Modern Vespa

Hi, i am having strange issue with choke. Usually when starting from cold choke should be "active" and should be providing more fuel/air to engine. Engine should have higher RPM until warmed up and then choke would close and engine should be at steady idle RPM. My scooter was doing so some time ago. At some point I have noticed RPM would not go up right after startup, so i suspect something happened with choke.
Carb/jets cleaned numerous times. Electric choke thing itself is working- it extends as it should be, also have swapped it to new one, still no luck. Choke jet (also called starter jet) is pressed in and i cannot find where to buy it, so cannot replace it.
Have added bung for lambda sensor and can see that mixture gets slightly richer during cold start, but i still do not get higher RPM.
New carb is bit expensive and i would be more happy to find exact cause of issue i'm having instead of just replacing carb.

Scooter Derbi Boulevard 2008
Engine 125cc LEADER 4t 2v
Carb looks like Keihin CVK 2703A

Anyone experienced same issue?
Should i have higher RPM at cold start?
Any ideas?

Have read numerous articles about CVK carburetors and have found one very interesting statement: pilot fuel screw should be adjusted only when throttle valve is adjusted in such way that it closes transfer ports (transfer ports should not be visible from engine side of carburetor). It is not ok for transfer ports to provide fuel during idle.
So i have tried it and all i could get was way to low idle rpm and there was no way i could get it up to normal ~1600. Best result was with pilot fuel screw 1/2-1 turns out. This should indicate too big pilot jet. Have ordered set of new jets to play with, should receive them in week or two.
I could achieve stable idle with 3/8 turn in of idle adjust screw (thus exposing transfer ports), but then pilot fuel screw makes no big difference for engine RPM like it should.
And back to choke issue, it now makes sense: in order to have stable idle, i have to slightly open throttle valve, this in turn reduces pressure difference between choke circuit intake and outlet, thus air doesn't flow (and doesn't pick fuel on it's way) through choke circuit as it should.

Are you checking transfer ports when adjusting your CVKs?

Did you play with the idle screw?

Yes

I am no expert. I believe that your rpm s should be higher right after you start the bike specially in cold weather. And then go down as the engine warms up. I'm sure someone more knowledgeable will chime in.

Have played with pilot jets, went from 42 to 35, now pilot screw adjustments makes some difference, but still choke circuit is not engaged during warmup.
I have vacuum gauge laying around and i remembered it can be used for carb tuning. Hooked it up and made pilot screw adjustments to have highest vacuum. But now googling around i have noticed that my vacuum reading is way too low, only 7 inHG, while it should be 15-20. Low vacuum might be due to worn rings (will do compression test), air leak (have checked couple times) or wrong valve timing.
Another thing which might indicate issue is low mpg, usually i have 40-50 mpg. 90% of time i ride on full throttle, but still mpg i think is too low.

Can i ask someone who has got vacuum gauge and 125-150cc scooter to check vacuum reading at idle? Or maybe someone already has measured it before or know typical reading?

Now i am totally confused. Valve timing is correct. While i was there, have rechecked valve clearances, both good. Compression is ~9 bar or 130 psi, no air leaks, but vacuum still ~7.

Any suggestions?

Cracked intake manifold? Bad intake gasket?

Just checked, all good. But i found very small leak through blanking plate which is located at carb outside near idle needle and transfer ports. Have added some sealant on top, will test bit later.

Still bad. I think i will have to try new carburetor.

So far i have found couple problems:
Choke, although being new, was not fully closing and was letting some air through. Swapped to old choke and saw little improvement.

TPS (yes, my carburetor has TPS!) might have been out of adjustment, thus ignition was bit retarded. Manual doesn't have instructions how to adjust TPS. Googled around and found several different voltages for TPS at throttle fully closed. For some bikes it is 0.4 volts, for some 0.5, but one article (https://www.apriliaforum.com/forums/showthread.php?232509-HOW-TO-idle-tune-an-old-gen-(Denso-equipped)-bike-with-an-EGA&p=3039937&viewfull=1#post3039937) suggested setting it to 0.6 and it worked best. Hopefully new carburetor will have TPS pre-calibrated.
Maybe someone could measure or knows what TPS voltage should be set for 125cc 2V aircooled Leader engines?

I could use ignition timing light to verify TPS adjustment, but workshop manual is bit confusing. Manual states to check timing like in below picture



but in picture timing marks do not coincide. Should it be something like this at TDC?:

Back to vacuum. Looks like i got lost in measurment units... 7 inHg is same as about 18cmHg, which is not best, but within specs
Still waiting for new carburator to arrive.

Regarding TPS, if anyone will need to adjust it, i have found procedure in Aprilia Sportcity One 125 workshop manual. Sportcity One 125 has same Leader engine.




I was also struggling to find correct wiring diagram for my Derbi Boulevard (same as Piaggio Fly). Workshop manuals all had wrong wiring diagrams, but fortunately Sportcity One Wiring diagram fits