Modern Vespa

Dear all, I got leader 125 engine in replace of my lx 50 2T, before I disassemble the cylinder I managed to photograph the top of the piston through the sparkplug hole (the photo). Besides burn cake I could locate some clean places with some debris seen, I wonder is this means that the engine had some meltdown parts or other major damage which will cause its full rebuild? Compression was 120 psi, after wet test with spoon of oil it shows 150 psi. The oil plug is stuck so can't inspect the debris in the oil at the moment. Thank you a lot if you take the time to see this.

Actually pretty good news - tho incomplete info to work from.
1. From what we can see the piston is black. Means it wasn't run lean.
2. That's likely NOT debris. You are shooting a close up photo through a hole. Those rings in top of piston show a lot of magnification in your photo. So the small black divots are likely no more than caked carbon.

Pull the head off - it's easy - and shoot a pic of the piston top if you want more info about how well jetted and what condition the piston is likely in.

But at first glance - I would say - it looks pretty good!

That looks like old detonation to me. Maybe happened during a long WOT run at some point?

Thank you all for replays. I will definitely post a photo when I disassemble it. Not sure if I need to recalculate the base gasket thickness anew or, can I use the thickness readout from the old gasket? Would you advise coating the piston crown with a non-stick anti-carbon coating? That maybe would prevent detonation in the future?


Merry Christmas!

You don't necessarily need to recalculate your gasket thickness after a rebuild if you re-use the gaskets.

What are your current port timings, ignition timing, and squish, btw?

I'm swapping for 4T 125cc engine, so there is no port timing I guess? Ignition timing will be by reference points and I'm not sure about squish since I didn't remove head yet. But I will do it now, since new rings best to buy after you check the old ones I'm not going to reuse the old gaskets, just thinking whether is possible to read the old gasket thickness and use the same value for the new one. For instance, if the piston appears to be good enough to use it onwards, I'll clean the crown, and doing so I will change the compression rate a bit. But that should be within factory limits I guess. I love 2t on the classic models where air turbine makes it's musical tone. I'll rebuild this one and I will see if I have leftovers of myself for a new project, Vespa Rally or similar hopefully

I finally started the engine (leader 125)! Carbeurator's membrane is hardened so it doesn't react to throttle quickly, but there's no smoke and compression is 140 atmospheres when measured after the engine run for 2 min. I wonder if I need to disassemble it to change the piston rings? Old oil doesn't show debris. I will wash it now with a semi-synthetic to see maybe it wash it out something.

Sorry it took so long to open the scalp, I finally did it. Cylinder walls, from what I understand, has some minor water marks, piston was almost clear and I could clean it with the napkin, same with head. Only carbon-deposited places were valve surfaces. Intake valve doesn't drip carb cleaner, I have tested. Thank you for your expertise!

That's not a 2T motor. That's a 4 stroke auto. Thread should be moved to General Discussion…

sorry, I will try to delete this