HELP! - HP4 flywheel for ET3


this is my first message… Compliments for the forum!

I need your help for a problem I have with SIP HP4 flywheel for ET3.

I have a Vespa Primavera (no ET3) with Polini 130 and Zirri Silent exhaust. Last year I changed the original ignition with the electronic ignition of ET3 (I replaced stator place, flywheel and ignition unit).

As the original ET3 flywhell is too heavy, I decided to buy from SIP the HP4 flywheel, that I received yesterday. I mounted it by simply replacing the ET3 flywheel, but it does not work properly.

The motor works very well till 6000/6500 rpm, then starts to backfire and I cannot gain more speed. It seems to be an electric problem, I tried to adjust carburation but nothing changes.

Please consider that with original ET3 flywheel I can reach at least 8500 rpm.

Is there something I missed? Reading SIP website I understood that you simply had to replace flywheel, and nothing else… Is it true?

Please help me, I spent a lot of money and my Vespa does not work!

Thank you very much for your cooperation!

PK stator plate needed also

sorry just read ur post again „hp4 for the et3“ and not the pk one. are ther 2 woodruff keyways on you hp4?

Thank you for your answer…

Yes you understood very well!

I have the right article (50011200), I already tried to change spark plug, to modify spark advance, to fit carburation… Nothing happens!

It is an electric problem, probably my new flywheel does not like my ET3 stator plate! Please consider that if I install my original ET3 flywheel the engine works properly again!

For Polini 130: if you mean the key’s housings, my flywheel has only one of them…

I’m afraid that my SIP flywheel could be faulty…


if I understood well from your writing you have a primavera no ET3 but you have already changed the stator plate and flywheel with the ET3 ones.
If you have done so, you can use the HP4 flywheel for ET3 small cone that SIP has in catalogue.
If you have the right article (art. : 50011200) you can use with the ET3 stator.
If you are sure you have the ET3 stator plate and the right HP4 flywheel and you have your lamented problem, change first the plug (why not, let’s try with the most simple thing), and if no better then you should replace your ignition unit with a new one (art: 85000000).


I Too had this problem with an ET3 stator and the HP4 flywheel. I found that the only way to resove it was replace the stator with a PK one.