Modern Vespa

Hi All,

Was hoping for a bit of advice on this issue i'm having

Scoot is a Vespa Sprint V with a 3 port Pinasco 177, cut crank, SIP road exhaust and a 24mm SI carb

I've been struggling to get the carb tuned for a while. I recently just bought a brand new carb, drilled out the fuel hole and tried to set it up using the great guide on this great NSM wiki https://modernvespa.com/forum/wiki-guide-si-jetting-chart-how-to-tune-an-si-carb

I can get it to idle and rev out on the stand but can't get it to run right while riding.
Once its nice and hot, It will only rev out with the choke on, and it will only idle with the choke off.



Currently set up with 55/160 idle jet and 160 BE6 132 main jet
I've gone as high as a 155 main jet but still need the choke to rev out.

I suspected an air leak but couldn't find any by spraying starter fluid anywhere.

Not sure what else to try. ANy tips would be appreciated!

Thanks for reading

Raph

For starters I would change the air corrector to a 120. That will richen the main jet circuit and recommended for kitted set ups.

How does 0-1/4 throttle feel? Do you have other atomizers and pilot jets?

What is the petcock stock or something else? Is fuel flow good to carb and correct fuel line length no kinks? Fuel cap clear and venting?

If the engine does not rev out with a 155 mj or unless with choke does seem like insufficient fuel.

What type of ignition and timing?

Really needs a pressure test. I would guess a seal is leaking.

Yea i'd say pressure test too. Supers have unique seals that can catch on the crank during installation. Did you remember the gasket under the airbox and then the gasket under the carb?

Thanks for the suggestions. WIll take me a few days to try these out.
I think the 120 air corrector sounds logical but might be covering up the real issue. Will order one anyway.

How does one pressure test? Some kind of pressure gauge into the spark plug threads?

To answer some of the questions:

1/4 throttle was bogging pretty bad. I stepped down to a BE6 and its much better.
BTW the engine revs out fine on the stand, just not when riding it.
Rev from idle is very responsive

Fuel tap is a SIP fastflow, I'll check the line and flow rate first
Ignition is vespatronic. Never had an issue with this setup.

I'll comment on the pressure test. You need to blank off the carb inlet, exhaust and or spark plug and pressurize the combustion chamber. Any air escaping will either make noise or produce bubbles when hit with soapy water. In a perfect world you can have a gauge and watch the psi but quick and simple works too. It either holds air (5 psi) or not.

Going back to the carb, your jetting is close enough to have it run. How many turns out on the screw in the back? What about the gaskets?

These combos in the pic are from a previous thread by my set up is like yours and I used this to dial mine in. That said mine is a 2 port 177cc (3 port with the 3 rd covered for my VNA) definitely go to 120/140 on ac and richer on the idle so you don't ramp seize coming out of wot

Ps
I'd play with your jets before tearing things apart to px test but go there if its impossible to dial in.

Confirm with that you don't have an air leak, with a gauge, first. You will never jet your way out of an air leak. Search for "getting serious about air leaks".

If there is a seal leak, every time the engine is run, some damage will occur.

Finally found some time to have another go.

I started again with the setup c17age suggested and that got me really close.
Only issue I had was hesitation/bogging at 1/4 throttle, and when shifting from 2nd to 3rd at full chat. it would just cut all power, backfire (or maybe sounded like blowback) then keep going.
Changed the mix tube to a BE5 and that mostly got rid of it. Still when the engine isn't fully hot.


Only other small issue I'm still trying to tune out is that it will tend to die when coming to a stop. It otherwise idles fine on its own. Raising the idle speed doesn't seem do help much.
Any tips one this?

Thanks for reading!

You're describing a crankcase air leak. Riding it is doing damage to the expensive bits.

OP: Gonna say it one more time, then you can go ahead and blow it up: Find and fix your air leak.

Serious About Air Leaks

Send it! Parts retailers love this kinda stuff.