Modern Vespa

Ok. So my buddy just bought a 1979 restored P200E. The guy who sold and restored it knows his stuff and put a Sito pipe on it. He did NOT up-jet the bike so no big deal. Obviously the bike needs to be up-jetted from standard idle jet of 116. (I have a 78 P200E with a SIP road 2 and jetted to 122). In any casemy buddy took the bike to a scooter shop in the LA area (don't want to talk bad on) but the guy (at the shop) told me he DOWN jetted to 104?! Of course this goes counter intuitive to having a bigger pipe / more fuel to the carb needs a bigger jet. Right? I dunno the guy says he tried a bunch of different jet sizing's and that it "ran best" at 104. But the bottom line is it starts to bog out after a while in 3 and 4. Ultimately my buddy is taking back to the shop to have this issue resolved but has ANYONE heard of this down jetting business? Just curious to see what others have to say on this (and make sure I'm not loosing my mind or just don't understand the reasoning). Thanks.

nah, you're not losing your mind. that should go UP from a 116 not down to sub 110! I'd be hesitant to even take that back if they're that off track and clueless.

if it's running a 20/20 that's a different story. but still...

anyway, back in the day 118 with a sito used to be the jam here in socal.

Thanks for the reply. I agree with you 100% and yeah I was thinking that this guy has a full shop and I can't understand this. (BTW yeah it's a 24 carb not a 20) so yeah I am baffled.

Bogging could be a hot motor thinking about locking up with a small main jet. Pull enrichment circuit (aka choke) on while riding and see what happens with extra fuel.

I put a Malossi on my P200 with the Sito that came with the motor for a while. I had it running a 120AC/BE3/122MJ stack, so if you're running a 160 AC, then should listen to Greasy and start dialing in your main with a 128 or 130 and then walk it down (order a set of BGM main jets) until it's no longer bogging WOT in 2nd gear. You might be surprised how big that main winds up being.

The only reason a competent mechanic would put a 104 in your bike is so he can sell you a new top end & installation after it seizes.

^^ What he said. They need to earn money on a new top end after it seizes. I don't know how a mechanic will tell you to go lower on fuel when you are actually running an exhaust that will make the engine breath more air. I ran my P200E on a 118 or a 120 main jet.