi and then, after oil was put into the cylinder, about 120 psi.
Please tell me, what is the standard psi for these engines? One source online said about 15-20 times the compression ration of 10.5:1 or about 158 - 210 psi. Several youtube videos showed 50cc compression at around 160 or so, on up to 190 with a cold engine. One source said a MINIMUM of 100psi if it's to be considered usable at all, and another said 120 MINIMUM, but a healthy engine seems to push much higher, well over 150. I am pretty sure 90psi is crap, but I want a standard number to compare it to when I tell my client what's going on with the bike. I'd also feel better with a standard number in hand before I start taking the top end apart. I think we had once discussed that a good start would be 14.696 psi times the stated compression ratio of 10.5, which would be 154 psi. 70 would only be 45% of that, and would still only be 58% of 154.ALSO This is a Bintelli Sprint 50cc with EGR. Does anyone know if this takes the standard cheap-chinese-ebay heads and cylinders for 139QMB's? I also need to address the 64mm vs 69 mm valve length question. This one has 69mm valves, confirmed by measurements. There are videos on youtube that show you how to tell the difference just by looking at the base of the springs and the height of the valve retainers above the top edge of the head.
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Post by pistonguy on Jul 7, 2015 9:44:16 GMT -5
I dot have any specific info on exactly what you have. However Psi is on the Low side but Plenty Enough to Start and Run. How bout some Scoot history i.e. How many miles on her? type of Oil?
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Post by scooter on Jul 9, 2015 20:44:41 GMT -5
I dot have any specific info on exactly what you have. However Psi is on the Low side but Plenty Enough to Start and Run. How bout some Scoot history i.e. How many miles on her? type of Oil? 7,000 miles or probably KM. I can't see the dash right now. Standard mfg recommended oil at last oil change, possibly ran low on oil for up to 5 months depending on how long it was ran "off the diptick". Could have been days, months, or weeks. It's practically new, otherwise. Another compression tester read 70psi and I don't know why. Putting oil in the cylinder made it go to 120 or so on the second gauge.
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Post by pistonguy on Jul 9, 2015 21:32:37 GMT -5
Are you Holding the Throttle Wide Open when checking Compression? 70 PSI is Much More concerning to me now Especially since Dumpin Oil in the Cylinder Brought it to 120. You may need more than New Rings at this point. Since we have a Unknown of How long she ran low on oil If you have to go down that road Please have the Piston to Cylinder Clearance Checked. NO Calipers! Calipers are for Carpenters. New Rings on a Clapped out Piston will last a very short period of time. Thats why this Clearance is Critical to Proper Ring Seal and longevity. Carry On
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