Modern Vespa

I am rescuing this bike from the grave yard and have totally rebuilt the motor. It starts and runs but is spurting fuel out the carb air intake so much so that it clogs the air filter. The crankcase is building pressure forcing fuel up and out of the carb. It has good compression, the piston arrow is pointing down to the exhaust, the exhaust is new and still spews fuel with it removed. New crank, piston and rings, jug and head. all new bearings and seals throughout, new 20/17 carb and gaskets. Could incorrect timing adjustment (22 degrees btdc) cause this issue?? Any input is appreciated---------------Bill

Moved to NSM.

Long ago I used to have a 1962 or 1963 GS160.

Running the engine of that GS without the air filter on and giving it good throttle when at idle, then indeed fuel was blown in your face.
These old 2-stroke engines were not the most energy efficient machines. But they were very simple.

From what I read everything seems OK with your engine.
And no eventual incorrect timing will have an effect. It is all in the time between opening the inlet port and opening the flushing port and that timing is completely mechanically fixed.

But you say "it clogs the air filter". What do you mean with that and what does happen then?

What kind of air filter do you have? If I remember correctly mine had some kind of foam element inside.

Thanks for your reply and info. The filter is synthetic (the filter material cannot be replaced) from a vintage vespa dealer in Georgia, "scootermercato.com. I believe because it got so saturated that it actually affected the idle. I say this because it was idling when I re-installed the filter and the revs dropped as soon as it made contact. When I lifted it the revs increased. Small variation but indisputable. I will attempt to put some time on the scooter that I have dubbed "gangrene" as the photo depicts. Then I will report back of any issues. Best regards, Bill

Ohh that's a nice scoot!

Speedy used to fire fuel back out the carb!
Those were the days when it used 10lt/100km... I just ignored it, but think it had a PHBH28 on it.

Is that a horn on the front?

If you're getting spitback out the carb, you probably have ignition timing issues.

What's your ignition timing set to right now? And are you still on points, or have you installed an electronic ignition?

Still on points and timing is 22 degrees BTDC with a new jug, head, crank and piston all standard sizes. Still has 6V stator and new coil. Motor is running well but carb spurt does not seem right to me--I enlarged the tailpipe to 3/4" thinking back-pressure issue which seemed to lessen the spurt. Bill

what's the condition of the rotary pad?

if you haven't been in there, there is a way to check by removing the air box and filling the intake passage with oil while the crank is rotated so that the intake is blocked.

Please excuse my ignorance but---What the heck is a rotary pad?? This is a piston/port motor--no reed valves.

A VBB2 should be rotary valved not piston ported…
"Should be" that is.

Yup. And the rotary pad is the area of the crankcase that that crank seals against to prevent spitback. It's not at all uncommon for it to have been damaged, especially on older bikes, by something falling into the intake.

Rotary pad is this slightly elevated area around the intake port that the crankshaft lobe seals against. If it's damaged, it will not seal properly and crankcase pressure will leak out towards the carb.

Thanks for adding the pic Finyoshi. Exactly what this needed.

Thank you for educating me on rotary valve motors. My bike a 1963 vbb2t has a vbb2m motor that is not rotary valve is this not the correct motor?? Best regards, Bill

A vbb2m would be the correct motor for a vbb. Can you share some pictures of the motor?

Everything you've shared thus far would seem to support it being a stock VBB motor.

1963 vespa 150 vbb2m motor--hope this will show what you need to see..Bill

To be honest, not really. The pad doesn't look terrible in the picture, but can't say for sure.

Looking at the overall crusty state of that motor, though, now I'm wondering if the crank isn't at least part of the problem, too. I can see it a little bit in the back of the picture, but do you have any close(r)-ups of the sealing surface on the web?

If you haven't done a leakdown test, you definitely need to do that, even though we probably already know what it's going to tell you.