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Post by dster5 on Feb 10, 2021 0:36:58 GMT -5
Restoring a US MB5 but big learning curve as I’ve never done anything like this before! Stock cylinder, new piston and rings, good spark, new Chinese carburetor, but compression only 90 psi and won’t start. After the piston etc exchange, I tried to start it but no go even with a drill. I hadn’t gapped the rings so I took it apart again and under the head gasket I found new black film/ comes off with acetone. Is this due to bad head gasket? The gaps were fine and I will be replacing the gasket, also replacing the reeds although they looked good with no air gap.
Any ideas for a noob? Thx!
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Post by aeroxbud on Feb 10, 2021 5:11:02 GMT -5
Welcome to the forum.
It's hard to say what that film is. But the surfaces should be clean. If it wasn't in there when you assembled it, it must be a leak.
Did you torque the head down in sequence? It needs to be in a cross pattern. It's also best to torque to a lower pressure, then increase till you get to the required torque spec.
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Post by geoffh on Feb 10, 2021 16:05:10 GMT -5
Welcome to the forum,the arrow on the piston should point to the exhaust pipe,just a thought??
Geoff
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Post by ThaiGyro on Feb 10, 2021 20:34:45 GMT -5
Yeah...90 is way too low. As the guys stated above...rings installed up side-up? Not sure about the MB5, exactly, but should have two rings...30mm x 1.5mm. They both should have a locating pin to seat against, thus less room for OOPZ!
That is one good and fun engine, so best of luck!
You may also want to measure the cylinder bore...ID/runout. If you do not have a "runout" ball gage, you can get an idea of cylinder wall dimension, by measuring what you can with your calipers, top side and bottom. Not a great method.
Another tell-tale method...pencil mark your cylinder walls with....uhhh a pencil. (#2) Make marks up and down...plus all around every port. Slip in a super thin sheet of baking parchment...fitting to the walls as tight as you can. Then try to slide the new piston inside going top to bottom or visa-versa. If that works, take some photos! We all can see!
If that cheap method gets you no where...look at your crank seals. Haha, kind of ass-backwards trouble shooting, but you get the idea!
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