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
- 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?