Vespa 150 Super (VBC1T) backfires and doesn't start
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Lurker
Vespa 150 Super 74'
Joined: 16 Nov 2019
Posts: 3
Location: Vilnius
Sat Nov 16, 2019 2:54 am quote
Fellas

I need your advice! I have Vespa 150 Super (year 74'). I bought it 3 month ago, it was stated as completely restored. Anyhow after 200km it stopped, it kept starting up but was bogging oh higher revs. Now it is not starting even. I pulled it to my garage and tried to clean the carburetor and check the spark. The spark was working fine, the seller sent the new carburetor (as he thought it might be a problem). After replacing the carburetor the problems remained. I checked the points and cleaned them with sand paper, didn't help. Changed to new spark plug, didn't help.

Please check couple of videos:


Video (choke is pulled out, stops after several seconds)
https://youtu.be/4lhhLeKZLqI

Video (choke is closed, backfiring while kicking)
https://youtu.be/CwzzcRQqk_o

Maybe someone from community allready had such problems, please share an advice.
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Ossessionato
Joined: 22 Oct 2011
Posts: 4078
Location: Tega Cay, SC
Sat Nov 16, 2019 7:36 am quote
First: don't sand points, you will ruin them. Yeah, I did it when I was 15 years
old, when I was desperate, had no money for new ones and filed off the tips.
But the hard coating is on top and when that is gone - thru wear or filing
(sanding), they will wear out much faster.
Second: Check the gap and timing.
Third: Pull the jets, give them a cleani g and let us know the numbers they are.
Fourth: what does the plug look like? Wet and black, I bet. You should not have
to leave the choke engaged for for than 5 to 10 seconds. And what is the
number on the plug, what brand?
Come on back and let us know. Good Luck
Member
P200e
Joined: 29 Jul 2018
Posts: 36
Location: Orange , Ca
Sat Nov 16, 2019 8:11 am quote
Dont forget to check the flywheel woodruff key too !
Member
Joined: 05 Feb 2018
Posts: 42

Sat Nov 16, 2019 10:00 am quote
had this same thing happen on a smallframe. Tried all the stuff suggested, but ended up being the condensor even though it was new... replaced this and it was fine. You may as well do the points and check the woodruff key at the same time
Lurker
Vespa 150 Super 74'
Joined: 16 Nov 2019
Posts: 3
Location: Vilnius
Sat Nov 16, 2019 11:49 am quote
The carb is completely new SI 20/15, I replaced it yesterday with no effect. Sanding the points was an advice from seller, as I'm green in this field I followed an advice. The spark plug is black, picture attached. Anyhow I was driving first 200km on rich petrol (5% mixture of oil) due to engine was rebuilt maybe that made the spark plug black. Attaching points picture.
Probably I need to take off the flywheel to check the gap and timing? Do you have any advice how to check the timing? Is it possible to do with simple tools?
Is there any way to check the condenser or just change it with points without checking?
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IMG_2157.JPG
Spark plug

IMG_2295.JPG
Points

Member
Joined: 05 Feb 2018
Posts: 42

Sat Nov 16, 2019 2:17 pm quote
Maybe dont get into the chasing stuff. One thing at a time.

Im not sure how you check the condensor, but there will be a way. Probably not worth doing though, they are cheap, just do points and condensor and see if that works. Backfiring and a failed condensor go together.

You do need a timing light etc to check timing. Who built your engine? If it was running ok when you got it, and then started backfiring, will probably be ok. When you take flywheel off to do points condensor, check the woodruff key, as if this gone or bad, there will be no timing at all.

I dont reckon you need to run it in at 5% oil. Will definately make your plug look like that. Even worse with you using the choke. Thinking on this, first and easiest check is a brand new spark plug with 2% mix. You may have just fouled a plug!

And with the carb swap, check your jets! See if the same on both and post the sizes here. I dont know what they should be, but lots of people will.

And found this thread which helped me with my condensor diagnosis. Good luck!

Backfire, backfire, backfire, dead
Ossessionato
Joined: 22 Oct 2011
Posts: 4078
Location: Tega Cay, SC
Sat Nov 16, 2019 4:25 pm quote
I'm surprised it ran at all if you were mixing at 5% . It's to be 2% for all rotary pad engines. At 5% it will foul the plug and the gas mix will be off. The engines are fairly simple and you will pick it up. Everyone on here started out where you are now, and if you keep an open mind, you'll never stop learning.
Lurker
Vespa 150 Super 74'
Joined: 16 Nov 2019
Posts: 3
Location: Vilnius
Sat Nov 30, 2019 6:30 am quote
Hi all, thank you for support! I spent hours trying to check everything!
The reason - worn out woodruff key! Right after I changed it, started from the first kick!

Thank you community!

EA99281B-4A91-44F4-ABE8-32CF189F96BE.jpeg

Molto Verboso
1963 VBB2T
Joined: 07 Nov 2012
Posts: 1183

Sat Nov 30, 2019 7:03 am quote
For a rebuilt engine I would think there would have been signs of a worn key when it was put back together.
Is a split wooddruff key a common thing?
Nice to hear you got it going, do you have a pic to share of the ride?
Ossessionato
73 Rally, 74 Rally, 76 ET3, 80 P200, 63 Li125, 65 Li225 Silver Special, 86 Elite 80, 2015 HD Road Glide Special, 2011 Ural Tourist
Joined: 18 Jan 2012
Posts: 3306
Location: Oceanside/ SF
Sat Nov 30, 2019 8:01 am quote
Beside wrote:
Hi all, thank you for support! I spent hours trying to check everything!
The reason - worn out woodruff key! Right after I changed it, started from the first kick!

Thank you community!
Congratulations on finding the problem. Do you have a torque wrench and flywheel holder? You always want your flywheel torqued to spec. Otherwise you will shear the key again and eventually damage the crankshaft.
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