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@matt58 avatar
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VB1
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Location: Tacoma, WA
 
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@matt58 avatar
VB1
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Location: Tacoma, WA
UTC quote
Hi, I have a 1958 VB1 wide frame, which used to belong to Scooter Daddy for those knew him. My problem is that the engine is racing when I sit on the bike. If I get up, it slows down, and if I sit down it races again.

When I first noticed that it was idling too hot after warming up, I thought it must be sucking air. A Vespa mechanic disagreed and noticed that it slows down when I take my weight off of it, so he inferred that it must be cable tension. Yet he was unable to fix it. Investigating on my own at home, it seems to me that neither the throttle nor the choke is budging when I sit on it or get up. This stands to reason, since on this model, the carburetor is attached directly to the frame, not the engine, so nothing changes in the route of the cable when you put weight on it.

The bike sat unused for about six years before this issue arose, so I would expect that some seals have dried up or something.

Any opinions? Is it likely that it's sucking air when I put weight on it but not otherwise? Does that give any indication of where the leak might be? Could it be anything else?

And can you recommend anyone in the Pacific Northwest to sort it out? I have already tried a well known mechanic in Tacoma, and that didn't get me very far.
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Ossessionato
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Location: Racing Capital of the World
 
Ossessionato
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Matt,

did your mechanic get the bike warmed up and then remove the throttle cable from the carb? that is exactly how i would test it. . maybe by sitting on it it's pulling on the carb inner in some way.

Here i am a little confused. In no way is the carb attached to the frame. It is attached to a steel manifold that is attached to the engine; hence, the carb is attached indirectly to the engine.

if not a throttle cable issue, look at fuel line-is it getting pinched when you sit on it?
@daleb avatar
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Hooked
'55 VL1
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Location: Omaha, NE
 
Hooked
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Location: Omaha, NE
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Has that got the choke lever under the seat?
OP
@matt58 avatar
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VB1
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Location: Tacoma, WA
 
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VB1
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Yes
@daleb avatar
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Hooked
'55 VL1
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Location: Omaha, NE
 
Hooked
@daleb avatar
'55 VL1
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Location: Omaha, NE
UTC quote
Gotta wonder if that's not involved. Maybe it's partially choked until you sit on it, which opens the choke. It's worth a look anyway.
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@matt58 avatar
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VB1
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@matt58 avatar
VB1
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UTC quote
Fixed it! The end of the throttle cable was bumping against the top of the carb compartment. 🤦

As one member pointed out, I was wrong to say that the carb is attached directly to the frame. It is attached to a long metal intake manifold that is bolted to the engine. This goes through a big bushing into the carburetor compartment of the frame, which opens in the center of the step-through area. The throttle cable comes through the frame and curves up before it attaches to the throttle lever on the carb. My cable extended about an inch beyond the fastener. When the bike is sat on, the whole carb assembly tilts upward relative to the frame. The excess cable past the fastener was bumping against the top of the carb compartment, depressing the throttle lever. All I had to do was bend the excess bit of cable so that it doesn't bump anymore.

Something to watch out for if you've got an old wide-frame that's not a GS!
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