Modern Vespa

So it appears that my last build did not have the best break in. 88 PSI compression after only a few hundred miles. This is a breakout from this previous post:
Compression through Carb
I will do a light hone and a re-ring this weekend.
I hear about running richer during the break in. How much richer and how to obtain it is why I'm here.
I am posting my current jetting. What jets are what and what jets do I change for a richer scoot.
Also, I could use some help identifying each jet. That way if I need to order larger or smaller jets, I can identify them by text description.
Also, I assume a larger number is a larger jet.
I almost forgot to add this is a stock VBB2 150 no aftermarket pipe and a stock 57MM Bore. I live at sea level if that matters at all.

From top to bottom


You have your air corrector
140
Next is the mixer or emulsion tube
BE1
Then the main jet
96

Choke jet is next
60

Then your idle jet
42/160 (which is a very lean jet, though not sure if that's a normal size for a VBB)


Jack is the resident expert on where to start for you. I'm still new to trying to tune a carb. I just turned my VNB into a VBB so I'll be following the jetting advice. What's your carb size and exhaust?

Carb Size, see attached. Pipe is stock OEM from Scooter Mercado

Chippie-

What cylinder and piston is on there? New kit? Deflector style piston? Unknown?

You have a VBB correct? Posted jetting for that says 140, E1, 100 main. Did you change your jetting?

Stock domed piston, no deflector style piston. All stock parts via Scootermercado.
I was not sure of the amount of overbore if any so I opted for a new stock cylinder, piston and rings. I kept the head.
Not a kit. I wanted to bring the scoot back to factory as close as possible.
I did not change any of the jetting. This scoot was a basket case and that is the jetting it came with. It has run well the past few hundred miles. The plug looked great so I never messed with jetting. But as I stated earlier, I would like to break it in a tad bit richer. It would be nice after the break in, I can put the jetting back to OEM. And yes VBB

If you want to make it richer, just change the main tip 2-4 points.

The 88psi compression has me worried. For a brand new kit, compression should be much higher. In the 110-130 range. Can you verify with another tester?

If it is that low, you've done something wrong on install or break in. A poor break in can cause seizes which will show low compression. You really have to baby a motor during break in for at least 100 miles. Hopefully that was your case and not the other 😬.

I think I have a 100 jet in a bag somewhere. Larger number equals richer, is that correct?
If you have a chance, read the breakout posing at the top of this post.
90 PSI with one tester and 88 with another tester.
It was having all the symptoms of low compression but low miles on the motor. That confused me a bit thinking it was something else before I did a compression test.
I fixed a transmission problem and must have damaged the rings re-assembling the motor. It ran great before the transmission repair.
Any input on the lean 42/160 idle jet?

chippieboy,

Here is a link decoding the SI Carb jetting sizing. This is a must read if you want to understand SI Carb jetting! You will learn that bigger numbers don't always mean larger!

Hec

Wiki [D] Guide: SI Jetting Chart + How to Tune an SI Carb#:~:text=Idle%20jets%20are%20marked%20with,ratio%2C%20in%20this%20case%202.90.

Compression testing a 2 stroke is often inaccurate. If it's really 88psi the kickstart will feel like there is little difference from plug in or plug out. If too easy to kick, it will be hard to start.
As this is stock, how you want to jet it? For speed or for economy, they are not the same.

I want to jet it rich for the break-in. After that, back to normal or were is was before. As for Speed vs. Economy. No idea. Economy does not rate much. Speed does not either. I use it primary just to put around a beach town. Hardly get into 4th gear.
It does seem easier to kick after splitting the cases. Also, previous to the transmission repair last weekend it was 1 kick warm, 2 kicks with choke cold. Now, many, many kicks.

Let me ask you this, during your break in, were you riding and then it magically died on you? If so, that's too hard and you probably seized it.

If no, can you pull the head and take a picture for us?

Actually, the scoot ran great and the plug looked great for months. I installed the incorrect (not knowing it was wrong) selector rod during the initial rebuild. Recently the transmission started jumping in 4th gear. I hesitated to split the cases but the jumping was getting worse. I split the cases, installed the new correct selector rod, new cruciform, new seals and new case gasket. Afterwards it was hard starting, poor or no idle, 1/3 less power, way more blow by coming out of the carb and sounded a bit more raspy from the pipe. I could not figure it out until I did a compression test. I must have done something to the rings putting the cylinder on or did a bad break in.

Based on this, don't split the cases yet. Maybe the rings moved upon installation of the cylinder and they aren't resting against the pins in the ring groove on the piston. Pull the head and cylinder by removing the cylinder head studs. Post pictures of the cylinder/piston.

Hec

Hec, that is the plan, I will leak test it tonight and then remove the cylinder in a day or two.
I really don't want to remove the motor and/or split the cases. I really, really have a hard time getting the crank bearings in the correct spot putting the motor back together.

Doubt you need to open the cases again, probably just need to focus on the cylinder. Easy enough to remove the head, pull the studs and lift the cylinder off.