Modern Vespa

Hi Everybody. Been a lurker for some time and decided to join to ask y'all to ponder the most unusual mechanical issue I have come across in some time. I purchased an ET4 that's been idle for a few years from a friend. I put in a new battery, cleaned the carb using the generally accepted MV process, and replaced the gas. It fired up and ran like a champ - without the air filter on. I completely disassembled the entire air intake system cleaned it inside and out and re-installed it without the element. The scoot would start and idle fine, but not accelerate over 20mph. Clearly there is an air restriction. So I took the entire air intake system off again and replaced it one piece at a time, taking time to inspect the airway for any occlusion. First the pipe from carb to air filter box.... ran great. Then the box itself without the air filter or access cover for the filter element. No problem. Here's where it gets interesting: I replaced the access cover for the filter element on the air filter box WITHOUT putting the element in and could not get over 20mph... sputtering, gutless performance.

I'm stumped. How could this be?

PS- EVAP was removed by previous owner.

The symptoms (disregarding the experimentation with the airbox) are identical to what happened to me when my main jet was clogged.

Perhaps your main jet is clogged and the freer airflow sans cover is leaning up the mix allowing it to overcome the partial clog?

You know this now, but it's not a good idea to run it without an air filter--you never know what can get sucked in there.

Some folk have had to clean the carb up to three times to get it completely clean - as muckite can still be getting though the fuel line from fuel filter etc.

Ok, I can see that. Gremlins can be hiding in there. I'll tear it down again and overdue the soak time and air pressure. Perhaps I should change out the fuel line also in case it's deteriorating from the inside out. I was surprised that the inside of the carb was varnish free and quite clean despite having sat idle for a good 3-4 years prior to this resurrection.

Thanks for the replies!

Have you misplaced the spring that returns the slide?

Good question. The needle float assembly with spring came out and went back in without issue. Question for you: If that spring was missing would I observe the problem I am having? I would think there would be a stream of gas dumping out of the carb or down the intake manifold if that spring was missing. Thanks!

People who have tried to remove or modify the air box for performance gains have failed as the diaphraghm that lifts the slide relies on the negative pressure created by the airbox. The fact that yours runs without the airbox intact makes me wonder if a missing or broken return spring allows the slide to open with out the restriction of the airbox.
Unless the floatbowl needle is not working you wont get flooding

Ah, I misunderstood your original reply. I freshened the bottom part of the carb where the jets and needles are. Didn't mess with the diaphragm on top. That's next, I guess. Thanks - good suggestion.

Here's what I have done to get this ET4 running right:
-New battery
-Rebuilt the carb 5-6 times. Used carb cleaner and 100lbs of air pressure.
-Cleaned air filter
-New NGK plug
-Mobile 1 oil change up to the top hash mark on the dipstick
-New drive belt and variator weights.
-Inspected the intake manifold for cracks
-Adjusted the mixture screw any number of times
-Fresh gas
-Verified the choke plunger is mobile, but did not measure movement while applying 12v over time. Note that scoot runs poorly weather cold or warm.

I'm completely stumped and have spent far too much time on this. Again, this thing runs great without an air cleaner, but cant get over 20mph with the air cleaner in place. Symptoms include sputtering and unresponsive throttle. It starts and idles fine no matter what. Any additional thought would be most appreciated.

Hi Stick,
I'm pretty new around here and but I'm reading over this and it reminded me of something that clicked in my head when looking at the Technical Manual about the immobilizer. I don't know if that's the same with you, I mean - can you kick out the revs now? I know the chance is slim but you may have done something to the immobilizer when tinkering. I thought this might have been an issue with mine before I rode it. My lock looks like it's been replaced and I think they may have by-passed the immobilzer somehow.

Luckily this wasnt an issue for me and I can get about 60MPH when I fully wind it on! (I'm currently looking at dropping the oil, replacing the filter and cleaning the carb this weekend to see if I can get a bit more!)

Hope this helps! I hate thinking about Sick ET4s

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Thanks for the reply. I really appreciate it. I'm confident the CDI cant be the issue because without the air cleaner assembly installed I can hit 55mph. As soon as I put the entire air filter assembly back on (even without the foam element) I cant get out of my own way. This is very odd.

And yes, I have the brown and blue keys.

I think I'll just sell the damn thing. I've wasted a lot of time trying to get it running to spec. Man, illogical mystery problems are infuriating.