Pk rotary valve (for Curare)

Hi guys,
this is my first time on your forum and I wish to thank everyone who can help me. Curare’s advice would be very much appreciated. I’m tuning a Pk125s with a Pinasco 121 kit or a Polini 130 kit. I don’t want to change the crankshaft or to cut it so I thought about modifying the intake port but I found out that I have only 3 mm to work on. So the question is: is it possible to enlarge the intake for 1-1,5 mm on each side ensuring a correct functioning?
I apologize for my English but I’m Italian mother tongue.

hello aplus,

yes don’t worry, we understood there was a confusion, but well done to correct yourself since it can happen that somebody read it and take it for good.

I would like to add:
It happened to me more then once that with a PK125S case (engine serial number indicating this model) I have got the 24/25 cranckshaft you normally have with XL and ETS.
So guys be careful with PK and before ordering the crank check not only the model of PK but check the crank you want to replace or the case where it sits.

hello aplus and welcome,

first of all tank you for the consideration you have given me, but here in this forum there are other people of very good knowledge and contrary to other forum when somebody is not sure of something is the first to say, so you would not risk your engine.
So in the future you could ask without specially addressing it to me.

For your question:

you can enlarge the inlet port but be very very careful. Not more then 1mm each side or you have to mount a reed block after since the compression plate will not be able to work properly.
And then you could make it longer to alter the timing. Maybe 2mm in the front position and 2mm in the back if you are using the pinasco, (Pinasco is more or less an improved original cylinder so if you don’t alter its porting you can’t go much further since it will be useless) 2m front - 5mm back if you choose to go for the polini.
This is since you don’t want to touch the cranckshaft. But it is still a little better with a cut cranck. The problem with this kind of induction is that in a vespa engine the crank shoulder are very small.

Hi fella,as we conceed here Curare know’s more than most(bloody scientist[:D] )

I’m not sure off hand but I think the 50 flywheel uses a smaller taper on the crank than the 125 aso I’d advise looking for an ETS or PKXL flywheel if that’s the crank you’re using.

On the subject of opening inlets. i wonder what the maximum opening is for reed valve conversion on the basis we are relying purely on crankcase compression. i know zirri anf fabrizi work has involved massive milling and welding of casings to accomodate huge reed manifolds. it would be interesting to know what the most extreme versions that have been carried out.

I think OZOAPS rotax engined smallframe is a classic work of art, but would be interested to note who has worked vespa casings to the maximum .

Thank you very much. Curare I apologize for having asked your advice but since I new in this forum and I took a look on your blog I thought you were so experienced to ask you direclty my question. I also appreciate very much your respect for the knowledge of other persons.
I will enlarge the inlet only to extend the opening time. This is a daily rider scooter that will be used in long rides. So it must be very reliable and durable. That’s why I will preferably buy a Pinasco. I also will change the flywheel using that of the Pk50s and use a Polini right hand exhaust.
Thank you again and, as we say here in Italy, Buona Vespa a tutti!

I’m sorry mates,

I confused the word bearing and the word seal. I’m not a perfect English speaker and I apologize with everyone.
Then, the taper of the Pk125 and Et3 are the same but there’s a different seal: 20 mm on the Pk125/50 crank and 19 mm on the Et3/50 Special crank.

Hi Juank,

speaking of flywheels I think to have clearer ideas than about modifying the inlet port.
For what concerns the Pk50 (s) flywheel I’m sure that it’s possible to use it on the ET3, Special and Pk125 (s) crank. This last without changing the stator plate. I owned a Pk50s with a complete Et3 tuning and used its original flywheel. In fact the taper is the same for all this models. Instead for what concerns the flywheel bearing there’s a difference. 19 mm for the Et3 and 20 mm for the Pk. I tuned many 50 Special using the Pk50 flywheel and stator. The weight is of 1,9 kgs but can be reduced to approximately 1,6 kgs without mechanical problems.
The flywheel of the PK50XL can be mounted on the ETS/PK125XL crank because it’s the same taper. Actually I personally own a PK125XL and mounted this flywheel (the electric starter version) having a better acceleration even with the large flyscreen and the top box we use here in Italy. No counter indications for the maximum speed (the same). No changes on the electric system.
If someone has a different experience can add his personal notes.

Viaggiare a „tempo di Vespa“