OP
UTC

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Vespa PX 125
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Location: London
 
Member
Vespa PX 125
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Location: London
UTC quote
I have just got my bike back from the garage. Took it for a run, it has a whole new engine re-build. I have been riding for about 1 hour now since I got it back, then all of a sudden, the engine started to rev like mad.

If I put it in to neutral I the engine acts as if the throttle is full open. As soon as I put it in 1st it moves forward but as soon as I try to get in to 2nd as I pull the clutch lever in it revs like mad, then I get it in to 2nd and 3rd then 4th.

Obviously when I get a red light I am completely stuck and have to turn the engine off and then start in 1st again. I had to do this in order to get myself home.

The only thing that is not normal is that the throttle does not feel right when I turn it it feels very loose for about half a turn then a little stiff, it moves all the way around but first half turn is loose second half turn is stiff.

The bike has just come back form the Piaggio dealer which are not that near, I am thinking of riding up there tomorrow and give them a few angry words, it will be difficult but I think I can get it up there, 9 miles away.

But does anyone know what has caused this? Is the cable stuck or could it be something else? Thanks for the help.
UTC

Hooked
Too damn many
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Location: Seattle, WA
 
Hooked
Too damn many
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Location: Seattle, WA
UTC quote
It sounds like the throttle slide (inside the carburetor) is sticking. Usually this is caused by torquing down the carb bolts too tight. You can pop off the air box cover and air filter and look down the carb throat and see the slide--it should spring back to closed on its own when you release the throttle. If it doesn't, try backing off the bolts that hold the carb a bit.

Could also be that your throttle cable housing is not seated properly where it enters the carb. Check that, too.
@monza avatar
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63 VNB
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Location: Kansas City
 
Addicted
@monza avatar
63 VNB
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Location: Kansas City
UTC quote
+1 on the sticky slide, that would explain the slack feel in the cable.

OR, its an air leak.
OP
UTC

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Vespa PX 125
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Location: London
 
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Vespa PX 125
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Location: London
UTC quote
Thanks for the replies, I was thinking that it was a cable problem but you are saying it is probably a stuck slide, I just had the bike back from the garage so like you say the bolts might be too tight.

I have the Haynes manual for the bike:

http://www.amazon.com/Vespa-PX125-Scooters-Haynes-Manuals/dp/1844253899/ref=sr_1_9/002-7390491-6921628?ie=UTF8&s=books&qid=1188310098&sr=8-9

If anyone else has this manual can you point me to the right pages in order to see the diagrams.
@christos_x avatar
UTC

Molto Verboso
No Scooter no more...
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Location: Brighton, England
 
Molto Verboso
@christos_x avatar
No Scooter no more...
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Location: Brighton, England
UTC quote
josef wrote:
Thanks for the replies, I was thinking that it was a cable problem but you are saying it is probably a stuck slide, I just had the bike back from the garage so like you say the bolts might be too tight.

I have the Haynes manual for the bike:

http://www.amazon.com/Vespa-PX125-Scooters-Haynes-Manuals/dp/1844253899/ref=sr_1_9/002-7390491-6921628?ie=UTF8&s=books&qid=1188310098&sr=8-9

If anyone else has this manual can you point me to the right pages in order to see the diagrams.
Yeah mate you want page 66 image 5.2 at the bottom. Loosen those bolts off a bit and see if that helps, you'll only need to loosen them a touch, too loose and your carb wont sit properly and you could get an air leak between the carb and the carb box.

If the bolts a loose already then this could cause you air leak and high revs, tighten em up (but not too tight)

also make sure the two air filter screws are not too tight, if they are this can cause your slide to catch and not close fully.


good luck, let us know how you get on.
⚠️ Last edited by christos x on UTC; edited 1 time
UTC

nothing at all
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nothing at all
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UTC quote
you got it back from the garage

you mean you paid someone to f--k up your carb

did i read that right? or you had it in your garage?

whatever,don't try putting in gear at high rev or your asking for bigger issues
@louispain avatar
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Cosa and P200E
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Location: Spain
 
Hooked
@louispain avatar
Cosa and P200E
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Location: Spain
UTC quote
Check the spark plug, see if it´s running lean.

Is it a Spaco carb?

I spent 2 days last week with my Cosa (p200 engine) trying to rev its heart out at . Thought the slide was sticking, air leak etc. Stripped the carb cleaned it. Still revving. Turned out to be a bit of fluff in the tank breather.

When I reset the mixture screw I set it a 1 1/2 turns out. When the scooter warmed up the revving started again. Checked the plug, much to lean. Then I learned the Spaco carb mixture screw is 3 - 3 1/2 turns out.

