No compression new top end?

Chris7420

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Jul 23, 2021
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So my buddy’s bike the engine is a super tight fit so he’s unable to put a air filter on it (I’ve told him he should buy a shorter/angled intake) and we think he sucked in a pebble or something, I wasn’t there but he says he was riding and all of a sudden the top end started making a squealing noise, he shut it off for a few minutes then started it back up it was really hard to start and smoked a bunch when it did start and had no power it would only go like 5 km/h and when he’d pull in the clutch the engine would die as if you hit the kill switch also had low compression. So we took off the top end and there was a little chip in the piston and a pretty deep gouge/scratch from the top to the bottom of the cylinder. We put a different piston and cylinder/head on it, not a brand new one it came off a parts bike that had good compression low hours (both Zeda engines) and with the other top end it still has low compression. Any ideas on what the issue is? I thought maybe the clutch wasn’t working right sense when it was running with the original top end after the squealing when we think it sucked in a rock it was dying when he pulled the clutch in as if he hit the kill switch (before swapping I took it for a ride and yeah it barely had enough power to pull my weight and if I pulled the clutch it died instantly) so I released the clutch cable from the clutch arm so the clutch should be fully disengaged but it didn’t make a difference we also tried adjusting the flower nut a full turn each way in small increments and testing to see if it made a difference but it did not so I don’t think the clutch is the issue. I’ve changed a top end before on these engines and on dirt bikes/atv’s I believe I did everything right and I don’t see where I could’ve went wrong. Anyone have any ideas?
 

wrench

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Aug 20, 2019
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A picture of the chip on the piston and damage to the cyl - wall would help

It might not of sucked in a pebble.
What you describe is exactly what would happen if the piston wrist pin circlip tab broke off. Did you inspect both Circlips?

The engine case should be flushed out looking for that circlip tab or pebbel and piece of piston that chipped off. If ya don't get it out it can damage the other cyl and piston

The head or cyl deck probably isn't flat on the used cyl and head you are now using
Might be why ya got low compression
 

Chris7420

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Jul 23, 2021
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A picture of the chip on the piston and damage to the cyl - wall would help

It might not of sucked in a pebble.
What you describe is exactly what would happen if the piston wrist pin circlip tab broke off. Did you inspect both Circlips?

The engine case should be flushed out looking for that circlip tab or pebbel and piece of piston that chipped off. If ya don't get it out it can damage the other cyl and piston

The head or cyl deck probably isn't flat on the used cyl and head you are now using
Might be why ya got low compression
I’ll get a photo of the damaged piston/cylinder, and yeah both the wrist pin clips were present and weren’t broken at all. And the used piston and cylinder had good compression on the engine they were taken off of so I don’t understand why they wouldn’t have good compression on this engine, is there any way I can test? Maybe turn the engine over with a drill and spray around on the cylinder with liquid and look for bubbles? I know you can use a leak down tester but I do not have one.
 

wrench

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Aug 20, 2019
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So Cal
I guess it might of been a pebble, It might of been crushed and blown out the exhaust.

You should be able to feel the air escaping around the head with your hands.
When ya start the engine while it still cool put your hands around the cyl head and feel for air leaking
 

Chris7420

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Jul 23, 2021
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I guess it might of been a pebble, It might of been crushed and blown out the exhaust.

You should be able to feel the air escaping around the head with your hands.
When ya start the engine while it still cool put your hands around the cyl head and feel for air leaking
It won’t even run with the new cylinder/piston
 

Greg58

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May 1, 2011
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If its a used piston and cylinder it's sometimes hard to get the rings to seat, even though the piston and cylinder were run together piston pin to connecting rod alignment could be slightly different causing the rings to run in a slightly different place in the cylinder. Even though you can't see them there are tiny scratches in the cylinder that match the surface of the rings, that's what we call the "seating" process, we should in fact be calling it the "wear-in" process.
 

Chris7420

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Jul 23, 2021
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Did you use a torque wrench to tighten the head nuts? 12-14 foot pounds is all it should be.
Haha so I actually put the old cylinder back on by accident, switched it around with the good one and all is well! I think I had a few too many beers the night I put the new top end on.