Modern Vespa

Over the years I've finally gotten to where I can feel when the engine's happy and when it's not and generally have a good idea what to do about it, but there's one last thing I've been afraid to dial in because I basically had a hard time wrapping my head around it.

My main jet stack is great and it's a pleasure to open the throttle enough to feel it kick in. It runs strong and doesn't overheat at any throttle position.

I have the idle set where it starts on one or two kicks and idles as steady as I could want, and when pulling up to a stop light the idle drops right back down to the RPM it's supposed to be (the TT says so).

But, right off idle and just before the main stack kicks in, is not quite right. It's kinda of gurgly and spluttery and I'm certain it's the progression circuit range. It's a mostly stock PX150 with a SIP Road, an SI20/20D, and the current set up is...

Main 160-BE3-104
Pilot 48-160
Slide Uncut #1

Correction:
The slide is not uncut, it is a #1 with a shallow scoop on the underside.

I'm having a hard time deciding if it's lean or rich in that range. Depending on which, I could do the following...

Swap out the mix tube with another.
Decrease the main air corrector to bring the main stack online sooner.
Change the pilot jet to something else.
Try a different slide.

I'm hoping the carb experts on the board will have few suggestions where to start. I can just envision spending an eternity trying all the possibilities and only going in circles.

I suggest you could richen the idle by half a turn, even though it makes it lumpy, and see if its better or worse. That should tell you which way to go. Also it is possible that its not purely a jetting issue and you could try restricting the air intake just before the bell mouth to create a more turbulent and faster air speed into the carb. Its surprising how much difference this can make although maybe not a permanent solution.

An interesting thread. Why? Because this scooter is factory spec (except 2 points up on the main jet) and it doesn't run perfect low down.

Lots swear by the Vespa stock jettings but in various countries/years there are different stock jetting versions.

One difference for the stock 150 was the pilot jet. There was the 45/160 once but the 42/140 was another alternative to the 48/160 (both of these have the same jet performance ratio 3.33).

I suggest you get a 42/140 and give it a go I'm sure it will be much better.

42/140? I'll think about that and give it a try. I see the F/A ratio is the same, but I assume there'll simply be less of both in that range. I imagine I'll have to back the mix screw out a bit at idle as well. Right now it's just shy of 2 turns (SPACO).

I should've also mentioned that I'm still using the original undrilled Piaggio air filter as well. I have a drilled filter and tried it once, but wasn't sure I wanted to upjet everywhere.

The factory jetting has to be on the rich side, as it has to suit every engine without going lean. That's one reason the drilled filter helps. Never seen any issue with a drilled filter on a stock scooter.

The jet performance ratio of 3.33 is how to compare one jet to another. This number means the jets flow the same quantity of fuel at full flow.

Assuming your mixture screw is set correctly at 2 turns with a 48/160 . When fitting the 42/140 the mixture screw will need to come out more (to let more mixture in to stay the same). This will probably put it at 2.5 turns. Which is exactly where it is supposed to be (Spaco)

I had a 42-140 on hand and gave it a spin.

The bike winds out much better now and the gurgly blubbering range is much reduced and pushed down to just off-idle, making it feel like there's still a bit of a flat spot.

Thank you for the suggestion, Jack221.

Question: with the pilot jet A/C at 140, and the main A/C still at 160, doesn't that create a low fuel flow zone in the overlap? This bit has always baffled me.

Also, the CHT says the bike is also running a bit hotter which is good, because it was definitely running too cool, barely getting over 200F unless I was going uphill in 4th. At my next opportunity I'll do some WOT plug chops to make sure all is well.

Glad she's running better pdxjim. I'm only at work, so plenty of time to talk scooters

Slight rich just off the throttle stop means the mixture screw is out too far or the 42/140 is still a bit strong. If you have a 45/160 give that a go.

The size of the pilot air corrector or main air corrector have no connection to each other or any interaction. The overlap point is adjusted by the main air corrector and atomiser but that's quite an art and a lot to explain.

I expect there might be a flat feeling point at somewhere after the 1/4 throttle mark?

If the gauge is working ok, 200F means the plug is running quite dark. If it is the power will be down and the economy will be bad.
To sort it, I'm sure you know already but the next thing to do is reduce the main jet one jet at a time until you like the temperature better.

On an SI carb the main jet seriously interferes with 1/4 throttle, so I would get the main jet better first before any more pilot changes.

The overlap point is adjusted by the main air corrector...
I knew this... smaller = sooner, but could never figure out exactly what the pilot A/C would affect other than fuel/air ratio.


On an SI carb the main jet seriously interferes with 1/4 throttle...
I did not know this. I'm thinking I probably ought to put the 48-160 back in and focus on the main jet first.

The overlap point is adjusted by both the Air Corrector and Atomiser. There is no linear rule for this. A weak AC and rich Atomiser will still be rich on transition but lean out faster. It's the combinations of both that provide the smooth transition.

The pilot AC only effect the pilot jet. Nothing else. It's such a shame the two parts are in one jet. If it was split they would be so much easier.

The 48/160 is too big for yours. Stick with the 42/140 and go back to the pilot once the main jet is a few sizes lower.