Modern Vespa

Hello all, new member here. I'm also quite new to the vintage Vespa game and even newer to the modification game.

I have a '63 Allstate 125 with a standard stock engine, 20/15 carb and jetting. I recently purchased and fit a Sip Racing Exhaust Road 2.0. I'm here looking for some advice on upjetting. I know a lot of sources have said solely to change the Main Jet. And I have a main jet kit on the way.

That being said, I also have a 20/17 carb on hand with standard jetting if you all think that might offer more performance.

After playing with it a-little, since fitting the new exhaust on, I've noticed that the bike revs to higher rpms quite fast. To the point where it feels like I must be overheating the engine because it feels like with only half throttle is screaming. Is that symptomatic of a mixture or jetting issue?

I know there's probably a bit to unpack here, excuse me for that. I'm new at all of this, and there's quite a lot of information to navigate and I'm still quite confused.

Thank you all for any and all insight/advice in advance.
I hope you're all well.

This is for the not so modern Vespa forum guys to answer. Moderators have been informed and it will be moved to the proper forum.

Morning. Before you switch to the 20/17. I'd say stick with the 20/15 and get comfortable playing with that carb and how your scooter feels. The less variables you throw at it right away the easier it is to detect problems.

Two stroke exhausts help the engine breathe better and are tuned to provide power at certain parts of the rev range. Some box exhausts like the SIP provide a nice linear power curve. Other exhausts provide power all at once and will snap your neck back.

As far as jetting goes, you have your idle jet, the first 1/4 throttle, mid throttle and then wide open throttle WOT. Aftermarket carbs you can really mess with. Standard Vespa downdraft carbs are pretty dumbed down so you cant really dial them in. Gas weed eaters have a smoother acceleration that most of my Vespa carbs. Dont worry if you cant rev out your motor to max RPM's in the lower gears. Third and 4th gear should rev out fine though.

The way to mess with jetting at a certain RPM on your 20/15 is to swap jets. There's no needle or anything else. Just a dedicated idle jet, a mixer tube above the main jet and then the main jet itself. Normally the jetting is pretty conservative so thats why you only hear about switching the main jet, not anything else when swapping exhausts.

Getting back on topic for your carb. I just searched around and saw that you should have an 82 main jet tip. Is that correct? I could be looking at the wrong scooter. Normally when you add a box exhaust you want to increase your main jet tip 2-4 points. Did you do that? Since the exhaust is "breathing" better, it is leaning out the air/fuel mixture. Leaner motors run faster, but create more heat which can cause damage. I personally go 4 points higher than stock when adding an exhaust. I dont care about the extra fuel used.

You mentioned the bike runs to high RPM's much faster than it used to. How are you determining that? Butt dyno? Your ear? SIP pipes have kind of a roar to them which is much louder than the stock Vespa exhaust.

If its revving up on its own and not coming back to idle you may have an air leak elsewhere that needs to be addressed.

Let me know

Thank you for all the insight. It has actually helped a lot.

To start at the end of your response; As far as the higher RPMs, I determined it just by riding it and listening to the engine when i throttled, opposed to the exhaust. (if that makes sense) The general feel of the bike when throttling can only be described as a hairline throttle. I seemed to only be able to twist the throttle about 1\4th of the way before the engine case sounds and feels like its racing to the moon.

To your advice about the jet sizes; I took my 96, or maybe it was 98, off of my 20/17 and put it in the stack from my 20/15 (it was the only larger jet i had on hand and wanted to just see what it'd act like). And its actually running quite well. Throttling is good, no bogging, idling is good, RPMs seem and feel healthy (going solely by feel here). Spark plug looks alright after a long ride. That being said, based on your advice of 2-4 sizes, is there any adverse affect of 14-16 sizes larger? I have some smaller jets coming, so I can dial it back, but for future reference is it unsafe?

Thank you again for all your help.

It's not unsafe. The worst thing that could happen is you'll foul your spark plug easier. Take a few spares with you in your glovebox in case you get stuck somewhere with no spark. Obviously change the jet as soon as you can and throw a fresh spark plug in so you can how the combustion is.

As far as the racing sound, we need a video of it running or you can find a tach to see what RPM's youre at vs. noise.

Hi MJRally - are you still there? I could really do with some help! I have a PX125 engine and a sip 2 twin pipe but it does not run right.

Let me know buddy!

Answer a few question for me please

1. Is the motor still a stock 125?

2. What size carburetor is in it?

3. Get a screwdriver and unscrew the big main jet stack. Read me the little numbers stamped into the stack. There should be 3 numbers.

4. How does it not run right? Bog? Scream bloody murder?

Hi - thanks for getting back to me!

It is a std PX125 engine with 20mm carb. I will get to the lock up later and find out the jetting.

It just pulls irregularly and hics/splutters when going through the gears. It's ok top end, but I use it mostly for pottering around town to the beach at 20-40 mph in traffic.

Ok. Check the jetting and report back.

If you have the tools, pull the spark plug and take a picture of the bottom of it. Post it up.

Usually most people have fuel problems/ dirty carburetors, then its something electrical ie. Loose connection on the spark plug, loose terminals @ the CDI etc. etc.

We'll start with fuel and then work our way to electrical if that doesnt solve it.

Mixture screw adjustment, got a good link?

You may find this of interest and can help you understand these unusual carbs. You're on the right track.