Engine problem thread for the Corvid 19

Loop

Member
May 27, 2020
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Ok, so I had an issue yesterday the motor lost power and got to the point that it even had no compression at all. Today I went to the shed after letting it sit all night so I didn’t lose my temper lol.
And I tried kicking it over and the compression is back, so that may have been overheating ( I noticed my gas isn’t blue for some reason even though it came from the proper can) I pulled the plug, the cap was broken of course so I need to get a new one I think I have a plug wire set here I can get a cap from. I put it together and barely got a spark but that is probably the broken cap.
Anywho I took the exhaust off and looked inside and this is what I saw,,
8710E514-4C3D-4B73-A67A-7EA4021E82B0.png
D211FAA9-ACDE-49A9-83A9-6516A7B23D68.png

any thoughts?
 

FOG

Well-Known Member
Mar 3, 2019
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Looks like it got too hot and seized the piston. At an elevated temperature the piston expands to the point where it gets too big and more or less gets stuck in the bore. They can even lock up completely.

I know this from an RD400 that locked up on me at 75 mph. That was a slide for life. Once they cool and the piston contracts and you can usually start 'em up and cripple home, but they're wounded.

This is typically caused by the carb jetting being too lean. Lean mixtures run fast and hot and can even melt a hole right thru the top of the piston! My brother killed a Zundapp that way when we were young and dumb...

Looking at the plug is a good way know. Light gray is is dangerous and you can also use magnification to look for aluminum globules on the plug too. Bad sign. Using a lot of oil in the pre-mix tends to make them look black. What you want is tan.

Parts are cheap tho so I wouldn't even try to salvage the top end, but if you want to, you can actually sand the rough spots off the piston. There will normally be aluminum deposits on the bore too and it's just amazing how well muriatic acid cleans that up. Rings are probably not salvageable tho. They lose their temper after overheating. You can tell if they've gone soft when they bend instead of breaking.
 

Loop

Member
May 27, 2020
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image.jpg

that’s the plug, I think it probably overheated, the new carb is probably the culprit, I guess I’ll try to get a new top end for it.
any suggestions for where to get it and what kind to order? It’s a flying horse motor if that matters.
 

Loop

Member
May 27, 2020
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Looks like it got too hot and seized the piston. At an elevated temperature the piston expands to the point where it gets too big and more or less gets stuck in the bore. They can even lock up completely.
The piston never seized, it just lost compression.