Low compression two stroke

[PotatoHead202...]

Unfortunately my CZ 175 has now developed low compression (75psi). It was running without major issue until last week when it became very difficult to start from cold after being left since the weekend. I thought the issue was a flooded carb. Same thing happened on Sunday but it otherwise ran without issue albeit a bit lacking top end. Come Monday and today it absolutely would not even fire unless I bumped it where it would rumble quietly for a few seconds.

The top end was purchased as a set Inc barrel, piston and rings. Has been run at 25:1. I haven't felt it sieze up at all but there is some scoring on the piston. The barrel feels absolutely fine and cross hatching still visible. It's had about 80 ish miles or so since the full rebuild Inc new crank seals.

Any ideas what I can do now? I'm at my absolute wits end with it today!
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Honda VFR1200F, Honda CBF500 ABS, CZ125 now CZ175

[Nobby the Bastard]

Judging by the scoring, something was in the bottom end that shouldn't have been.
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[stinkwheel]

Is there any play on the small end? All the needle bearings still in there?
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[PotatoHead202...]

Bearings all seem good. I can't understand what could have gotten in there in the build. Was absolutely coated in assembly lube.

Interestingly, someone has mentioned that the compression figure is about in spec (well, about 8 psi off) for a stone cold engine.
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Honda VFR1200F, Honda CBF500 ABS, CZ125 now CZ175

[A100man]

The rings look quite bad for 80 miles.. Did you check the end gaps when fitting?
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[PotatoHead202...]

[stinkwheel]

Is it just me or does it look like that piston is slightly bulged out just below the bottom ring land?

Are all three piston ring locating pins still present?

Anyway, it's seized for some reason. Maybe it doesn't actually do 60mph?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Nobby the Bastard]

This bit looks very much like something was trapped between piston and barrel.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[PotatoHead202...]

[PotatoHead202...]

[Nobby the Bastard]

Well it left a mark on the piston....
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[Easy-X]

Unless the supplier was very specific (and trustworthy) I wouldn't just assume the ring gaps were set correctly and just lob it all in. Where did you get this top-end set, if you don't mind me asking?
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[PotatoHead202...]

Partdek CZ or Jawashop from memory
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Honda VFR1200F, Honda CBF500 ABS, CZ125 now CZ175

[stinkwheel]

Last time I had a Jawa rebored, the "set" of pistons came back from the engineers with an L and an R scribed on them and a note to say they were a) Slightly different sizes so don't mix them up and b) Measurably oval so he'd bored it for the widest diameter.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[PotatoHead202...]

[Nobby the Bastard]

[A100man]

[PotatoHead202...]

OK then.

Here if the barrel for you.
____________________
Honda VFR1200F, Honda CBF500 ABS, CZ125 now CZ175

[stinkwheel]

There shouldn't be vertical scoring on either the piston, rings or the barrel. I'm pretty sure I can vertical lines. It's been seized, maybe didn't seize solid but it has nipped at some point.

Are those rings free to move about in the lands any more?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Robby]

If you made it look like that in 80 miles, it was tight and hot the whole time.

The parts were shit when new, so there's no reason for anything currently available to be any good.

You may get away with honing the barrel, otherwise it's a rebore. It needs another piston and rings. Measure stuff before putting it together, particularly the ring end gap.

[PotatoHead202...]

[Bhud]

You've got to know when to hold 'em, know when to fold 'em, know when to walk away, and, know when to run.

[MCN]

It looks like the piston overheated, grew to big for the bore and seized.

The rings would disintegrate during that process.

Either fueling issue.

Oil mixture/injection.

Overloading.

Wrong piston/rings for the bore.

A piston have quite a bit of free play between the crown, skirt and the bore to allow for free movement and thermal expansion.

The cylinder will require re-bore or at least honing to remove any marks and restore the cylinder wall surface texture (Cross Hatching to retain the oil film).

It is not the end of the world.

Engine out time.

When it's out you can thoroughly wash and de-crap all the casings/housings before clashing it together with your new bits.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[Bhud]

80 miles since full rebuild...

If you fix it again you would have a working CZ175. Well isn't that great.

Just scrap it, or hold for a few years until its value skyrockets. The market is bad at the moment so it's a holder or a scrapper.

If now isn't the time to call it a day, then when is? Maybe it's got another 80 or even 800 good miles left in it before something else goes wrong.

It's just like those gambling ads. When the fun stops, stop.

[blurredman]