Modern Vespa

In the process of starting my VBB engine for the first time. It has a 177 kit and a new 20/20 carb.

It starts and runs and sounds great, but ONLY if the slide in the carb is nearly halfway open. If I let the slide close any further, the engine shuts down.

I can see fuel coming out of the nozzle when the slide is half open, but no fuel seems to be coming out when the slide is open any less than that.

Obviously, under the circumstances, there is no way to set idle with the idle screw. The slide has to be much more open than the max range of the idle screw. I can only start and run the engine if I'm pulling on the accelerator. If I let the throttle cable go, the engine dies, no matter where the idle screw is set.

Main jet is 114 and the atomizer is a B4.

Anybody have any guesses about what's going on?? The main symptom is that no gas is being pulled out of the carb throat nozzle unless the slide is way open.

Here's a pic of how far the slide has to be open for the engine to run at all (a little hard to read; sorry). Thanks for any and all wisdom!!

Sounds like a booger in your idle jet circuit. Doesn't take much. Idle jet openings are very small. Try blowing it open with carb cleaner or compressed air. If that doesn't work you can try a very fine wire, but be gentle, brass is soft and easily damaged.

Thanks. A similar thought had occurred to me, but I know so little about the circuitry of these carbs that I didn't know if that scenario was a real possibility. Will now study & mess around. Merci!

OP here. Well, THAT was an interesting discovery: The idle jet was never drilled through. The tip has a dumple but inside it's still solid brass!!

THAT would exactly explain the symptoms. Will try another idle screw and give the engine a kick in the morning.

The sizing pin just bangs against the brass (pic below).

Thanks!

Mystery solved - good job!

It's not supposed to be drilled through. It's a top and bottom circuit…

^ +1

Also, the mixture screw looks all the way in. On these newer carbs with the fine pitch thread, it needs to be at least 2 - 2.5 turns out…

Jeez. Not supposed to be drilled through? Okay, new to me. First carb jet I've ever met that isn't supposed to be drilled.

Just called a guy who knows this stuff inside out and will see him later today. He said that it depends on the carb—that old carbs used a jet that wasn't drilled, newer 20/20s use a drilled jet. I have no idea.

I was hoping this was gonna solve my problem because the scenario made sense—no fuel was coming out of the nozzle until the slide was halfway open—i.e., i was giving it a lot of gas by pulling on the throttle cable. Intuitively, it made sense to me that that's when the main jet was kicking in, and that until then i should have been getting fuel from the idle jet but wasn't. Right now whatever I do with the idle screw is completely moot—it won't open the slide far enough for the engine to run.

The carb is new, and i got the impression talking to this guy that, although early carbs of this model used idle jets with no holes, this one should use one with a hole.

He also said he didn't think the correct idle jet would solve my problem.

So, no idea. I'm in a deep hole looking up!! We'll see what happens.

Also, if this jet is supposed to have no fuel hole, why is it marked 40-130? My understanding is that the first number refers to the size of the fuel hole, the second to the air holes (I did run a measuring pin thru the air holes and they spec'ed out).

Not being a wise-guy. But why would they stamp a fuel-hole measurement if the jet isn't supposed to have a fuel hole? Possible, I suppose, but seems odd.

It is a world of mysteries.

If you look at the jet, the air comes in the top two holes in the side and the fuel through through the bottom two.

If it makes you feel any better, I had the same no start condition on the VBB today because the idle jet was blocked. Cleaned it and first kick start.

Thanks Chandlerman. But do you have any idea why the jet would be stamped "40-130" if there are no 40 holes anywhere?? All four of the side holes are 130. ??


A dweller on the threshold, seeking wisdom.

The 40 hole is the hole in the tip. I think Safis's point was that the hole is only supposed to go as deep as the lower two holes on the side. It is not supposed to go through to the top two holes where the air comes in. Put another way, the fuel and the air do not mix inside the jet. That is a common misconception with the mainstack too.

Okay, NOW I finally get it, I think. I went over to SF Scooter Centre, where they work on a lot of vintage, and the guys walked me through it. Yes, for the life of me, I couldn't figure out why I couldn't see THROUGH the jet. Never encountered that before.

That said, the mechanic at first was unable to get a jet-measuring pin into my idle jet, so it in fact may have been wonky all along. He also recommended a larger idle than the one that came with the 20/20 (because I upsized to the 177 cc kit).

So, will try it tomorrow. If the problematic symptoms disappear, I'll assume that the first jet was plugged.

Thanks everyone. I'm now at least SLIGHTLY smarter than I was this morning.

Yes, typical idle jet for a 177 is a 55/160. Some want even richer. My Malossi uses a 52/140.

