OP
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Member
1985 PK50S
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Posts: 23
Location: USA / Germany
 
Member
1985 PK50S
Joined: UTC
Posts: 23
Location: USA / Germany
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Hi Everyone - I am enjoying getting to know my recently purchased 1985 PK 50 S and learning how to start fixing issues with it.

I discovered a new one today that I'm hoping you might be able to give me some ideas for where to start looking.

The engine generally runs OK when I'm riding it around. Today, however, I tried to take a passenger with me for a ride around the block. When the extra weight compressed the suspension, the engine choked out and shut off. I couldn't save it by trying to start driving either. I then tried sitting myself further back on the seat and noticed the engine would start to struggle.

It seems that fuel to the engine is getting cutoff when the suspension compresses. I checked the fuel line from the tank to the carb and it is not pinched at all. Perhaps something is happening between the carb and the cylinder?

I suspect the carb needs a teardown and tuning though as well. When in neutral it sometimes starts with a "runaway throttle" where the engine keeps increasing in speed until I put it in to gear and start driving again.

Scratching my head again with this bike
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Ossessionato
Vespas 1964 GS160, 1965 SS180, 1977 V9A1T, 1983 PX150E
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Posts: 2537
Location: Siam
 
Ossessionato
Vespas 1964 GS160, 1965 SS180, 1977 V9A1T, 1983 PX150E
Joined: UTC
Posts: 2537
Location: Siam
UTC quote
Most likely the fuel line inside frame is routed in such a way (incorrectly) that when the suspension compresses, the fuel line pulls on the choke cable which then floods the engine. You just need to route fuel line so that it doesn't interfere with the choke cable.
OP
UTC

Member
1985 PK50S
Joined: UTC
Posts: 23
Location: USA / Germany
 
Member
1985 PK50S
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Posts: 23
Location: USA / Germany
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Thanks for the idea!

I pulled the tool box out and tried flexing the rear suspension while looking in there. Didn't see anything moving around, but the fuel line and choke cable are right next to each other. Do I need to shorten this fuel line a bit to remove some excess?
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OP
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Member
1985 PK50S
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Posts: 23
Location: USA / Germany
 
Member
1985 PK50S
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Location: USA / Germany
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Another pic
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parallelogramerist
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parallelogramerist
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Maybe also check that your spark plug cap isn't being smooshed against the frame.
OP
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Member
1985 PK50S
Joined: UTC
Posts: 23
Location: USA / Germany
 
Member
1985 PK50S
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Posts: 23
Location: USA / Germany
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Ever since it cut out on me yesterday, I can't get the dang thing to start again. I have probably kicked the starter 100x at least and it used to always start on the first kick.

Checked ignition on, fuel valve open, no kinks in fuel line, choke on, reseated the spark plug connection. What the heck is going on - any advice on where to check next?

It doesn't even come close to starting or start/sputter, so something essential is not working.
@moto64 avatar
UTC

Molto Verboso
'64 Motovespa 150S (177) , '65 VBB, '66 Allstate SF, '66 180SS
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Location: S.Salem, NY
 
Molto Verboso
@moto64 avatar
'64 Motovespa 150S (177) , '65 VBB, '66 Allstate SF, '66 180SS
Joined: UTC
Posts: 1782
Location: S.Salem, NY
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You might want to check the operation of that 'choke' cable. What it actually does is pull a plunger up in the carb and allow more gas to enter the idle circuit. The outer sheath is not anchored at the lever and maybe it has pulled out of the fitting on the carb which can put a strain on the inner wire and delivering too much gas to the mix. A proper cable is cheap.
That said, all of this is inside the frame and should not be affected by the up and down movement of the rear end.
OP
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Member
1985 PK50S
Joined: UTC
Posts: 23
Location: USA / Germany
 
Member
1985 PK50S
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Posts: 23
Location: USA / Germany
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I went out investigating a bit more tonight. Checked that the spark plug is getting a spark - no problem there.

When I looked inside at the manifold, it appears to be "wet". Would it be possible that the connection between the carb and manifold is not sealed?

To check this, would I remove the fuel tank and then the carb?
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@mjrally avatar
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Veni, Vidi, Posti
73 & 74 Rally, 76 ET3, 80 P200, 06 PX150, 59 Ser 2, 65 Silver Special, 90 V5N 50, 2015 HD Road Glide Special
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Veni, Vidi, Posti
@mjrally avatar
73 & 74 Rally, 76 ET3, 80 P200, 06 PX150, 59 Ser 2, 65 Silver Special, 90 V5N 50, 2015 HD Road Glide Special
Joined: UTC
Posts: 5078
Location: Oceanside, CA
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Since you've been kicking forever, pull the spark plug out of the cylinder and let it air out for a while. Might have flooded the cylinder. Doesn't hurt if thats not the case.

As far as the wet area where the manifold bolts to the cases, its a common dirty spot on small frames. It's the lowest part of the cases so gravity brings oils/ gas down to that area. Wipe it down and check on it in a few days. See if its leaking, or if something is dribbling down to that area. There's 3 nuts that hold that onto the case. 2 are easy to get to, 1's a pain. You can check that those are all tight while youre down there.

If you havent already, go ahead and pull the fuel tank out. Youre not going to hurt anything doing that. A little tip, if you have a wooden bench or rail, insert a screw into it vertically. You can hang your tank via the bolt holes onto that screw.

While the tanks out, check that that carb is secure in its collar and if youre up for it, pull the carb out and remove the banjo bolt on the right side of the carb and check the fuel filter for grit. If there's plenty, consider a carb clean.
OP
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Member
1985 PK50S
Joined: UTC
Posts: 23
Location: USA / Germany
 
Member
1985 PK50S
Joined: UTC
Posts: 23
Location: USA / Germany
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Good news - I got the scooter running again!

Even though the spark plug did have a spark, it must have been weak. After I convinced myself it had a spark initially, I pulled the carb and installed the fabric washer, etc. Still didn't start.

So today I just put in a new spark plug to try something new. And it fired up!

Now....to my next question (never ends with this bike haha).

During idle the RPMs kept running away like it was racing in the Indy 500 (without me touching the throttle). It would calm down again if I applied the choke or turned the idle screw. But it would only settle for a minute or so before taking off again. I eventually just killed the ignition and went searching for ideas.

Is this likely something in the carb that needs to be cleaned out or replaced?
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