Modern Vespa

WHY ARE YOU DOING THIS TO ME??? When I got here you seemed happy enough for a short run without suddenly starving, and needing me to choke you to even keep running. I will ask some others with experience to help me cure your grouchy mood.

The motor is a 4 speed 90(intermediate primaries for plenty of offroad power) w/ a stock 125 top end, a 20L carb and a Polini kidney pipe. I enlarged the ports and matched them to the cylinder. It took a while to dial it in initially, but for the last 5 years it has been a dependable powerhouse that idled smoothly and started easily! This was with a 95 main jet which seems like it should be way too big,

Now it has started acting starved or way over rich & bogging at WOT and midrange revs when it usually would be pulling hard. So I've 'rebuilt' the carb, checked fuel flow, always good spark and worked my way down on jet size, trying both an 85 and 80 but just got worse. Next up I'll try a 100 and see what that does, but...


I hate to think it's a failed seal w/ no fast fix here, and still more days here to ride . Can I sort of compensate for a tired seal w/ a bigger jet without risking damage to the motor if I resist the urge to usually wring it out WOT thru the gears whenever possible?


The motor build, pretty tame & mostly stock parts.

What's the ignition ? You know what they say..... One of the coils going bad? Points? CDI? Wire connections?

Stock points and serviced a couple years ago, or maybe less than a month riding time.

I checked all that best I could but the symptoms appear only with throttle action when it seems starved under load. Drop down a gear, or pull the choke, both seem to help. Always getting nice blue spark when kicking it over checking the plug.

i've had a couple spark plugs that visually made good spark, but wouldn't allow my P2 to start.

Yep, I've seen that too and tried 3 different plugs, same results. So far, every time I prime it, it fired right up.

Gear oil level ok? No white smoke?

Will check gear oil level but no white smoke.

Fingers crossed there is still oil in there

Float ? Level matters...

Check timing?

All's well that ends well, but first: the float is good , no gas inside and you can't easily adjust the level. The timing should be good, I set it last time I had to pull the flywheel to deal w/ a sheared key and now taper lapped in and torqued up tight. Gear oil checked and seems to be plenty. But the jetting issue seems to have been the root of my problems.

I put in the 100 main jet thinking that's GOTTA be too big. But no; now it goes like a scalded cat, pulls hard under load, revs up nice, starts easy and idles smooth. What more could I want? I'm still surprised that it wanted that huge jet.

Some electrical problems are fuel oriented...
What do you think caused the need for a larger jet?

Good question, worst case I suppose would be a bad seal that is letting in more air, so needs more fuel. Maybe bring stuff to leak test the crankcase next time I come. Or tuff to transport the whole bike back to my shop, but just the motor I could handle. We'll see how the next few days of riding go then decide.

Killer photos! Yeah, been there done that with the prickly pear 😆. Glad your scoot is pullin now.