Modern Vespa

Hi guys,

My et4 125cc (with new Keihin carb) has a hesitation at acceleration in lower rpm range.

I´ve done a compression test with the engine still in the scooter.
It´s not easy to get a compression tester connected, but somehow I managed.
The compression reading I got at full throttle was just under 5 bars.

The big question is, how much compression should I have with the decompression system still enabled ?

Has anybody ever tried ? Any numbers floating around? thanks

Eventually I took the motor out of the frame,
Disabled the auto decompression system on the camshaft

And did another compression test....

So to be accurate, with the auto decompression I got 4.8 Bar
Without the decompression system I got 5.1 Bar

So it´s definitely the top end where things go wrong

Disabled how? 5.1 bar = 74 psi, doubt engine would even run with compression that low. Might want to check valve clearance if you haven't yet.

I disabled the decompression system by removing the tiny axel with the centrifugal-weight fitted inside the camshaft. Then led a pair of jumper cables directly from the battery to the starter engine and did the test.

74 psi is indeed pretty bad, everything is set and adjusted as it should be.
At this point I´m guessing a warped cylinder-head or some issue with the aftermarket RMS cylinder I never never should have bought (It just doesn´t fit properly)

maybe at higher rpm the compression restores itself to a somewhat respectable value.

To be continued

I wonder if that's enough? Completely removed and replaced with big washers on mine. And WOT while cranking for test? Not opening throttle will also yield low number. For reference mine is right at 200 psi. This with high compression Malossi kit so I'm guessing 180ish would be a good number for stock specs.

decompression mechanism removal

That does look very neat , well done.

I´ve made up my mind and bought a malossi 190 kit. thanks for passing on the pressures, that will be a good reference.
Hopefully I can get ¨the kit my multivar 2000 and polini upgear¨ to sing the same song.

I am still not sure what rollers to buy for the multivar, I find the malossi ones wear off to quickly. Maybe the piaggio ones...