Modern Vespa

Hey all - I joined this forum a couple years ago but never posted. We had a 2006 LX150 that was a joy to ride, sold it to buy a Moto Guzzi V7, but now we're back on a Vespa, this time a 2007 LX150. Our last one was a strong little runner - so I know what they should feel like. This new scoot is having a problem that is leaving me quite perplexed...

This LX150 has 2,200 miles on it, and was last registered 5 years ago, so it's been sitting a while. When we checked it out it cranked right up and idled perfect. When I test rode it I noticed it would bog when wide open. The seller was selling for a friend, who he said mainly used this bike for bicycling events as a pace vehicle. He thought it was just old gas - and I suspected a simple carb clean would get it back up to snuff, boy was I wrong! Anyway, I haggled on the price a bit and rode it home - doing about 40-45mph indicated as long as I stayed under about 3/4 throttle it was running ok.

Ok, so I get home and I start to tackle the problem. I'm just going to list off what I've done so far over the past month or so:

1. Cleaned carburetor - as suspected the main jet and emulsion tube were partially obstructed. Tested the auto-choke. Ran slightly better but the problem persisted...
2. Replaced fuel tap/petcock
3. Replaced fuel line
4. Replaced vacuum line to petcock and SAS valve
5. Capped off EVAP vacuum line
6. Replaced Intake boot gasket
6.1 GAVE UP AND BOUGHT BRAND NEW CARB
7. Cleaned and inspected airbox - old filter was clean - replaced anyway
8. Checked intake tube from airbox to carb for leaks
9. Sprayed carb clean and map gas all around to try to find a leak - no change in rpm
10. Replaced Spark Plug (NGK CR8EKB - gapped to 0.8mm)
11. Replaced intake boot anyway (thought maybe it had a hairline crack that would only draw air at full throttle)
12. Checked compression (~160psi w/ throttle closed)
13. Checked valve clearances - all good
14. Replaced coil w/ Racing Coil from Scooter Parts Co (829671R)
15. Replaced spark plug AGAIN - Denso U24ESRN this time
16. Ran without Exhaust (wow that's LOUD )
17. Ran without Fuel Cap

Still doing the same thing...

I'll try my best to describe exactly the symtoms, as I know it's really hard to diagnose something like this over the internet. The bike starts up and idles perfect. At partial throttle openings, it revs up and acts like a perfectly healthy engine. At between 1/2 to 3/4 throttle you can get up to 40-45ish mph and everything seems fine.

At greater than 3/4 throttle, the engine bogs and loses power, surges a bit, and continues to accelerate but slowly - getting little spurts of power. Once you get up over about 30mph things start to smooth out and it starts running better. It will eventually get up to about 50mph and, while still surging a bit - is running quite a bit better. One other note - when you wack the throttle open you get a little spurt of what feels like nearly full power.

This behavior REALLY makes me feel like it's running lean or starving for fuel - either the carb isn't getting the fuel it needs, the needle isn't raising up like it should, or I have an air leak. However, the carb is BRAND NEW - the fuel tap and clear fuel line are band new, the vacuum lines are brand new, etc etc (see list above).

About the only thing left is the CDI. Unforutnately there's no way to test that and it's expensive. There's also a pickup at the stator but I would imagine that would be more of an all or nothing spark situation.

Mainly posting to vent about the situation but I'm all ears if someone has any other suggestions. I have not checked the CVT yet but judging by the engine sound an how it's directly related to the thottle opening, I'm hesitant to think I have a problem there...

It's a good looking little scoot - help me get it running like it should be!

I would suggest biting the bullet, opening the CVT, checking the rollers/clutch, and replacing the belt.

What you're experiencing COULD be explained by a slipping belt or glazed clutch - particularly the belt, which cannot possibly be operating at original spec after 15 years.

You mentioned that it bogs while you're on the road but you never mentioned how it revs out on the center stand. If it is bogging from revving out on the center stand, definitely look towards the CDI as you mentioned. If it revs out nicely on the center stand then I would definitely looking in to the transmission.

It definitely revs out better on the center stand vs. with a load. I think I just had a revelation... It's always felt like it's lean or starving for fuel when wide open. I have new vacuum lines, a new intake boot, have verified no leaks in the intake tube going to the airbox... however.. I just threw some tape over the intake in the airbox, and I now have full power wide open....

Either my airbox is leaking somewhere that I can't see or.... what?

Brand new stock carb... but would adding this tape increase vacuum and make the slide operate better?

Your problem sounds indeed as starvation, lack of fuel, when you fully open the throttle.

Looking at it from the cylinder onwards then everything behind your carburettor should not influence your engines behavior. It should run perfectly without airbox, although not advised in order to keep dust and so on out of your engine. So forget about air leaks in the airbox.

Unless the air filter is heavily clogged. But you experience better behavior if you block part of the incoming air, which again points to lack of fuel. By blocking part of the incoming air there is a bit underpressure in the carburettor and then the airstream sucks a bit more fuel.

The root cause must be somewhere in the fuel getting into the airstream in the carburettor, but you have already replaced most if not all of the parts involved.

Carburettors is a long way back, but back then all carburettors had a system to temporarily enrich the fuel-air mixture when you suddenly go full throttle. I know it as an acceleration pump, but I do not know if that is the correct english term. If that acceleration pump failed then happened exactly what you describe.

Problem SOLVED! Turns out my airbox was missing the small gasket that is supposed to go in between the two halves. I bench tested the airbox by taping up the inlet and outlet, and adding compressed air to the crankcase breather hole. Felt a bit of air coming out between the two halves. Added some butyl rubber tape to seal it up good and now it runs perfect! Item number 3 on the diagram.

nice! it's always fulfilling when you finally get it sorted out, even if it was something simple all along!

great job!

Thanks Greasy! I finally posted today in frustration and fear that I was going to have to spend even more money on a CDI that may or may not solve the problem. Glad I experimented with taping up the airbox today! Amazing how something so small can make such a big difference.

Thanks for the report, as that's a new one for me!

New to me too. Any idea how this missing gasket could have led to the phenomenon NitroToo experienced?

I'm also curious about this as I ran a 150 without half the airbox attached and it ran fine.
Is there anything else that could have gotten nudged/tweaked...resulting in success?
I'm also very happy that you got things working.

Yes. In my experience the engine should be running perfectly without the airbox.

The carb is designed to accommodate a certain amount of resistance associated with bringing air through the air intake, airbox, air filter, and venturi.

If the air intake is increased in size due to a lack of a big gasket seal, there's much less resistance to the engine vacuum and more air comes in. The engine runs lean, and quite possibly too hot, because the proper ratio of incoming vaporized fuel also cools the cylinder. (Running the engine with the cover off the airbox definitely creates a very lean mixture that can seize the engine.)

A very basic 2-stroke mod is to drill holes in the air intake and increase the main jet size inside the carb. More air + more fuel = more power out, if you get the ratio right and aren't at the point of saturating the exhaust output.