Modern Vespa

HI Team-
I need your brain power and collective experience.

I've got a 2009 Vespa LX 4T wiht 12,000 km's and it idles well, starts good, but loses power at half throttle. I've read several similar posts that seem to have common themes from what i am experiencing, but the problem remains elusive.

This winter I discovered it was pissing gas from the gas line, So i replaced the gas/air lines which i gather is common. I also ended up changing the fuel pump at the bottom of the gas tank as after replacing both hoses it was leaking from the clamp. After new gas lines and fuel pump, the scooter was hard starting, especially when warm. Before touching the original leak the scooter was running perfectly. Next step- I took the the carburateur to a local shop to be ultrasonically cleaned, and replaced the jets. i also had new tires put on for the season. NOw the scooter chokes at half throttle, but idles and starts just fine. some posts mention exhaust could hinder exceleration. and the exhaust must have been removed to put on new tires.

I discovered that if i remove the air house to the filter box the scooter will have full power when accelerating. However with the airhose to the filter box in place, the scooter loses power at half throttle.. even with out a filter in the filter box?

is it possible that the hose to the filter box can interfere wiht the carb intake? IS there a trick to have the intake valve seated correctly? I've tried using a Zip tie to make the houe fit as snug as possible to the carb.

I need some super guru Vespa help, love to get the scoot running as before.

+Oil level oK
+Spark plug recently replaced, gap good, spark good
+I've even purchased a second hand carb and i have the exact acceleration loss wiht the other carb. ( some posts indicated it could be the carb diaphragm

Did you plug all the vacuum hoses back in their proper place when you replaced them?

I would check: 1) the carb diaphagm and 2) the valve clearances. You say you had it cleaned, but it almost sounds like a clogged main jet. But since it also happened with the second carb, just try what I mentioned already. And good luck.

HI MJ RALLY-

vacuum hoses are back in their proper places.. I had a doubt if the diameter of the newhoues could mess with the air and fuel delivery.. everyone and their grandma tell me no. I purchased black gas line from an automotive store and brought a sample of the old gas line to compare.

HI Tierney-

I swapped the diaphragm with the second carb, and have the same results. I have not checked valve clearance but it seems odd that the scooter has full power when air intake hose to filter box is not attached. I feel like the hose is blocking the intake to the diaphragm?

Did you verify the rubber diaphragm is seated in the top of the carb? The spring pushing on it makes it difficult sometimes. A trick is to blow into the carb with your mouth on one side and block the other side with a hand. If the rubber is seated you'll hear the slide lift and no air will leak out. It doesn't take much blowing force.

Is the snorkel coming off the filter secure on the carb? Those LX/Fly carbs absolutely need that snorkel on them to run right.

The rubber diaphragm is well seated. i tried blowing into the carb as you mention, and hand blocking the other side i couldn't hear the slide lift... but i bet i look pretty funny

The snorkel from carb to filter box is secure, i feel like theres a touch of play on the carb side, almost as if the groove/lip inside the snorkle is blocking the air intake to the diaphragm on the carb side. ?

you mention vacuum hoses in your first comment.
+one from fuel pump to intake boot,
+one from the fuel pump to carb
+carb to intake valve
+filter box to carb
anything i'm missing there? it seems pretty straight forward.

So after racking my brains, and cleaning the carb a few more times i continued to have the same results of engine bogging after more then half throttle. Here is the solution that has me back on the road riding. I've removed the stock airfilter and put an after market filter directly on the back of the carb, and the issue with of lost acceleration is a thing of the past.

Right on! Thanks for telling us what worked.

Did you ever check your valves? If your intake clearance is off then having the open filter will allow more air to flow and solve the valves being off. Make sure with the open filter you up jet your carb or you will be running to lean, overheat and seize your engine.