vespa gs 150 vs5
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Member
Joined: 18 Apr 2020
Posts: 18

Tue Apr 06, 2021 9:28 am quote
i apologize if i dont get the terms right, i cant get it to idle without applying the throttle. ive been slowly trying to fix all the little issues.

i think the problem is the idle/air adjustment screw. the slide? in the carburetor is completely closed with the screw turned all the way in. the screw doesnt want to seat all the way in. ive only tried to hand tighten it, im afraid its stripped.

it unscrewed easily and i did see a little bit of metal on the tapered end of the screw. i pulled the carburetor off and cleaned and while doing so i found a tiny bit of metal shaving that came from somewhere.

i bought a new idle/air screw needle and it threaded in easily but stopped in the same place.

so my questions are,

how far should the screw be seated in, can i just buy an idle screw with a longer needle?
is it possible to tap the last few remaining threads, what would the size and thread pitch be on that?

carb.jpg

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'64 Motovespa 150S (177) , '66 Allstate SF, '65 VBB, '66 180SS
Joined: 27 Mar 2013
Posts: 848
Location: S.Salem, NY
Tue Apr 06, 2021 11:49 am quote
I am guessing that the upper screw is the idle mixture screw and the lower one is the slide stop or idle speed screw. Are you saying that the speed screw does not stop the slide form closing and you can't set the speed ? Have you pulled the slide out to see if the screw is interfering with something or not contacting the slide for some reason ?
Member
Joined: 18 Apr 2020
Posts: 18

Tue Apr 06, 2021 2:04 pm quote
sorry i should have just taken it off and looked at it, the threads are messed up a bit.

would you know a fix for this? or what size the idle screw is? it looks like m5 or around that size.

the carburetor is a dellorto ub23s3

carb.jpg

Member
Joined: 18 Apr 2020
Posts: 18

Tue Apr 13, 2021 5:20 pm quote
the thread pitch seemed to be .75, m5 tap was too big, m4 too small
i got 4.8mm on the idle screw. instead of trying to order a m4.5 x .75 tap i just made a tap out of a spare idle screw i bought, it worked.
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2:6
Joined: 11 Jan 2007
Posts: 7703
Location: San Francisco
Tue Apr 13, 2021 6:06 pm quote
FYI that is a rare and expensive carb, not one to make bodge modifications to.
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