Engine problem thread for the Corvid 19

Loop

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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

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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

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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.
 

Greg58

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May 1, 2011
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Also take the engine off the bike and wash out the crankcase with mixed fuel, pour it through a old tee shirt streached over a bucket to see what comes out.
 
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Loop

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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.
 

Loop

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May 27, 2020
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Update,
Ok I put the new cdi on today, drained the gas in the tank and as I suspected it was yellow. Somehow my mix can was filled but had little to no oil in it, the overheating makes sense now. I oiled it, a little heavy just to be safe (8oz/gallon) and I had to tighten the choke lever, it was super loose put the air filter back on and it took a little work to start it but it started, I had to adjust the idle a bit not sure where it is supposed to be set it was really high out of the box btw I turned it down so it sounds more like it used to and ride up and down the road a few times, It feels good but probably has lost a bit of power because of that overheating. And the exhaust is out of place a bit now i need to readjust it.
The oil issue was my fault, I should have checked it better but was In a hurry last week.

I plan on building a second off road type later on and there will be a pre ride checklist lol.
 

Loop

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May 27, 2020
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I think the new carb may be part of the issue, it seems to not want to be adjusted and surges easily when warmed up but bogs more often than I like.