Modern Vespa

Hi all, something for you to muse over......

I fitted a Polini 208 kit to my TX200 (Spanish T5 with P200 engine) and ran it in correctly as stated for 350 miles.

I then decided to see how it performed top end, I was travelling at 70ish mph with approx 7k RPM when it soft siezed on me.
(I also have a SIP Speedo fitted with CHT gauge and was showing 135 celsius).

It was only a nip up and after a couple of minutes, it started and got me home.

It got me thinking though after reading online that max revs for a P200 is 6000rpm, but the T5 Speedo hits the red line at 7500rpm.

I appreciate that there are multiple factors for a soft sieze but I was wondering if a larger clutch drive gear would lower the max RPM?
Standard P200 clutch gear is 23 tooth, it's possible to get a 25 tooth.

Any thoughts on the above appreciated

Someone will be along soon to ask you about carburation, exhaust and jetting. I don't think your rpm's are excessive.

7-7500 is not excessive for a properly tuned touring P2.

What are your carb and jetting specs, Oil mix, spark plug?

Along with exhaust, crank, any info you have on port timings and intake timings, build quality (i.e. pressure test?), etc.

Getting to a meaningful root cause of "why did I seize at 70 MPH?" is going to require a lot of info.

In terms of "what is the red line?" it's 8,000 RPM's with that cylinder (really, that piston). The upper piston ring is 2mm, and 8K is when 2mm rings start to float.

Off the top of my head, I'm going to guess fuel starvation from one or more of: a too-small main jet, undrilled float passage, some other choke point in the fuel flow.

Maybe your ignition timing too advanced, but I'm hesitant to blame that until we have more details.

With more modern top ends, the red line on these motors is "more RPM's than you'll ever get it to turn." The Polini 208 ain't a modern top end, though.

I'll get back to you with carb jets as I can't remember off the top of my head, the rest though....

Polini 208cc (cast iron) cylinder with 1.5mm spacer at bottom, NGK B8ES plug
Polini 60mm crank
BGM Pro 26mm Si Carb (standard air filter with heart drilled out)
BGM fast flow fuel tap
Ignition timing 18° BTDC
Pinasco touring clutch with Mallossi clutch springs
Mallossi Clutch drive plate (24t)
Mallossi Primary Drive gear (63t)
T5 4th gear
PM Tuning Evo Exhaust
PX200 Engine cases (no porting, absolutely standard as don't want to ruin them )

edit: carb jetting -
Idle: 50/120
Air Corrector: 140
Mixer: BE4
Main jet: 135
Mixture screw 2.5 turns out
98 octane fuel, 3% mix
Mannol semi synthetic oil