Modern Vespa

I know it's a big question but....
I've got my p125x running pretty good now after rewiring the stator, new points, adjusting the timing, and using non-stock jets. But i know it should be even better; still got 'low power' (will it die?) when at low revs / moving off from idle. So, i'm thinking the jetting is not quite right yet, but not sure how to tweak it even further.

Carb, engine + exhaust is all stock for a '78 p125x, but the new 20/20 Spaco carb jets were not 'stock' (according to the Haynes).

The Spaco jets were;
starter:60/100, idle:45/140, main:96, air:140, atomizer:BE5

Haynes for a p125x 20/20 carb is;
starter:60/100, idle:48/100, main:98, air:160, atomizer:BE3

My jetting now is at;
starter:60/100, idle:38/120, main:96, air:160, atomizer:BE4

Anyone care to suggest where my jetting should go to from here??

cheers

Dunno if it'll help you but you can find a list of stock jetting for all models here: https://modernvespa.com/forum/post1570651#1570651, 5th chart down.

isnt your idle jet 10 points smaller than that suggested by your haynes manual, meaning its running lean.

Why dont you try lapping the underside of your original carb flat and replacing the spaco out with the old stock carb with its jets and see what that gets you.

thats all ive got to offer

Yep, i've done that

It runs pretty similar to new carb with the old jets in.

It is the idle jet i'm focussing on as it was the idle & low revs that were my problem (not sure if that's the right route though!?).
I'm using a 45-120 right now and it seems to be closer to what i need.
Trial and error, i guess is the only way.

you say its got low power at low revs. But does it tick over ok?

youve swapped out your carbs and got yourself a nice new lead for your plug?

it could be electrical still. Man i dont know.

[quote="TrumpyScooter"] Yep, ticks over nicely. It's only when moving off from idle, that there's a point where the clutch lever is coming out, and the revs can drop off almost to nothing, unless i'm giving it some excess throttle. I'm getting used to giving just a bit more throttle at lights but that drop in revs does not feel right.

I've pretty much checked everything off (timing, fuel line, fuel tap, compression, float, carb) and it's the jetting left that seems to be the solution now.

[quote="Oliver Tamborine"][quote="TrumpyScooter"] Yep, ticks over nicely. It's only when moving off from idle, that there's a point where the clutch lever is coming out..

are you saying the drop is during the process of letting the clutch lever out? if so, that's going to happen.. giving it throttle as you're letting the clutch out is normal.

Yep, but this is separate from the 'normal' effect of the clutch coming out. It was a lot worse, before i swapped out the stock jets from the new carb. Last week the engine would die going from idle to low revs. Different jets have made a noticeable difference, along with rewiring the stator, adjusting the timing, and, putting in a new fuel line. I'll keep playing i'm sure the sweet spot is just around the corner.

Copy that.. if you think you're close.. play with your air mix screw.. 1/8 turns at a time.. 15 seconds before each adjustment.. be patient.. ride it, try again, etc..

try and get it so when you blip the throttle (bike running in neutral) blip the throttle and let it go, and hopefully the motor will come down quickly and smoothly with 2 nice exhaust pops at the end.. (someone told me that many years ago, and when I used SI carbs, if my idle jetting was right it would do this) blip! brrrrrrrrrr pop pop! good luck!

good tip! Thanks

Id suggest the stock jetting as haynes (and in your old carb?) unless youve done other modifications.
Now that youve done all those other things and adjusted that mix screw nicely, with all stock jetting im sure it will run better than ever.

You want the idle to come down quite quickly when you 'blip' the throttle (like, twist it quickly using no more than 1/8th throttle)

Compare the slides in the carbs too. Especially the tiny cut away / notch in the corner. Are they identical? This could also have a similar effect.

The jets in the old carb were definitely not stock. I guess the previous owner had something smart going on

I've got 48/160 jets on back order (no-one has them in stock right now) so i can go 100% what Haynes states. The cylinder and exhaust are all stock p125x.

right now i have;

starter: 60
idle: 45/120
main:160
air:96
atomizer:BE4

it seems pretty zippy at low and mid revs, and i'm reaching 55mph at full throttle. Plus, the spark plug looks good (maybe a little on the lean). So i'm hoping the stock jets *will* feel even better.