Modern Vespa

Have you read this?
Wiki [D] Guide: SI Jetting Chart + How to Tune an SI Carb

What carb?

20/20

I read in another post that going low enough on the mj, requires a different AC. Thats something i never saw in any jetting guide, and I assume the opposite is true as well.

I tried a 140, and upped the MJ. Took it for a good wot run. When I stopped at a light, it smelled like burning clutch, and there was a little smoke blowing around, coming from the engine.

I have the same kit. The AC120/MJ118 combo is way too rich. That feeling like you are hitting a rev limiter at WOT is the splutter that confirms it. Try an AC160. That paired with the MJ118 should be pretty close.

Ill try it tomorrow. Im just scared to jump to a 160, since everyone seems to recommend the heavier 120ac, and heavier idle jet. Can I just keep an eye on the temp gauge to avoid seizing?

When I tried the 140ac, it was way peppier on takeoff, but that smoke billowing out after a wot run really worried me. Smelled pretty bad.

Have you checked your timing is accurate and steady at higher RPM's?

Get the bike warmed up, then strobe it at 7-8K RPM's. A dying CDI can cause your symptoms and will be obvious on the strobe. If it's really bad, you try revving it at WOT and it just won't do it.

Once you've confirmed it's not ignition, you can get back to chasing fuel system issues. Next thing I'd check is that you have good, consistent fuel flow, because intermittent starvation is what usually causes the surging your describing.

While you shouldn't need to with a 115 main (which seems lean for a Malossi 177, IMO, but I've only ever run their 210), you might drill the float passage if you haven't already.


Timing is 19BTDC, no cdi. Fuel tap, and line are new, bowl is drilled.

When i first got the kit, i ran 120ac/BE4/118-120, it was always spluttery up to 1/4 throttle, but felt good through the mid range. Never got the chance to sustain WOT since I lived downtown. Now Ive got a longer commute, and plenty of long stretches to open it up.

Timing is 19BTDC, no cdi. Fuel tap, and line are new, bowl is drilled.

When i first got the kit, i ran 120ac/BE4/118-120, it was always spluttery up to 1/4 throttle, but felt good through the mid range. Never got the chance to sustain WOT since I lived downtown. Now Ive got a longer commute, and plenty of long stretches to open it up.

Plug is a BR8ES. Just changed my main to 115. So currently running 120/BE4/115. Cruised right along at WOT, no surging/bogging. Never even hit 250f.

That is the correct plug.

I am surprised the AC120/MJ115 combo is not too rich. Are you doing the WOT splutter test in 2nd gear? Because of wind resistance, you won't reach max rpm in 4th, and possibly not in 3rd either.

Its definitely rich. After I start her up for the first time of the day, It splutters quite a bit though the gears until it warms up. If I remember correctly, I think they give you a 105mj, and a 115mj in the kit. Im guessing the answer is somewhere between the 2.

Edit: Just did a plug chop with 120/be4/115, and brought the plug to the scooter shop. Slightly lean. Went back up to 118.

An AC120/MJ118 is roughly equivalent to an AC140/MJ125. In other words, 10 points higher than the max you should be seeing on the MJ with this kit. If it works, it works, but can't help wondering whether there isn't some extra air sneaking in someplace.