Modern Vespa

Here's a weird one.

My recently rebuilt '63 VBB had been running well, but I'm trying for even better tuning. I had run a 55/60 idle jet and things seemed okay, then I tried a 48-60 and the engine seemed a little happier.

Then, yesterday, wondering if I could improve the idle a bit more, I tried a 42/60. Didn't work. Engine bogged at low speed.

So I went back to the 48/60—and now my engine dies after a minute or so. I start it with the choke open, it fires up happily, then I push the choke in and it runs fine for a bit, with me blipping the throttle, and then—pffft. The engine sputters and dies. Every time. I went back to the largest idle jet, a 55/60. No luck. Same pattern every time.

It feels like, and looks like, the engine is just running out of gas. Then, after a bit, I can start it again—like gas is GETTING to the carb bowl, but not fast enough, so the engine burns what's in there and dies.

Notably, I don't see ANY fuel coming out of the nozzle above the slide. I'm new to these engines, but, uh, that would seem to be a problem, yes? What could cause that?

Here's what I've done:

Removed the tank and checked the valve. Seems to work as it's supposed to. Drained some gas into a bucket.

Checked the fuel line. No kinks, and, as it's clear, I can see it's full of fuel.

Loosened the screws in the banjo fitting on the carb. Fuel leaked out, so at least the fuel is getting that far.

Removed the carb and looked into the bowl. Fuel was at or just below the drilled hole at the bottom. Is that something? Should there be more fuel in the bowl when I check? It DID look like maybe the carb couldn't draw, because the fuel was at or below the hole.

Removed, checked and cleaned all the jets. Checked that the drilled hole at the bottom of the carb bowl —that leads to the passage where the jet stack is—was clear and not blocked.

And—nothing. I start the engine with the choke out, push the choke in, blip it a few times, and everything seems normal. And then, about a minute later, it sputters and dies. Every time. No matter the idle jet size. No matter how I adjust the slide or fuel-mix screw. Also, I switched out the carb float and float valve from another carb body I had. No change.

As I say, I have yet to see any fuel come
out of the nozzle above the slide, at any time and at any adjustment. That ain't normal, is it??

The engine worked fine yesterday morning, and now it doesn't. I'm stumped. Given my personal history, could well be something dumb & simple. But what? Whip it on me, experts. I can take it! (I think . . .)

do you mean fuel is below the hole at the bottom of the float bowl? see picture
If so you may have a stuck float needle.

Thanks. When I have opened the carb—-after the engine dies—the fuel level is right at that hole. I'm not sure what that means, but my first thought was, "Hmmm. It's sure looks like the carb can't draw fuel, or not much fuel, through that hole."

I've opened the carb twice and found the fuel at that level.

But I'm no expert. Is it SUPPOSED to be higher?? If so, that would explain a lot. But I don't know.

I thought that maybe the float or needle valve might be bad, so swapped over the top (banjo, needle, float) from a known good carb body that I have (unfortunately the fuel screw is in that one is mangled so I can't just switch the entire carb and test). But changing the float and valve made no difference. Still dies after one minute.

Still stumped!!!

Thanks for your interest!!

SOLVED!!

Read an old post here by someone who had the same problem. He said that the tank spigot exit was only about 1 cm above the height of the banjo intake. He had a big dip in the fuel line before the carb, and when the tank was low the carb had trouble getting enough fuel. He solved it by straightening the fuel line. Another guy solved it by running the fuel line thru the air intake.

Anyway, to test the theory, I topped off my tank and everything worked perfectly again. I also had a big U-dip in the fuel line before the carb. Straightened that out. Hoping that is enough to provide steady fuel even when the tank is low.

Onward!