Modern Vespa

My stock PX150 with an SIP Road 2.0 exhaust and a larger main jet bogs down at high rpm. When I slow down a little it runs fine.
It starts and idles fine.
Stock carb is clean.
Running NGK B7 plug.
Any ideas or theories?

Sounds like fuel starvation…

Any modifications done to the carb?

But you're riding stock engine I presume… I imagine it's not the most demanding setup.

Other issue could be ignition/CDI throttling the revs.

Carb has been thoroughly cleaned and up jetted when SIP pipe was installed.
Float and needle are perfect.

Stock motor

What did you up-jet to?

104 Main jet

With the stock pipe and 98 main jet, it also bogged down @ high rpm.

could be the good old blocked fuel cap hole... make sure the fuel tank is breathing.

Are you still running the stock fuel valve/mechanism (the one that goes on the bottom of the fuel tank). Have you looked into your air filter? is it the stock one with the blue and white layers? They tend to get dirty and oily restricting the air flow.

You main jet is spot on. I recently put back my stock cylinder on my PX150 because I am doing several exhaust boxes test and I have no problem with my top end although I would say that my carburator is not stock, I am running a 26/26 and I do have a BGM Fast Fuel valve.

Bogging down on the top can go both ways on the main jets. Typically too big of a main jet can cause it to bog down. Do you have other jets to try?

3 years ago I did a double engine swap on two scooters. Everything went smooth until I went for a test ride on the PX150. Turned out that when I was doing the fuel line under the tank the fuel line was pinched between the gastak and the body(frame) causing fuel starvation only in third and forth gear as soon as I would open up to try to merge to the hwy. The fuel line was pinched causing the fuel line to generate random air bubbles during the engine demand for fuel at full throtle. It wasn't that obvious in first and second because of the little time to get to red line but third and fourth would starve.

Have you messed with the fuel lines?

Hey, sidebar: just want to say "hi" to rajavw -- I didn't realize you were back.

For those not in the know, rajavw is almost certainly the earliest user on MV that's still active, besides myself.



Quite happy to see you around.

Hi Jess,

Still here!
Turning 55 in November.
Obsessed with these contraptions since acquiring a new 1981 P200E At Burbank Kawasaki.
Down to just 2 Red 2005 PX150. Mine and my son's.

I have not messed with fuel lines (yet) nor h have I investigated the fuel tap system.

Air filter is off white in color and pretty clean

Will check that after work today

Got it.
So this was already an issue.

Blocked fuel cap/fuel starvation is a distinct brrrrr... sound.
Do you have another CDI to try?

I'm feeling CDI failure, too. Usually, the RPM's where it starts to struggle will fall over time until the bike becomes unrideable.

Cheap and easy to swap out, though.

My buddies red 05 PX had a CDI failure at high revs a few years back. Stuttering at high rpm's like you described. Eventually a pickup failure on the stator as well. Replaced and moved on.

Glad I have a spare blue CDI to test after work today!

It was the CDI box after all!!
Thank you all for the continued support.

woohoo! Pretty much the easiest thing that could possibly have been wrong.

CDI failure ftw…!

Everyone's pretty good at the guessing game now.

Unfortunately the high rpm bogging has come back again!

bummer.

Where'd you source the new CDI from? It's possible it failed rapidly, but doubtful.

Alternately, did you also check for worn/rubbed/frayed wires from the Stator to the CDI? If wires are grounding or arc'ing to each other, that could cause similar issues, but only when there's enough vibration (RPM's) to cause contact.

Bullseye on this one. It was the CDI.

Replaced fuel line, still bogs at high RPM in 3rd and 4th gears.[/img]

Replaced carb needle and the bogging has completely stopped.