Modern Vespa

Other than engine oil and oil/air filters, I also wanted to change the drive belt as well. I have been looking at Dr. Pulley rollers and wondering what else I should change in the driveline while I have everything tore apart.

1. what size and weight rollers do I need?
2. do I use an OEN size drive belt?
3. how do I tell if the clutch is good?
4. anything else I'm missing?

Valve check, coolant and brake fluid if they are over 4 years old.

I use the OEM stock rollers, sliders, and belt
Clutch should still be good so inspect and deglaze pads
Watch the Scooterwest YouTube video on this

You mean the engine valves? I don't hear any irregular noises, I'm assuming they're good to go?

What noise do you get from an exhaust valve that is too tight?

Good point! I'll have to add this to my list of tasks during this service. Thank you for that chart, it is very helpful as I do not have a book that came with the Vespa. GTS Valve Lash Adjustment

Is there a reason you would advise to not use Dr Pulley rollers? I've seen lots of good performance posts on those, why do you prefer OEM rollers?

I put them in every scooter I have ever owned from my Burgman 400 to my 50cc Yamaha. I had them in my GT200 with a Leo Vince exhaust. It would run neck and neck with my friend's GTS300.

I usually combine Valve check, spark plug (denso iridium) and belt & roller service.
Double up scrap 2X12s under your center stand. Use another scrap 2x6 under the rear wheel. Undo the top shock nuts, and use the 2x6 under the rear tire to drop the whole assembly. Be careful of your coil wire while you are doing this. You should undo the plug wire first.

My experience with them over many miles and 2 scooter cannonballs. I have found all the OEM parts to be extremely reliable and see no reason to change a successful formula

Although the OEM stuff is fine, I like the extra acceleration with Dr Pulley rollers plus an extra 1mm washer between outer driving pulley and centre bushing. Top speed is maintained.

Dr. Pulley Sliders are kind of like installing a car tire on the rear. Most people who bash the idea have 1. never used it, or 2. have a bad experience from improper installation, wrong specced parts.

The stock parts work very well, that was good enough for me. I tend to not mess around with what's working well and 'play it safe'.

But that's just me.

Research carefully:

In particular be carefully of terminology: Dr Pulley make both round rollers and sliders. They ain't the same thing.

https://drpulley.co/round-roller-weight/
https://drpulley.co/sliding-roller-weight/

The ideal weights will be diff for each type. Also diff for each model of engine. Also the need/thickness for a spacer washer is different for each engine. What target RPM do you want during accel? Not sure what the peak torque is or the peak power is for the 250 (very diff rpm for these two peaks).

Personally I tried sliders (multi diff weights) in my 300HPE and interesting but went back to the rollers as the sliders struggled at 110+kph but were fine (but not worth it) at lower speeds. Maybe the lower cruising rpm that I got suggests I needed a washer? 13g, 11.5g, 14g sliders in GTS 300 HPE