Modern Vespa

Hey! The short story is that I wanted to convert my GT125 vespa to 200cc with the sip kit (cylinder+piston+decompressor). So far so good, changed everything, made sure the timing is correct and valves not touching the piston. Started up and it worked like a charm with ocasional pop which was proably due to small jett in the carburetor. Went for a short drive and when reaching 60km/h there was a loud noise and the result was bent crank shaft and piston in pieces. Now I'm trying understand what went wrong. In my mind three things could have contributed - 1) I didn't let it idle long enough thus not enough oil; 2) engine wasn't warm enough; 3) reved too high.

I would apprechiate your input because I have new engine and getting ready for take two with the conversion

I dont think it was any of the 3 procedures you mentioned, at worse those would make the break-in process longer or no issues at all (37 mph is not a fast speed for a new motor). Sounds like the piston was hitting something and at that speed it caused the piston rod to bend/break...

I also suspect interference...something hitting something else it was not supposed to.

Now that you need a new engine may as well put something really big in there. Ever since I saw the hayabusa powered Ape I just can't stop thinking about it.

A Hayabusa Vespa would be a crazy ride for sure

Check out youtube for videos of the hayabusa Ape. That thing is badass!

In my opinion :
1) I didn't let it idle long enough thus not enough oil; —> I don't think so. A lack of oil will destruct journals and bearings in first and also making marks on the cylinder surface


2) engine wasn't warm enough; —> It's breaking cylinder and piston surface and also bearings



3) reved too high. —> Normally, it's rev like the 125.

How are the valves?? How is the rod?? How is the chain?? look like if the piston is hitting violently a valve. Or a valve is broken. A crankshaft is not breaking like that.

Gory pictures of engine innards would be entertaining, and maybe useful too.

Almost 200 hp with a Hayabusa engine in a Vespa might add a bit more excitement than one could reasonably cope with.

Just a guess (from a former two-time Hayabusa owner).

^^^
This! This is the content I'm here to see!

Bring the gore!

Yes yes...I understand that the "busa" might be a bit much. The trick to carrying on is not getting carried away!

Reminds me of a story I heard John Madden tell about Ted Hendricks, the stork. Supposedly John Madden saw Ted throw $50 into the toilet and then reach in and grab it out.

When John asked him why he did that the Stork explained he had dropped some change into the toilet by accident and he wasn't going to reach in there for only $.35 cents...

https://www.upi.com/Archives/1990/12/19/John-Madden-as-a-stocking-stuffer-You-could-do/3454661582800/

So using that logic now is the perfect time to drop a big motor in the Vespa!

Too small jet caused it is my guess.

Running an engine rich is much much safer than running it lean. It will get extremely hot, and also a lean mix will detonate and self ignite at the wrong point which bends rods and destroys pistons.

Always better to go for too rich than too lean.

Hey, yes totaly agree with you! I'm looking for the busa engine already!

I'm hoping to put in the new engine this week. I'll take some pictures of the mess.

You may be right! I spoke to one mechanic and it was his guess as well. For me it's hard to put my head around a jet being a cause for destroying a piston but the bent crankshaft rod may be additional proof for that.

À jet too lean will destroy piston head and and valve port.

But to bent the rod, that mean something hurts between piston and cylinder head. A bit of the piston maybe??

Hey guys! Here are the pics. As you can see the valves were not the cause.

https://drive.google.com/drive/folders/1GHWBjDQYN0VkaSc5y0gDjKgy-yn9hJka?usp=sharing

Let me know what you think!

Now it seams the jet and lean fuel mixture might have been the cause for this mess.

Piston failure. How was the piston shaft?? Do you found the other pin circlip??

The piston seems to be burned on the exhaust side. It's mean maybe it's too lean or an exhaust valve stay open, or the both.