Modern Vespa

I wonder if the float in the carb bowl is adjusted correctly. The float level is adjusted by bending the little metal tab that closes/opens the needle to keep the fuel level in the bowl from getting too high/low. If the fuel level in the bowl is too high you can end up with a rich condition.

When the scooter is idling, are you seeing black smoke from the exhaust? Does the exhaust smell rich?

Good thought, but he replaced the carb already.

Good thought. Float is good in both carbs.

I know you replaced the fuel tap. But check to make sure that the new fuel tap is not leaking fuel into the vaccum hose. I've seen this happen once in the past. The unregulated fuel makes it down the vaccum tube and ends up getting dumped into the intake (bypassing the carburetor).

You've addressed all the most common problems. This one would be really rare, but it's easy enough to check.

Yeah...
I just re-read page one and there is no indication that you checked the vacuum line to the fuel tap.
There should be no gas coming out of the fuel line when the engine's off.
I know you got fuel out of the float bowl, but if the fuel tap is defective, you're pumping gas after the carb.
Just pop the vacuum line off from the intake....
Pop the fuel line off the carb....
If gas comes out without stopping, the tap is defective.
If no fuel comes out, apply suction to the vacuum line....that opens the tap and gas will flow.
If gas comes out of the vacuum line, the tap is defective.

I've confirmed the fuel tap is working properly but I have not confirmed there isn't fuel in the vacuum line. I'm moderately confident there isn't as I've had it unplugged enough times that I would have noticed gas by now but I'll take another look.

I may have missed it, but did you ever remove rhe rubber intake manifold to check for cracks?

It can happen - saw a Youtube video of a bike actually running off the vacuum line (hoses were accidentally reversed and petcock faulty - this was an ET2 though). Weird, I know!

I did. I sprayed the crap out of it with no effect and then removed both sides last weekend to inspect.

The plug suggests running rich. Pic is above if you want to see.

that is so odd, it almost sounds like you've got a vacuum leak in there somewhere. did you check the evap lines-- esp. the ones to the carbon canister?

that plug is dark dark. it's almost like you've got a stuck ring or something and it's sucking oil.

Hi,
(You WILL figure this out...)
How did you confirm that the fuel tap is working?

Your problem is pointing at an over rich situation, barring something exotic, like an ignition issue....
That could be one of several things....but you have to start out with a compression check and eliminate silly fuel/air issues.
(I know it doesn't seem silly right now...)

All the more reason to get a compression test

No gas in the vacuum line from the fuel tap.

Compression test around 130 psi.

is that with or without the decompression lever present, with or without the throttle open?

Throttle closed, no decompression lever that I'm aware of

decompression lever is a counterweight designed into the camshaft/cam drive assembly. don't worry about it for now.

make sure the battery and starter are tip top, throttle open, crank until the gauge stops fluttering. anything under 9 bar and you've got a problem.

Open throttle compression is just under 11 bar

well. you've got compression.

here's a shot in the dark- try running it without the gas cap. that's a total shot in the dark.

anyway, there's a red LED in the dash that blinks. when you turn the key to the on position what is the blink sequence?

2ndary questions:
-have you tested the stator?
-what coil did you replace with?
-did you check your plug cap, or have you replaced that as well?
-what type of plug (brand, part number, screw on or molded nipple)
-on the air filter, when you replaced the old one what was its condition like?

is there any service history of the bike? like, has somebody been in the motor or anything like that?

I'll try the gas cap trick. I think I tried it early in the process but I'll give it another go.

Led - single flash while it's off, turn on the key, light comes on for a second and then goes out. Doesn't come back on

I bought it from an older lady that had it serviced professionally. It only has 1600 miles on it. I don't think anyone has been in it before

Haven't touched the stator. Iridium plug from scooters west. It has a screw on end Same for coil. Oem replacement I believe.

The air filter was totally broken down when I got it. It was intact so I don't think anything was ingested but a possibility.

I'd have a look and make sure that there isn't any air filter detritus in the intake port or hanging the valves. it's unlikely, but a possibility.

when you checked the valves, how did the cam look? was there any scoring or odd signs on the surface?

given the state of the plug, it's almost as if the valve seals are shot and it's sucking oil.

when it's running is there any perceptible smoke? if so what color?

have you checked the exhaust? is it obstructed in any way? like, maybe a mouse built a house in there?

it's just so interesting. you have compression, you have fuel, and you have spark. it just won't go.

i keep thinking about that air filter too. I've looked down as far as I can see w/o finding anything.

the cam looked good to me. The thing doesn't smoke a bit. It starts right up. Idles like a dream. its the craziest thing. If it ride it, it won't go over 20mph on the flat.

