Modern Vespa

Greetings from greece!!!!

I recently got my vespa restored and upgraded to polini 177
my specs are:
polini 177 cylinder
polini box exhaust
24/24 carb

the carb seems to be the bane of my existence i cant get the jetting right
its a stock 24-24 carb with 130 main jet all the other jets are stock... since
i got the kit i have tried 112 118 120 122 125 jets but nothing seems to
work out the problems i get are the following:

120 122 125 jets strong power in the start and mid range / hard splutter in higher rpm (the problem probably was the filter it wasnt drilled properly)

130 got a new filter also a new flotter (the carb was new...) but now the bike sometimes doesnt start and shuts down in low rpm after it stutters a little bit also the rpm in 1/4 throttle sometimes climb really slow then it stutter etc...

do i need different mixing tubes and air jets for this settup to work???

for any clarification and if my description is not writen well enough feel free to reply so i can elaborate

Couple of things to think about. Did you do the engine work installing the Polini cylinder? Did you preform a leak test to confirm any air leakage? This will be critical in order to have the engine run properly and be able to dial in the carb. Lots of info to find here how to do a leak test.

Do you know if the carb fuel passage from the float chamber that leads to the main jet gallery has been enlarged? On a new carb that hole is generally around 1.5mm. In many cases on tuned engines this can be insufficient and behaves like a jet or blockage limiting fuel flow needed for main jets in excess of standard 120 main jet size. This passage is
commonly enlarged to 1.8-2.5mm.

Can you post all the jet sizes you currently have installed. All the jets work together in this carb so it important for the experts to know to provide guidance.

Have you done any plug chops? Any pictures of your spark plug?

How does the engine idle and return to idle after a fast run? Where do you have the air fuel mixture setting, how many full and partial turns out from closed? The more the screw turns out enriches the circuit. On a properly jetted carb this will generally be 2-2.5 turns out.

From your post it sounds like it's spluttering beyond mid range which sounds like a rich condition. Could be but let's see what others come up with I think more details are needed to provide better advice.

We have some members from Greece they may be helpful.

Figure out all the jets. The main seems huge for that setup

Greetings! Setup looks correct for a PX motor. 130 main jet in 24/24 is good! Did you confirm ignition timing? 19* is recommended.

If you still have problems, I would look at your motor for air leaks like Hibbert mentioned (cylinder base gasket, crankshaft seals on the flywheel and clutch side).

I had similar issues when I upgraded my PX125 with a BGM177 kit about 3 months ago. I had bought a brand new 24/24 carb, used the recommended BGM jetting and the Vespa would struggle throughout the rev range and refused to idle.

Clearly there can be many factors at play and I cannot say that my issues are the same as your issues, nor do I want to suggest that previous comments are invalid or wrong. I can only say what worked for me.

To prevent myself from going mad, I fitted the previous "newish" 20D carb which had been running perfectly and simply upgraded to PX200 jetting. What do you know, suddenly the scooter is running perfectly! Obviously I am not benefitting from the full power potential of the upgrade but I was never looking for a screamer and it proved that the new carb was the issue and not something else in the engine.

Original 20D settings:
Air Corrector 160, Mixer BE3, Main Jet 102, Idle Jet 48/160

"Upgraded" 20D settings:
Air Corrector 160, Mixer BE3, Main Jet 118, Idle Jet 55/160

I have no doubt that I would have been able to make the 24/24 work eventually, but since the scooter is running really well and I have gained a lot of power and about 15km/h top speed, I am reluctant to fiddle too much with it...

Is this Polini 177 cast iron or aluminum? Plug & play, or did you match the cases, open up the intake and cut the crankshaft?

Stock aside from main jet means AC160/BE3 and 55/160 idle jet. There is ambiguity in the word "stutter," but I agree the MJ130 probably is way too big for your setup, so I infer that stutter in this case describes an over-rich condition. Now that you have the drilled air filter, give the MJ120 another go. Might still be too rich at wide open throttle, but should clean things up a lot at low and midrange rpms.

The mixer and the idle jet may still need fine tuning, but you need to get closer on the AC/MJ combo before you can do that.

Unless you did some other modifications when you installed the Polini (opened the inlet, cut the crank and matched the case transfer ports to the cylinder), there's no way a 24/24 with a 130 main is going to work. It's way too rich.

I'd put in a 140AC and maybe a 116 and work down from there.

So I just installed a cast iron polini on my stella (for clarity its basically a p150 autolube engine with reeds instead of rotary and bigger transfers). My current setup is a SIP Road2, SI 20/20D, P200 airfilter (so with holes above jets). My carb is currently 55/160 160/BE3/120. My main and idle seem good. In between not perfect but rideable.

