Modern Vespa

Sorry in advance for the long post...

I'm a long time lurker here and I've had my 2007 Piaggio BV 250 (with a carb) for about 5 years and it always ran very well. In about 2019, I removed the under seat evap system.

Suddenly a few months ago, it started having issues idling. At the time - I thought it may have been related to the low temperatures on that ride (even though I've used it at lower temps in the past without any issues). I could keep the scooter running - by revving it a bit at stops and then restarting quickly, if it stalled (often).

The issue persisted, I thought it may have been some old fuel - it hadn't been used a lot for several months over the winter. I ran a fairly strong mixture of Seafoam and let it sit for a while. I couldn't get it running after that - so pulled the air box and used some carb cleaner as starting fluid. I could get it to run, but VERY poorly. I tried to flush all the Seafoam gas out and put some fresh fuel in and it still needed the starting fluid to get it to run.

I eventually remembered that with a carb - the air box missing would really change the air/fuel ratio, so I re-installed the air box and was able to get it running. At idle - it was running very poorly (missing and popping, wanting to stall), but once on the throttle it actually pulls pretty well and is quite smooth. At lower throttle positions, it is not smooth at all. On the road - it's hard to know there's an issue, until I slow down or stop.

With the air box back on and when cold, I can only get it started by pushing down on the accelerator pump on the side of the carb a few times and then it will reluctantly start after a few attempts (pushing down on the accel pump between tries). Then I have to fight with it to keep it running - lots of throttle, etc…

I pulled the air box again and can't recall the last time I oiled the foam air filter, it's VERY dry. I can see this allowing more air than it would have had when it was oily - but I only have K&N filter oil right now. It seems unlikely that this would change things so dramatically - but I'll get some proper foam filter oil and see if it helps.

I've been kind of just shooting in the dark looking for a solution, so I've also:

- changed spark plug with the same plug (NGK CR8EB/7784) - mostly just thinking it's been very long since it was replaced.
- thinking that it seems like it's lean - I looked around for a vacuum leak, but found nothing.
- pulled the hose from the evap port on the carb and left it open, but didn't see any fuel present.

It seems related mostly to the idle side of the carb - but I've not pulled the carb to do anything more involved. I'm supposed to take this scooter on a trip in about two weeks and hesitant to start tearing it apart. Maybe I'm overestimating how much work it would be to pull it. I can ride it right now - but it's a pain getting it started and trying to keep it running near idle.

Is there much I can do blow out the carb without removing it?

Any other suggestions?

Thanks for any advice.

You have all the symptoms of a clogged low-speed jet. Bite the bullet and take the carb apart for a look - it won't bite back.

Thank you very much for the motivation - I removed the carb, gave it a good cleaning and it now runs better than it ever has!

The boot on the accelerator pump was "mush" - it had left a black skid mark inside the carb and once cleaned up, I left the boot complete off. I don't know if I'll do anything about the missing boot in the future - but for upcoming trip, it seems to be running very well!

Thanks again.

Great! You did the troubleshooting work and just needed a nudge. The carb is not so scary now, is it? After a while you'll think of it as an old friend.

Those "carburetor accelerator pump boots" are readily and inexpensively replaced. I don't think they're Vespa-specific - somehow I found one on eBay that was labeled for a Harley and it fit perfectly. It was several years ago though and I don't remember how I cross-referenced it. They may just be generic.

I bet this one would work:
https://www.ebay.com/itm/284594909707

It wasn't great fun getting it out and back in - but I'm happy it's done. It did "bite" me a bit - the little wire hose clamps on the coolant lines are sharp!

I'll keep an eye open for a boot - but given the tight clearances, I'm not sure I could install the boot without pulling the carb again!

You should clean and re-oil the air filter according to the maintenance schedule. The stickiness of the oil helps keep crap from passing into your carburetor.

Thanks - yes, it's on my immediate "to do" list!

All I had on hand was K&N cotton filter oil, so I picked up some foam filter oil from a friend yesterday.