Modern Vespa

Hi all, I am trying to jet my new carb, after having several soft seizes while running reasonably hard.

While I believe that the problem was caused the carb bottom being shaped like a banana, after flattening the SI 20/15, I realized the float seal was bad...so I moved onward and upward to a 20/17D, which has on the shelf for quite a while.

I have seen the scooterhelp charts, and also read the very detailed wiki guide on tuning. It seems to me that the scooterhelp chart will only guide me back toward fuel efficiency/lean...and I would like a lower AFR then what I believe that would give me?

The motor is a 2 port, 150. Running stock exhaust (for now).

the wonderful table suggests

a/c - 140
mix - E1
main - 102

idle -42

the 20/17D came with a 160/E4, and a 90mm main. This seems too lean to me?

Am I right in thinking that the main should be a bit bigger? I am going to have to order some parts, so would very much appreciate someone suggesting a conservative (rich-side) range of jets to start with?

I have available:
A/C - 120/140/160
Mixer - E3/E1/E4
Main - 92, 90, 80
Idle - 50,42,40, 52/140

Would the 120/E3 or E1 overcome my small jet sizes and work with the 90 or 92? This is not a speed demon I am looking at here...

Thanks for any advice,
Jeff

What exhaust are you running?

the original, or as close to it as an indian repro gets?

thx,

the box is approx 3"x6"x9"..

I'd expect you'd need a 96 or 98 main. Your best bet is to just buy a set of jets so you know that they're accurate relative to one another and dial it in rather than slapping one in and hoping it stops seizing.

This is also assuming that your soft seizes were caused by bad carb setup and not air leaks.

Thanks, Chandlerman.


Is your numbering recommendation based on a 120/E1, I assume?

I was looking for a good starting range, and it sounds like you have given me a good place to start. I didn't want to order more than 5 or 6 jets if I can avoid it.

Your advice in new is well taken. Forced me to admit that I was being super cheap... . New ones are on the way...thanks for being my conscience here.

Is there any reason to consider a 140 AC?

My gut tells me that the smaller the better here, as the impact of air flow on the fuel rate through the nozzle is reduced. Am I making this up?

No, sorry, I should have clarified. That's based on a 160. You want the largest AC you can get away with, because more air == better atomization, especially at lower RPM's when it's not pulling much air. A ten jet set from SIP or BGM will cost like $20, versus $5-6 per jet if you buy them one at a time. And if you think a $5 jet (plus $12 shipping) is expensive, wait'll you start start shopping for new top ends. Not in your case, for reasons I laid out above. If I'm reading this the way you intended, you have it backwards.

Gotta think about the bigger picture. The carb works by generating a vacuum in the neck, which pulls the atomized fuel through the spray bar. The trick is to get the fuel as finely atomized as possible. The way that happens is by maximizing the amount of air through the jet stack (larger Air Corrector) which can still carry enough fuel.

Tuned motors literally can't get enough fuel into the mix for what they're pulling through the carb, so they have to reduce the amount of air to make more room for fuel.

The other thing to remember is that everything in carbs is a tradeoff. The carbs all try to minimize those tradeoffs, but the SI carb is pretty primitive in that respect. You have to go to a proper flat slide side draft like a PWK or a PHBH to get enough knobs to really tune it, and that's way, way, WAY out of scope for what you're trying to do here.

So helpful. Thx!