Now when I was on the stock cyl but same setup above, I ran the polini box and the sip road 2 box. The jets were the same as above save for the main. The Road 2 wanted a 108 and the polini a 102. So if I ran the polini now it would probably want a 116. Hope these data points help. Im not sure how stacks cross over between the si 20 and 24.

https://www.sip-scootershop.com/download/article/1/pdf/a9f5954f-6fff-43fe-9682-38199a5444f4/Tuning+summary+largeframe.pdf?contentType=application-pdf

A handy starting point for jetting is this pdf. Compare setups and fine tune to match your local conditions/ riding style. I definitely bias the richer side as the riding I do is country/fast/ Im fat! Someone who rides in the city/ gentle/ skinny wouldn't need the same jetting as me.

The fact that you have tried every jet from 112 to 130 and still have issues leads me to believe it's not the main jet causing your problems. What air corrector and mixer tube do you have in there now? What is the status of your stator ignition wires to the CDI?

That sip chart looks so useful but engine set up is all about the detail. And that chart hasn't got any.

Very good comment about riding style and weight. Fat people end up with leaner jetting. Out on the open road needs richer jetting. Definitely far from one size fits all. And why jetting on a dyno is almost pointless.

Less than 2 months ago I installed a cast iron Polini 177cc. First of all, this new kit came with absolutely no jetting of any kind. I had to guide myself with my other 3 Malossi kits( cast 166cc, cast 177cc and Aluminum 177cc) which by the way they all came with the whole jetting. The Malossis (all of them) comes with a 115 main jet.

I know there are many variable to cylinder kits flow and other variables to other modifications but in a similar competitor is putting 115 MJ on theirs I would suggest that since you are running a bigger carburetor than stock(24/24) and also a expansion pipe you should run at least no less than 120 MJ.

It is important to differentiate the testing. My Polini was overheating badly when I tested it on 20 miles close loop in the flat desert roads but little signs of overheating in short stop and go, small acceleration burst.

My suggestion is to feed more fuel and work your way down. Anything below 115-116 I think you are leaning it specially if you are already running a bigger carburator and an expansion chamber. is just my humble opinion, I just tested my Polini 177 recently with CHT.


Personally I would put a BE4 and a 125 for now and see how it feels. On the 115-116 it will run but I doubt it will hold up on a road trip at a sustained speed without overheating due to being lean. This is just my opinion off course, anyone can debate and argue with me but I am just saying, I just had a brand spanking new Polini fitted 6 weeks ago.

By the way, I wish you would have gone with Malossi. Not that Polini is a bad brand but the Malossi Cast Iron 177cc kit is much better quality and finish. The polini had an awful cylinder head finish, the combustion chamber looked like sand paper. The exhaust port was too small(29mm). The cylinder kits needed to be cut to fit the flyhweel/starter. The kit came with no settings or gaskets of any kind.

My malossi comes with engine to cylinder case, jetting, cylinder head gaskets and the finish of the cylinder head was simply superior with the CNC mirror finish cut. no cutting of the cylinder fins were necessary. I am just giving you a humble opinion. Polini 177 cc is not a bad kit(this is my second), they are tory as hell but not the same quality finish as the Malossi for the same price. my .2c

The engine was ported and matched to the cylinder the only thing that i didnt oversee is the intake witch i will port and match my self... since my original post i changed the ignition cut the gas tube shorter because it was to long and now the scooter runs great with no hesitation on high rpms . Next thing i will try is the intake as i said and a smaller mj with be4 because i think i can clear the power a little bit more through the rev range for clarification i also use a mazzy crank along with sip fast flow v2 and a little bit lighter t5 lml flywheel

check my reply to hibbert i got the scoot to run better i think it takes a little more tinkering and it will be perfect ( intake matching and leak test)

i got the scoot to run better if this setup doesnt work ill deff try the 20-20 carb setup that you use

i got it to run better through the rev range i think a little bit more tuning in the mixer and idle jets will make it perfect . High revs now dont flood the carb and low revs are great but can take a little more fiddling . Also the cylinder is cast iron ported and matched to the case the crank is mazzy cut and the intake is the only thing i didnt do my self i dont think its properly matched ...its the next thing i will be doing

i got it to work well but i think smaller is the way to go

today i just cut the gas tube since it was to long and i was using the sip fast flow v2 and also changed the cdi ...the scoot seems to run great a little more tinkering in the intake matching , mixer tube and air corrector will make it perfect imo (maybe a smaller mj too)

ill deff try the be4 and the 125 check my reply to hibbert to see what ive done to the scoot today to summarise i cut the gas tube and changed the cdi next thing will be matching properly the intake and try be4

^^ Matching the intake do you mean like opening the carburator box to match the opening of the rotary pad and the carburator itself? yes, that can help also.

You can give it a try. I am using a 20/20 carb on my Sprint Veloce with the LML engine and a Malossi 177 cast Iron and a Malossi exhaust box and it rips. I get it to 80km/h (GPS) in 9.5 sec. Power everywhere. Nothing wrong with trying it.