Modern Vespa

The setup:
PX200
Malossi 210 Sport w/head spacer
Longstroke crank
0.9mm squish
Polinibox
Standard gearing w/24 tooth clutch gear
SI24 carb (drilled out to 2.5mm), standard filter
160/BE3/130MJ, 52/140 idle
Jetting ever-so-slightly rich
Timing at 18 (strobed) degrees

Steps to reproduce:

1. Go out and rip around with this awesome touring setup
2. Start climbing a steep-ish hill in 4th at near-WOT
3. Discover said hill is just a bit too steep (speed drops from 110, to 105, to 100...)
4. Downshift to 3rd and stay at 3/4 throttle

Expected result: the engine gamely pulls the bike up the hill around 7500 RPM
Actual result: the engine makes an unholy rattle that sounds like pinking, and you roll off the throttle, and plod up the hill at 6000 RPM which makes the sound go away.

You don't need a hill to reproduce this -- it just needs riding under load in 4th and then downshifting to 3rd, and keeping it under load.

My first thought it's knock/detonation/pinking. This is the best close-up I could get of the plug and yes, there are some flakes in there, only visible by taking a macro photo.


(The oily ring is likely hanging around from the too-rich state it was in while I was dialling things in.)

So then: why is it so reliably reproducible under those conditions? I've read all I can about detonation and its evil cousin, pre-ignition, and gosh if it doesn't make your head spin. What are the next things to try?

1. Retard the timing by another degree or two?
2. Start messing with the squish?
3. Enrich the MJ?

There's another option, 4, "Get a properly tuneable carb", but I'd like to stick with the SI.

Thanks in advance for any advice!

Cheers,
JVM

Nice set up. Its going to be great.
Jetting is way too lean. Noise is detonation, most likely, having not heard it (any sound recording?).

AC120 BE5 125MJ would be where I would go next. Should be pretty close. No need to go any lower than 18 degrees timing on this set up (higher if anything). Change the jetting richer, what's the worst that could happen?

*** Lean alert ***

Swap out that 160AC for a 120 as Jack says (or even a 140) ASAP, and rejet the main to suit .... ASAP.

That detonation is a holed piston waiting to happen.

Hey fellas -- thanks for sounding the alarm.

I've got a 140 AC and a BE5 mixer tube and an assortment of jets going down to 120. I messed around with that Jack(ish) stack a while back but I never got it dialed in right and found that no matter what I tried it lost considerable punch and wouldn't rev out as far. Is that to be expected?

Also, is there any point in trying the 140AC with the standard BE3?

As I understand it, the point of this jetting approach is to get rid of the too-lean situation at higher revs.

https://www.cycleworld.com/story/bikes/kevin-cameron-explains-how-to-tune-carburetors/

https://www.cycleworld.com/kevin-cameron-explains-how-your-carb-works/

It's the lean situation at high revs that melts the piston.
AC140 BE5 could work but mid might get too rich. BE3 is too lean, forget that.
Pilot at 55/160 would be better with the 140.

Thanks Jack. I'd enriched the idle jet to the next-richest from the stock 55/160 (52/140) on the advice of many, making me less nervous rolling off the throttle after a long WOT stretch. I suspect it's overly rich at this point, given that the plug's pretty black (but not wet) after I let it idle for a while.

Could you explain how the AC140 + BE5 would be too rich in the mid, but the 120 + BE5 combo would not? As I understand it the smaller the air corrector hole, the richer the mixture.

If you still have your AC160 jetting it will be really rich low down. On an SI carb all the jets, atomiser, slide, filter work as a set. It's difficult to isolate any component. Adjust one part of the set and the remaining parts respond differently.
Fit the whole set AC120 BE5 125MJ 55/160 filter with hole 04 slide.

Thanks Jack -- got the AC120 ordered. Will give this a go in the coming week.

I assume that all this while I've been running relatively rich from 1/3 to 2/3 throttle, and above that was when I was likely in the danger zone.

Luckily I've been using it for puttering around town mostly, with very little highway/back-road cruising. I'll inspect the piston when I go at it soon, see if there's any damage.

Thanks skids for the links very informative and easy to understand.