'79 Vespa 100 Sport - Carb rebuild fail
Post Reply    Forum -> Not-So-Modern
Author Message
Member
'59 T/4 '79 100 Sport
Joined: 27 Mar 2021
Posts: 6
Location: Regina, SK Canada
Sat Mar 27, 2021 7:58 pm quote
Hello Friend,

(Foolishly) I left the fuel in the carb of my '79 over the winter and the Scoot wouldn't start this Spring. I decided to pull the Dell'Orto 19/19 and put it thru the Ultrasonic bath and reassembled with a full rebuild kit. I made sure all the jets were clear and reassembled carefully i.e. Idle and other jets are in all the way, Fuel/air is 2.5 turns out. Here is what I now have:

Pull choke and kick bike over: Bike attempts to rev to infinity and beyond. Mucking about with the choke I can get it to slow down to almost idle speed. If I push the choke in the bike stalls no matter what I do with the throttle.

With bike running (using choke as described above), any application of throttle rewards me with a throaty "Bwaaaaaaa" (you know the sound) and then the Scoot slowly dies.

Any thoughts? Might there be a plugged passage still after the ultrasonic or a bad jet or I need to play with the F/A screw or karma is catching up on me for owning a cafe racer (WHICH I AM SELLING!)?

All ideas will be fully investigated.
Thanks for thinking,
Bud

Ossessionato
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 2234
Location: california
Sat Mar 27, 2021 8:57 pm quote
Quote:
Pull choke and kick bike over: Bike attempts to rev to infinity and beyond.
One of two things likely happening.
1. You have an air leak - carb to air box or airbus to cases is likely culprit. Did you pull the air box when you took the carb out?
2. You have a stuck float needle - and what you are experiencing is lots of fuel being fed.

Start by ensuring your flow is operating and the needle is sealing. You can look down the carb with the top off and see if any fuel is dripping into the throat from the brass feeder tube that is poking into the throat above the slide.

If no - pull the carb again and the air box.
Take the jets out and take off the float chamber top.
Take out the idle mix screw also.
Blow compressed air through the float bowl and also down in to where the jets screw in. Do same where the mixture screw has been removed.

Make sure that the gasket didnt get pinched or torn. Are they new? Make or buy a nice gasket and ensure the air box is well sealed to the case, and same for carb to the airbox.

Re-start and see if that solves.

*edit - closer read: bwaaaa! sound - I now better understand I think. You are fuel starved. Something is clogged. Perhaps take apart jet stack and blow out the individual components as well. Your choke is bypassing the main circuit. Its something in your main circuit most likely.
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5802
Location: So Cal
Sun Mar 28, 2021 7:28 am quote
Classic smallie air leak.

Did you remember the felt ring between the intake and the carb?
Ossessionato
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 2234
Location: california
Sun Mar 28, 2021 9:24 am quote
Quote:
Classic smallie air leak.
Oh.
Smallie.
Oops.
See SoCal's advice.
Ossessionato
VSX, Stella 177, VNX1T, V9B1T
Joined: 20 Jul 2018
Posts: 2961
Location: Staten Island, NY
Sun Mar 28, 2021 9:41 am quote
SoCalGuy wrote:
Classic smallie air leak.

Did you remember the felt ring between the intake and the carb?
I'll be embarking on a carb rebuild of one of these next week.. so this is a great tip to be aware of!

Hope it solves the op's problems. I was with cm22 thinking of was a clogged idle or main circuit, but wasn't sure about it.
Member
'59 T/4 '79 100 Sport
Joined: 27 Mar 2021
Posts: 6
Location: Regina, SK Canada
Sun Mar 28, 2021 5:33 pm quote
'79 Vespa 100 Sport - Carb rebuild fail - THANK YOU!
Those are some great ideas.

It is literally plus 18C today with 10 cm of snow and 100kph winds forecasted for tomorrow. I crap you negative. It looks like I will have plenty of opportunity to tinker in the garage tomorrow night.

Thanks for the great leads. I will report back!

