Modern Vespa

Well - not sure if this is where you are focusing right now - but you sound rich down low. first 1/4 throttle 4 stoke.

As a general rule of thumb, in determining rich vs lean, when you have it under load - say going up a hill, and it cleans up a little and stops 4 stroking as much, it means you are rich. The hill demands the richer mixture, and thus that additional load starts to use up some of the rich condition's 4 stroke.

The opposite is also true. If you enter a flat or down hill, and it starts 4 stroking that's also a sign you are on the richer side at that throttle position. The motor is not demanding the richness and it starts to 4 stroke.

If your not sure, find a decent hill you can use to add load and get a better sense.

Nice test guide CM. I'll use that myself.

Thank you. That's a good test and makes a lot of sense.

Keep in mind, this is exactly what you want - a mixture that's on the rich side under load.

If you lean the mix to eliminate 4 stroking on downhill or low-load conditions, you're in danger of having a mix that's too lean when you put it under load - e.g. WOT uphill.

I don't think you can have it both ways, at least not with these top ends and carbs that we run.

The value of the test is also that it tells you whether the stuttering is rich or lean. It can be hard to tell from the sound alone, at least for me.

I'm happy if any 4 stroking is confined to really low throttle, especially on a road going scooter.

Finally making some progress. Jack has been a huge help with working through the jetting.

I needed to lower the clip before the idle mixture screw would respond the way it should. Really cleaned it up. Rides great up to 3/4 throttle with no main jet. So now to find the correct main, probably followed by some fine tuning.

It's surprising how late the main jet shows up to the party. In my usual riding, at least with this engine, it's intermittent and only for short periods. It seems like the use of a fuel pump depends a lot on whether you ride a lot above 3/4 throttle. I have one in my box of bits just in case.

No.

It depends on whether you want to be able to keep riding when your tank is low. It's not a performance item, its a motor safety item. Fuel flow on a low tank level is pretty poor with very little head of pressure. If you kept riding until empty without a pump, it will lean out, and you could potentially seize like my friend did. Couldn't save the kit.
With a pump it will keep delivering the same pressure until there's no more fuel, then the motor will cut out quickly. I know because it happened to me a couple of times.

The pressure you are looking for is only 2 or 3 psi (going on memory!) so we're talking absolutely minimal.
Only enough to ensure a reliable flow.

That makes sense. I ran the float bowl dry once on the stand before changing jets. It took a long time and ran lean for ages before it finally quit. I can see how that could do a lot of damage at speed.

Pretty close to calling this done. Some fine tuning left to do and the usual riding it further and faster until I trust it to not leave me stranded.

I'm very pleased with it. Lots of torque and more than enough top end for me. It pulls 4th nicely up hills and from lower revs without the need to frequently downshift. It should go well as a tourer. Jetting is pretty close at:

Pilot 55
Main 139
AV 264
X7 second clip.

Not much of riding season left but enough of the best part. I will post feedback about gas mileage, etc as I test those things. Thank you for all the help and encouragement.