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I got an S150 that had been sitting in someones garage last year. It had been sitting with gas in the tank and when turned over it would idle rough and just choke out whenever you gave it throttle

I cleaned the Carb really well, checked the air filter, and cleaned out the fuel tank. The bike turns over fine now, but it has no power at the top end. I can get up to 20mph before it caps out.

I checked the slide in the to of the carb and it had some resistance to it which i filed down. Now it slides in and out smoothly, but still no luck

Any ideas
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Still sounds like a carb problem. I have cleaned carbs several times in the past before I actually got the job done. Also, jets are cheap. Why not try changing them out.
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Check the carb diaphragm
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Tierney wrote:
Check the carb diaphragm
this.

the carb wasn't assembled or cleaned correctly.

if it's been sitting, checking the fuel tap, fuel and vacuum lines is also important.

however, given that it runs, I'd guess it's a carb problem.
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Molto Verboso
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Another vote for the diaphragm, which is on the "other" side of the carb opposite the jets, and is easy to get pinched on reassembly so air leaks in.

In my case mere disassembly and careful reassembly were enough to remedy the problem. I never saw what the actual issue was.
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Good luck and welcome to the forums from a fellow New Yorker!
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yes the diaphragm is what i was filing down for smoother operation

I've cleaned this carb intensely about 3 times now

any ideas what else it could be?
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This bike has 500 miles and i cant get over 30mph

Could it be a gunky fuel pump? Air filter is clean of debris.

I openned up and cleaned the transmission. Does this Variator look natural scored to y'all? with such low miles im suprised to see this kind of wear.

What about the EVAP system? i know a lot of people delete it. I tried capping it off at the intake manifold, but the bike wouldnt even run, it would just bog and choke out.
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Molto Verboso
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How old is the belt? My 09 S150 had trouble maintaining speed with the original belt when I bought it in 18. The belts get brittle and lose their ability to stick where held by the variator.
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Crim wrote:
yes the diaphragm is what i was filing down for smoother operation

I've cleaned this carb intensely about 3 times now

any ideas what else it could be?
Make sure the black rubber diaphragm is not torn or holed in some way.
You "filed" the diaphragm (actually, the slide) to make it move smoothly. If the very thin rubber diaphragm on top of the slide gets damaged, the bike won't have any top end.

Look at the diaphragm with a bright light behind it to check for tears or pinholes.
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^^^ This. Just take the top of the carb off as you don't need to remove the carb from the manifold/air filter.
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vario looks fine. if the belt was bunched up, maybe? but doubtful.

again, look to carb. I'll bet good money on the diaphragm not being sealed properly. second roll is fuel delivery/vacuum or fuel lines.
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Here she is yall. I'd hate to shell out the $100 for a new one, but she looks pretty good no? No pinholes with a flashlight behind it.
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Crim wrote:
Here she is yall. I'd hate to shell out the $100 for a new one, but she looks pretty good no? No pinholes with a flashlight behind it.
Seems okay.

Rubber soft and pliable? Alcohol fuels sometimes stiffen up the rubber.

Make sure the lip seats correctly in the groove around the body by gently using your finger to press it into place, then make sure the groove in the lid does the same. Check the lid for warpage with a straight edge before you put it back on, and tighten the screws a little at a time in a cross pattern.

Good luck.
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Ok, one more thought. I bought a GT200 about a year ago from a guy claiming the same thing - no top end. He was not interested in taking it to a shop and the nearest Vespa dealer was 3 hours away. I figured it would not take much to fix it and he wanted it gone right now, so I bought it and trailered it home. I checked the valves, spark plug, air filter, etc.. all in spec. I then turned my attention to the carb, took it apart for a quick clean up. No red flags, manifold was good, no air leaks - but still no top end. I then checked the vent on the side of the carb. Usually this is just left open with a short tube installed, sometime it has a small filter attached, or has a line that runs back to the airbox. In this case there was a small filter or an attempt t at a filter that was letting nothing thru. I disconnected it and the scoot came to life. I felt a little bad about how cheap I got it and offered to sell it back to the guy, but he refused as he had already picked up a GTS.
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And did you check the fuel and vacuum lines as Greasy suggested? If they look good, you may also want to put vacuum on the vacuum line to check for fuel delivery/petcock check.
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this is solid advice.

but if you've messed with the diaphragm and it's not sealed or seated properly, or the spring is binding or you didn't get the needle set back in? this is EXACTLY what happens.
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The vent I mentioned is for venting the air as the float bowl fills. No vent, no fill. You should also check the vacuum passage the runs from the top of the carb venturi that is normally closed of by the butterfly at idle. You can usually clean this from the throttle diaphragm vacuum chamber. If this is clogged, the slide won't rise.
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25BIKEZ wrote:
Make sure the lip seats correctly in the groove around the body by gently using your finger to press it into place, then make sure the groove in the lid does the same. Check the lid for warpage with a straight edge before you put it back on, and tighten the screws a little at a time in a cross pattern.

Good luck.
This may be something, I can't get the rubber to stay seated within that groove - maybe its been exposed to some solvent thats shrunk it? any trick to getting it to stay seated?
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They are never too easy to seat, a little dab of grease or sealant will help keep it in place. It's just as the rubber ages it is not as pliable. But make sure to check the vacuum passage and venting as well while it it apart.
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Mad lads, we've done it!

as per TIERNEY, a little sealant helped her seat proper and away we go

Thanks gents!
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Good man.
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