Bud
Member
'59 T/4 '79 100 Sport
Joined: 27 Mar 2021
Posts: 6
Location: Regina, SK Canada
Sun Mar 28, 2021 6:15 pm quote
SoCalGuy the felt ring was in place and I push the carb back into it when I was tightening it all up.
Addicted
'64 Motovespa 150S (177) , '66 Allstate SF, '65 VBB, '66 180SS
Joined: 27 Mar 2013
Posts: 865
Location: S.Salem, NY
Mon Mar 29, 2021 1:12 am quote
" I crap you negative. " ??
Member
PK50XL,PK100S,ET3
Joined: 26 Aug 2020
Posts: 48
Location: UK
Mon Mar 29, 2021 3:35 am quote
It might be worth checking the pilot jet again (there is a hole about halfway up it as well as the one at the end) and also check the pilot circuit as well to make sure its not blocked still.
Ossessionato
Joined: 24 Jan 2008
Posts: 2006
Location: Santa Margarita,Ca.
Mon Mar 29, 2021 7:11 am quote
From experience, the head and intake are prone to loosen over time. If you haven't in the past, I'd suggest giving them a snug up and even replacing the intake gasket. If you omit the felt ring your carb will sit on the intake pipe another 3mm past the slits the air enters from.
Addicted
'79 P200, '74 VBC Super, '80 100 Sport (sold to new home!)
Joined: 23 Jan 2012
Posts: 750
Location: 32 37' 40" N, 117 2' 53" W
Mon Mar 29, 2021 8:54 am quote
my smallie did both of the symptoms above.

air leak -- carb was not connecting to the intake far enough. SoCalGuy showed me that sitting someone heavy on the seat will compress the shock and push the intake forward so as to get more carb clamp space. clamp was cracked and did not hold too tight either I think. replaced that too.

fuel tank was crusty and shyte was clogging the tap. Tap assembly inside the tank was also rotten and so replaced that....all good!

I know its frustrating, especially when it "ran when parked"
Member
'59 T/4 '79 100 Sport
Joined: 27 Mar 2021
Posts: 6
Location: Regina, SK Canada
Tue Apr 06, 2021 7:57 pm quote
Success!
Took the great advice you gents gave me:

1. Took the carb out, disassembled and took an air gun to it all.
2. Put it back together and reinstalled with white lithium grease on the felt ring.

Note that I put my hand over the "throat" tube to the engine, kicked it over and could feel "the proper amount of suction" so I figured the tube itself was sealed well.

Kicked it over and the scoot started with the choke on. WAY easier than before. Then I also dialed the idle speed WAY back as it was somehow way too high (probably made the original issue even "more bad"), then adjusted the A/F screw until it was purring like a kitten. I think greasing up the seal and ensuring that everything was clamped up tight made a huge difference but it was probably a bit of all the above.

THANK YOU.

Now I have to figure out why my '59 T/4 bogs and then backfires when throttled up... Probably another carb issue!

Never a dull moment in Scooterville.








Style Maven
74 50s x3 78 P200 x2 84 Cosa PK50XL2 58 AllState 68 Sprint 80 50special '66(?) Super125
Joined: 22 Nov 2010
Posts: 8147
Location: seattle/athens
Tue Apr 06, 2021 10:54 pm quote
Moto64 wrote:
" I crap you negative. " ??
I know, huh?

So what is a 1959 T4? I'm not familiar with the designation.
Is it a VBA variation? I have a '58 VBA that looks a lot like that.
Ossessionato
1979 P150X, 1983 P200E, 1988 T5, 1995 PX200E, 2011 Yamaha Fazer 600 S2
Joined: 02 Aug 2015
Posts: 2253
Location: Veria, Greece
Wed Apr 07, 2021 9:44 am quote
V oodoo wrote:
So what is a 1959 T4? I'm not familiar with the designation.
German made VNB (Augsburg T4)...

Check out the unique horn...
  DoubleGood Vespa T-Shirts  
Post Reply    Forum -> Not-So-Modern
[ Time: 0.0381s ][ Queries: 27 (0.0196s) ][ Debug on ][ Thing Two ]