Vespa primmy probs

[redroo]

Good afternoon,

Looking for some help if you can, I’ve got a Vespa primmy 125, 1974.
I was wondering if anyone know why my scooter kicks over fine normally first time, ticks over fine and revs fine until I open the throttle fully, when I do this the engine bogs and struggles to run and even cuts out some times, we have stripped the carb and cleaned it, put new fuel in and checked spark which all seems fine. (Still 6v as tried to keep it original points are new ). Any ideas or suggestions are greatly appreciated.
Thank you for your time.

Ruby.

[Deleted]

Could be a few things from experience....

I always start with undoing the fuel cap, flip it right over and open, see if that helps, tank breather can cause starvation problems.

When it starts to bog when you open the throttle- pull the choke out and see if that helps.

Have you adjusted/moved miture screw or tried adjusting mixture?

[redroo]

Hi, Yep done all of that, I’ve adjusted mixture, cleaned carb at least 5 times/ jets, ticks over perfect, it’s ok if choke is out- (runs how should) only when choke is in it just won’t rev out.. could this be a jets/ carb issue causing starvation? Thanks

[Deleted]

Jul 31, 2021 15:29:02 GMT redroo said:

Hi, Yep done all of that, I’ve adjusted mixture, cleaned carb at least 5 times/ jets, ticks over perfect, it’s ok if choke is out- (runs how should) only when choke is in it just won’t rev out.. could this be a jets/ carb issue causing starvation? Thanks

Well if you've confidently cleaned fhe carb five times, tried adjusting mixture screw I dont think the problem lies there....

If it runs fine with the choke out and you cant adjust up to it I would suspect an air leak....

Has it stood for a while? Last had a rebuild? How long have you had it? Timing hasnt been moved?

[tzsteve]

adjusting the air screw etc only affects the bottom end
do you have too big a main jet fitted?

if it was weak it would tend to run fine for a while get a bit hotter
then
misfire / detonate / pink etc.
then burn a hole in the piston

[jonnysnatchsniffer]

its most likely the fitment between the carb and manifold, its a bit of a shit design, what with the manifold and the shitty ally thing that connects to carb and manifold, and rubber bellow, there is a felt ring that can sometimes come out when you take the carb off, make sure its there and the carb is fully pushed home, if thats all sound then it could be the flyside seal or if its a bit smokey the drive side seal
id say you definately have an air leak somewhere as its ok with choke on, look for oilyness at the bottom of the barrel and also the head and exhaust and the inlet manifold where it bolts to cases
you can do a leak down test with a few home made bits and pieces and a foot pump