Kitted Vespa - Do they all do this?

OK, so I need some help. I've been trying to rid the 4-stroking from my vespa for ages and I'm close to throwing in the towel.


  • PX200 engine 
  • Cast iron Polini 208 kit, port matched transfer ports in the casings. 
  • A Worb5 head machined for the long stroke crank (it has a 1.5mm squish and the plug is moved toward the centre). 
  • A 60mm racing crank
  • SIP 1.5KG full size balanced racing flywheel
  • SIP racing clutch (very nice)
  • SIP clutch bearing actuator
  • Max. CHT temp. 160 Degrees C
  • Max compression 150psi (warm, WOT).
  • Ignition timing 19 degrees BTDC (about 2.5mm with the long stroke crank)

Dellorto 28 PHPB with a Ram-Air foam filter. 

  • Main - 128
  • Pilot - 55
  • Choke - 70
  • Slide - 60 (heavily modified to make it more lean - I've tried 4 difference slides )
  • Needle - x2 (middle ring)
  • Atomiser - AV262
  • White float
Port Timings
  • Exhaust duration: 174 degrees
  • Transfer duration: 125 degrees
  • Blowdown: 24.5 degrees
  • Squish clearance: 1.9 mm

The problem is as soon as I am moving and I position the throttle "just" off idle, the engine 4-strokes, I'm talking about a tiny amount of throttle. A fraction more and the engine pulls hard all the way to the top. I spend a lot of time in traffic though so I spend a lot of time in that area of the throttle.

The thing that I can't get my head around is that everything I read tells me that 4-stroking is due to being too rich (well actually the cause is that the scavenging effect of the 2 stroke process isn't getting a chance to work on the amount of gas in the cylinder, so we could look at retarding the timing which I have also tried with no joy).

If I lean out the idle any more it is just too lean, very slow to die down to idle speed, hunting, uneven, no power to pull off. But if I get the mixture "just right", the idle is nice and constant at around 1500 rpm, drops to idle in around 2 seconds, good even beat.


So, I guess my question is does anyone with a similar set up NOT have any 4 stroking just off idle?

Oh well, no replies..... I guess I'm still on my own.


I've been working on this problem for over 10 years so another 10 wont hurt [;)]

Hello Dylan,


Long story short, according to our technician Max you should change the pilot to a 52 jet. You're running too rich.


Post back if this was successful please (also of course if it wasn't).







EDIT: I just saw Martin's reply, above. I wish I had turned on notifications for this post. [:)]Thanks Martin!


Anyway, I ended up solving this on my own in the end and came to the same place, so I thought I would copy and paste from my blog.



4 Stroking - Solved

I had a bit of an epiphany moment when I realised that the idle to 1/4 is actually adjusted in three ways, not two.

Once I looked at the diagram a little closer I noticed that.

1) The air/fuel screen controls idle to 1/4 movement through a hole down stream from the slide

2) The slide cut out effects the mixture in that range as well. 

3) However ALSO the size of the idle jet effects fuel coming out of a hole just inside the slide. 

I had been thinking that the mixture screw would be enough to compensate for the difference between 55 and 50 pilot jets. In fact there are 2 sizes between those, being 53 and 54. 55 was too rich, and always was. 50 was too lean and I couldn't make it rich enough. It's obvious when I think about it now, but I thought the screw adjustment would be enough.
I have since ordered the in-between sizes and had a chance to install 53 today and the 4 stroking is almost completely gone.

Now the changes I make to the mixture screw are apparent and obvious.  I am able to find the 4 stroking by increasing the idle speed, lean out the mixture till the revs increase and even out, and then back it off (richen it) by 1/4 of a turn which helps pick up off idle.
I'm going to try a 54 when it arrives from Italy, but I am so close now I am happy. 

Happy days!