It could just be bad mixture adjustment.
OP
UTC

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Vespa PX 125
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Location: London
 
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Vespa PX 125
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UTC quote
The engine has just been in the garage, Piaggio dealer, it had a complete engine re-build. I rode it home from the garage fine.

Then this happened, was riding it along and the revs shot up as if the throttle was stuck wide open.

I have only had it back 3 days! I will let you know what happens. I am planning to take it straight back to the garage tomorrow morning.
@mdchanic avatar
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Hooked
GS 160 /4
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Location: Maine, USA
 
Hooked
@mdchanic avatar
GS 160 /4
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Location: Maine, USA
UTC quote
If those barstuds bollocksed it up, they've gotta put it right.

- Eric
@eldorado avatar
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1980 Vespa P200E, 1981 Serveta Jet 200, 1982 BMW R100, 2003 BMW F650GS
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Hooked
@eldorado avatar
1980 Vespa P200E, 1981 Serveta Jet 200, 1982 BMW R100, 2003 BMW F650GS
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Location: Chicago
UTC quote
Well, from what you say, it was running fine right after you got it back. So it was okay (as far as could be told) when it left the shop.

My first thought was air-leak, except that you felt like throttle had no response until halfway. So...

When you're in gear, did it pull hard like the throttle was half-open? Sometimes a hot engine with an air-leak will act like that as well. If it won't shut off unless you choke it (even with ignition off, an overlean mixture can run the engine so hot it'll ignite without the spark plug - dieseling), it's likely an air-leak.

Since you just had a rebuild, some of the bolts may have loosened themselves slightly as you've ridden. Go over every engine bolt you can find and make sure it's snug. Make sure your head is properly torqued down as well. Do it even if you don't think it's an air-leak.

With the bike stopped and engine off, does the throttle still feel half-open? The slide may be stuck, like others have mentioned. Whether it feels like that or not, open the air box and take off the air filter and see if the slide at the bottom of the venturi is most of the way closed (it should be open a bit to allow idle). Open and close the throttle and make sure the slide moves smoothly. If it doesn't, clean the carb. While you're in there, you might as well check the carb mount bolts and make sure they're torqued properly. Too tight can warp the airbox giving an air-leak, I've been told, though I've never experienced it.

Assuming it's not the carb, try starting the bike. Ride it a bit to warm it up. Don't go far from home. If it starts to rev high again, slowly pull out the choke and see if it dies back down to a rough but more normal idle speed. If it does, see how it rides in gear. If it handles pretty normally, aside from the rough idling, it might be an air-leak. you can try spraying carb cleaner at the different engine seams. If it revs high when you spray a certain spot, the carb cleaner's getting sucked into the engine and igniting, and you've found your air-leak. Also, drain a bit of your gear oil. If it smells like gas, your clutch-side seal is leaking. With the gear oil drain hole closed, open the fill hole. Drain a little gear oil so it's not at the top, then start the engine. If you feel suction when you place your finger over the gear oil fill hole, clutch side seal again. Fly side seal is harder to diagnose, but much easier to replace. Though it shouldn't be the seals if you just had a rebuild.
⬆️    About 12 years elapsed    ⬇️
@peeteboy2 avatar
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Hooked
1980 P125x 2002 Bajaj Chetak
Joined: UTC
Posts: 151
Location: Southern California, but Crete is where my heart is!
 
Hooked
@peeteboy2 avatar
1980 P125x 2002 Bajaj Chetak
Joined: UTC
Posts: 151
Location: Southern California, but Crete is where my heart is!
UTC quote
josef wrote:
The engine has just been in the garage, Piaggio dealer, it had a complete engine re-build. I rode it home from the garage fine.

Then this happened, was riding it along and the revs shot up as if the throttle was stuck wide open.

I have only had it back 3 days! I will let you know what happens. I am planning to take it straight back to the garage tomorrow morning.
Ok 12 years on and still no follow up to what happened?
So not helpful. I've got a similar problem and would have loved to hear the end of this story
@subetherbass avatar
UTC

Ossessionato
1997 Italjet Formula 125, 2 matching N.Z. '69 VBC Super, 177cc Racer, VespaCross Bodge, Puch SRA150, Piaggio Zip 100! & others
Joined: UTC
Posts: 4721
Location: Australa, Mate
 
Ossessionato
@subetherbass avatar
1997 Italjet Formula 125, 2 matching N.Z. '69 VBC Super, 177cc Racer, VespaCross Bodge, Puch SRA150, Piaggio Zip 100! & others
Joined: UTC
Posts: 4721
Location: Australa, Mate
UTC quote
When it revs up, can you TURN THE MOTOR OFF, or do you have to crash it into gear to get it to stop revving?

If the latter: likely blocked main jet.
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