Modern Vespa

I recently got myself an ET4 125 engine to swap into my Vespa LX50. After compression testing we got around 8bar (120 psi). Is this compression number anything to worry about?

Greetings:

That does sound a bit low for a modern 4-stroke, at about 8.1:1. It should run, but perhaps sub-par on power. There may be specs for the compression ratio published online. Be sure to hold the carb wide open (or remove it) when checking compression.

Did you undo your decompression cam when you took the reading? If you didn't 125psi is pretty good.

Greetings:

PinkScooter makes a good point, as the decompression mechanism lowers the apparent compression by leaving a valve open a bit past TDC at low RPM. On my ET4 with the Malossi 187 kit, the starter struggles to get the motor to spin on the first stab of the Start button, 1 time in 3, WITH the decompression mechanism functioning. I'm not sure the starter could turn it over at all without it, making an accurate assessment challenging.

Thank you for your replies, the decompression cam was not undone I believe. So with it not undone it looks that 120 psi is quite a healthy number. I'll look more into it, but after some testing it seems that the engine is in quite good shape. Cheers once again!

I had this. Messed up my starter motor eventually, and had to get extra cranking power from a gel battery.

It explains so much now, thank you guys. I got the same thing - ET4 125 engine to swap 50 cc 2t into Lx body, so naturally I checked the compression and, while doing so with kick starter I get 120 psi tops, then getting a little bit more active with kick leaver I couldn't turn the engine at all, then putting oil into the cylinder I somehow get 150, but at this point, I don't know whether I finally managed to turn it fast enough to make decompression shut off or, was it an effect of the oil. Then I turn it with the battery connected to the starter directly and, I get 150 psi constantly. Still don't know whether it is the effect of the wet check or was it the cause of the fast starter spin. I wish I could undo the oil now, doubt it will disappear before I can fire the engine. p.s. "automatic valve-lifter" they call it in manual! - jjjjjj, I see now.