Exhaust has plenty of flow. I don't have a camera to run through it but its a nice solid exhaust when you hold your hand in front of it.

I'm beside myself. My wife is starting to make fun of me for not having it going yet! She's asking if I've finally met my match

Hey there - I'm having a very similar experience with an '07 LX150, although I can get a bit better than 20mph, the engine bogs down/loses power with any throttle over 3/4 open. If I hold it wide open, it bucks and surges a bit and slowly accelerates, and gradually improves as I get over 30mph and the revs increase. I've replaced just about everything except the CDI and the stator. I'm going to make a separate post about mine but I just thought I'd pop in and say you're not alone fighting a mystery

per the other thread: 2007 LX150 Aggravation - Fuel, Air, Spark... and magic?

something to have a quick look at!

good luck!

Latest update. I was indeed missing the airbox seal. It obviously helped but didn't get it to 50 or 60 mph. I'm up to about 25mph. Gas cap loosening had no noticeable impact.

I will note the air screw has very little effect from 0 turns out to 2+ turns out. A small increase at about .5 turns out and it all but quits at 2.5 to 3 turns out.

Have not removed the head to look at the intake. Trying to find a borescope to use before I go there.

man, that's confounding.

interesting on the A/F miss screw. it should die all the way seated, at .5 out it will run but be noticeably hunting on idle, right around 2~3 is where it'll pick right up and you'll notice it feels "right" beyond that it'll get all choppy and boggy.

anyway, at this point I don't have much left in the bag of tricks. I'd say if you can get your hands on CDI to test I'd go that route or splash the cash for one-- the non-immobilizer units aren't an arm and a leg and you don't have to re-key.

btw, I don't remember if asked and answered: how many miles does this thing have on it?

1650 ish miles. Next to none in my expectation/understanding.

Maybe there is something down the intake blocking the intake valve? I have no idea. I'll be glad when I find it though...

Thanks for the suggestions along the way.

yeah man, at this point I'd suggest going back over all the work and double check to see if maybe you inadvertently missed something-- which is a smart move and doesn't cost you anything but time.

then I'd try a CDI before tearing stuff apart. at least with a CDI you don't have to pull the engine! 10min and you'd have an answer!

Did some more troubleshooting. Boresxope down the intake. No obstructions but the to of the valves was either carbon or oil covered. The edges looked pretty good though. I couldn't get anything down there to get a sample.

Put it backed together. Have it a once over. Disconnected the enrichment circuit and fired it up.

Jumped to life, idle was fun. Held WOT for a couple minutes. Slow accel but it did get up in rpms. Really felt like it should go higher. Happened to look down at the exhaust. Cherry red.

Send like it's running lean. I'm going to try some bigger mains and see what happens. This is with the new factory carb.

You saw the other post in the last two weeks in which a missing air box cover gasket was the source of the problem?

I didn't see it. Thank you. I had the same problem but didn't solve my problem

what carb and main jet?

I have the walbro and a new Keihin, stocker. I think its an #82

so it's a keihin with an 82 on there? what's the idle?

I don't know the idle. It's whatever comes in it with aoem replacement.

The carb is a brand new OEM replacement from scooterwest.

confirm the carb type and jets, otherwise everything else is just a guess.

Original carb is not currently installed. Walbro WVP or F (can't tell). Main jet is an 80. Can't tell the idle jet. eyes too old, stamp too worn. Camera won't focus well enough on it to zoom and see. Assuming its the original size.

Currently installed Carb - New Keihin CVK with a main jet of 82. Again, can't tell the idle jet but pretty confident the standard OEM size in it as I bought it new from scooterparts as a factory replacement.

I'm happy to use either carb as they both run about the same. The Walbro may be a touch better on response but the Keihin has better top end currently.

The problem is neither one revs up right. The Keihin will get to about 25mph. The Walbro about 20. Even on the stand, it revs up and will gradually climb in RPM to the peak but it should be much faster than it currently goes. The glowing red exhaust makes me think its really lean.

I'm wondering if my air filter is more breathable than the original letting more air through. I'd like to go up in jet size to see if it helps.

Thoughts?

what air filter are you currently running?

https://www.amazon.com/dp/B07HXLST5Q?psc=1&ref=ppx_yo2ov_dt_b_product_details
Polini Air Filter (Blue); Vespa 150, ET4, Leader Engine

I know we've been focusing on fuel delivery but another shot in the dark:

You earlier mentioned changing the rollers—what did you change them to? If the rollers were already a bad size and you swapped in the same ones, the problem would persist.

Asking because there was a thread on here recently where a roller change drastically reduced the top end.

I'm not a fan of that air filter, I'd install a stock one and see if the problem persists.

you say that the exhaust got red hot just idling? how long was it idling?