OP here. Put in the new idle jet, and it made no difference. Engine will not run unless the slide is at least halfway open. Any less than that, and the engine dies, I think because it isn't getting any fuel (I can see fuel coming out of the nozzle when the slide is halfway open or more, but nothing at all if the the slide is less than halfway open).

I'm stumped. ?? Thanks for any thoughts.

you need to clean out the entire idle circuit, not just the jet. Remove the jet and blast carb cleaner into the carb. Also remove the mixture screw and blast carb cleaner through it.

You should see carb spray blasting into the carb mouth once the crud is cleared out.

How many turns out is this guy?

Okay, will go out and blast the idle circuit.

The fuel screw is two turns out.

Thank you, gentlemen!!

Oy. Just went out and tried everything. Blew through all the carb holes with carb cleaner and also with compressed air. Also—duh—finally really stared at the circuits and began to get a better understanding of how (I think?) this carb works: saw that idle fuel came in UNDER the slide (as it does, under the butterfly, on the Webers on my VW) and that fuel from the main jet comes in from the nozzle in the throat, yes? Maybe?

Anyway cleaned and blew out everything the best I could, making sure, by holding my fingers over various holes, that compressed air could blow completely through the idle and main systems (i.e., I could feel air coming out thru the idle hole under the slide, and also from the nozzle above the slide, which I assume is fuel from the main jet).

Alas, the engine remains the same. Won't run at all unless the slide as at least halfway out. At that point I can see fuel coming out of the nozzle.

To me—and I keep being reminded of how deeply amateur I am—it seems like a fuel problem. Specifically that, for whatever reason, I'm getting zilch from the idle circuit.

Taking it to my Guru tomorrow. He seems to be genuinely curious himself. Will report . . .

Perhaps this diagram will help with diagnosis.


Hec

Who assembled the engine?? Was the piston installed correctly (orientation)?? Did you perform a leak down test??

I built it. My second one. Built a small frame motor last year. That engine works fine. But yes, I'm a fresh fish.

Haven't done leak down. Will tomorrow.

Hec—I have seen that diagram. But now I'll blow it up and CONCENTRATE!!

Thanks, everybody.

What happens if you hold the throttle wide open? Does it seem normal?

Is there a chance you installed the piston just like in a smallframe engine, arrow pointing backwards??

It's been awhile, but that's not impossible. But would the engine run AT ALL like that? The symptom is that it runs fine, but only with the slide half open. Would a reversed piston cause that?

Thanks.

Re wide open: It's like the engine runs normally, but the process starts ONLY with the slide half open. Runs a little above idle at that point, then accelerates normally from there.

It just seems to be getting no fuel until the main circuit kicks in. Like either the idle jet or circuit are blocked (I've now cleaned pretty thoroughly) or, when the slide is closed, not enough vacuum is being generated to pull fuel out of idle circuit.

In my very amateur analysis . . .

One last test, which won't help identify a fix, but will confirm the issue is fuel delivery. Squirt some raw gas into the carb mouth and try to start it with the throttle closed.

If it's an "idle circuit not delivering fuel" issue, it should start and run for a least a couple seconds.

Okay everybody: Problem solved!

I'll just cut to the chase: As SaFiS suggested, I had installed the #^{^{%damn piston upside down. Yes, there you have it. And I had been feeling so PROUD about making sure that the arrow pointed UP, as it does on my smallframe. I assumed ALL piston arrows should point up, because isn't that what any self-respecting arrow would do?? Only a pessimistic, insecure, depressed arrow would point DOWN, right?

Took the engine over to Joe Deason at Second Chance Scooters in SF today, told him the symptoms, the things I had tried that didn't work, and that Modern Vespa contributors had also suggested a leak test and the possibility that the piston was reversed. When he said, "The arrow is supposed to point toward the exhaust," I had a sinking feeling. I had thought the arrow meant "up." We took off the head and . . . there it was, pointing away from the exhaust. I felt relief and embarrassment, in equal measure.

Will reinstall the piston tomorrow and re-start. HOPING FOR THE BEST!!!!

Thanks, everybody, for helping me work through this. Very much appreciated. I'm sure I will have more questions during final assembly, so if everyone could just put the rest of their lives on hold and stand by at their computers, I'd appreciate it. Thank you!!!

I once did work for a "pro" motorcycle shop that outsourced Vespa engine work to me after they could not get it to run. result- piston upside down. This is a shop that told me they have more years of experience in motorcycles than my age. okay.

arrow goes towards exhaust. when nothing is on the piston, then you really have to know what you are doing and what to look for in respect to orientation.

best to you.

Don't beat yourself up, we've all been there.

Now starts & runs like it's supposed to. As Winston Churchill so famously said: "Shazzam!! And boom shaka laka!!"

You are lucky that it ran as shitty as it did.

Otherwise you would have likely snapped a ring and then needed whole new top end.