Vespa wont run; bigger piston size?

Hi guys, 

I can speak and understand German, tough writing is hard for me so I write the problem in English. Answers can be written in German or English.
A long time ago I restorated the engine of my vespa 50L (V5A1T). After this I got the engine running once OK but did not drive with it. Now 1,5 year later the frame is finnished but the engine won't start.

I have:

  • Spark
  • New sparkplug
  • Wet spark
  • Clean carb 
  • Orriginal settings
  • New floater needle
So I checked the compression when opening the exhaust and i can hear some air leaking. This is noticeble after replacing the (probably orriginal) PIAGGIO piston by a standard METEOR 38.4mm piston during restauration since i had signs of Blow By (Carbon deposit below rings) and tought the piston rings were worn. I was convinced replacing the piston by a bigger one will solve the problem, but after opening the cilinder i can see some vertical sctratches and i think maybe this piston is already bigger.... 
VIDEO: [View:]



  • Vespa will not start after restoring engine, dispite new piston + rings + floater needle + wet spark plug + starterspray 




  • Did run and drive on old piston (probably Paggio orriginal)
  • Old Piston had blow by signs
  • After restoring with a new piston + new rings, engine does not run (Meteor 38.4mm)
  • When removing exhaust air leackage can be noticed (video)
  • New piston shows already vertical scratches

Any advice what to do, and how can I determine the right piston size?

(I tried starting it with a spoon of oil in the spark plug hole, it had no effect)

Let me know!




Hi Menno,

OK: I don't think your problem is the piston because it's a Meteor brand.
Also your video does not show any unregularity and there is no reason why this engine should not start. Your dry trials are not representative for the engine running with exhaust assembled onto it.
OK, the piston is not in good shape, seems like it has been mistreated, but that's no problem. It would even run with only one piston ring.

You did not say anything about another important aspect: The ignition spark.
- is there any spark?

Possible reasons for NO SPARK:
- ignition points could be corroded
- wrong spark plug with too long spark plug thread
- ignition timing not correct
- ignition capacitor failure (it's a wearing-part which should be replaced periodically)
- spark plug cable not connected tightly
- ignition coil failure
- cables coming from the stator plate not connected properly or broken (check if ignition coil feed cable is connected correctly to the ignition coil).
- the ignition coil feed cable (usually red) is not only connected with the ignition coil itself, there is another red cable connected to them, which goes to the kill button... if the isolation of the killcable is broken somewhere on its way from the engine cable box to the kill button, it could result in a short circuit of the ignition circuit (this means no engine spark!).

SUMMARY of basic electrical checks regarding igniton spark:
-> check if all electrical connections of the ignition circiuit are in good shape & connected properly.
if yes, and still no spark:
-> disconnect the kill cable down there on the engine cable box, but connect the other 2, then try to start engine. Please tell us what happened then...


Hi All, Hi Werner,

Here some more progress about the problem:

Last week I bought a compression meter, found out that the compression is 6 bars. Not too high, but possibly just enough to get it running. I did mount the piston right (arrow to exhaust outlet) and I do have a bright spark and new spark plug, cable and connector. 

After this I did try to adjust the ignition a bit and result: I got it running (very badly). Also I saw when running suddenly some petrol leaking allong the spark plug. Possibly I have to flatten the spark plug hole since the spark plug + ring is brand new. I think the problem is a combination of bad ignition settings and low compression. But since it won't run I am not able to set the iginition perfectly. 

This week I wil go to a motorrepair store and will bore/hone the cilinder anough to install an oversize piston, and I will flatten the spark plug hole to make the ring on the spark plug seal.



but you do not have a compression meter: this is just a pressure meter. You say you measured 6 bars, but that's a physical value called "pressure". "Compression" means it's the ratio between the uncompressed cylinder volume and the compressed cylinder volume (with the piston at upper dead point). Compression is no physical unit, it's a unitless ratio 1:11 or something like this.


I would recommend you ask for help from an experienced person, because it seems to me that you are 100 % on the wrong track.
All that reboring and all the things you have described make no sense. It's only a horrible lot of cost!
Why don't you simply buy a new DR 50 cylinder for roundabout 100 Euro and put an end to this odyssey? I've never seen someone having problems like you. Let a professional fix your ignition (new capacitor and new ignition coil)!) and then see what happens.

In Germany we have proverb which goes "shooting at sparrows with a cannon (and not hitting anything)". Thats exactly what you do.