Modern Vespa

I'm currently jetting my VMC 177 Super G. I was running 120/be4/125 50/120 60mm stroke with a 26/26 Si , bellows but no air filter.

Would not splutter revving out in 2nd with a 125 MJ. Tried a 128 and couldn't make it splutter with that either. The 50/120 is too rich for me too, but I'm keeping it for now until some bigger mains arrive. In spite of the idle jet richness, it still pulls nicely down low. I'm surprised the AC 120 128 main didn't make it splutter.

Beautiful bike!

140/BE5/big mj or 160/BE3/big mj.
55/160 ij
Warm it up and play with the mixture screw, starting from 2.5 and then trial and error 1/2 turns.
From there, only change one thing at a time or you'll get lost.
Good luck!

Don't use a pilot jet any smaller than 50. Reduce the main jet until you can ride it but still rich, then get riding for few tank fulls.

*From that jetting. Don't change more than one bit at a time.

I have the same Super G but I have a SI 20/20 carb and a SIP road 2 exhaust so my jetting is most likely going to be too lean for you, but it might help you a little.

I could not get mine jetted at all with the standard carb-mounted air filter. I have a LML filter drilled out and I have removed the blue mesh on the outside, still could never get it to run right. I eventually just left it off and installed a UNI pod filter onto the opening of the airbox in place of the rubber bellows, and it has run fine ever since. If you do not ride near dirt/dust/sand you may be able to leave the bellows on and keep the intake air cooler but be careful as there is no filter in that case.

My current jetting is AC140, BE5 mixer, 130 main, 52/140 pilot, mix screw 2.5 turns counterclockwise (rich)

Thanks all for help!

I put in:
AC 140
BE 5
130 mj
50/140 ij
Mix 2.5 out

And go to work. Still very rich at the bottom but now it more rideable so I done 16 km to work without big problems. Can I try now 55/160 or better do it after few tanks out? Great! I subscribed to you threads and read them few times very carefully.

I also struggled with SIP T5 airfilter and after saw FMP post about it useless I go back to original Piaggio filter.

After way home from work I understood that idle jet still too rich. I can't ride in slow traffic without discomfort. Also lot of smoke and sometimes hard to start move.
I put in 55/160 made a 4-5 km ride and it is feel better. But still on the rich side I sure.
Tomorrow I will know more.

AC 140
BE 5
130 mj
55/160 ij
Mix 2.5 out

Play with the mixture screw.
Don't get caught-up jetting around a fixed 2.5 turns out or you'll go batty.

And when you change the mj you'll have to play with it again
Good luck!

I tried 2 turns out. It seems a little bit better with 50/140. Thats why I decide go to 55/160.

I also noticed strange thing - when scooter stay with working engine and then I sit on on it, the idle speed decreases by 300-500 rpm pretty quickly.

AC 140
BE 5
130 mj
55/160 ij
Tried mix 2 and 2.5 out - not a big difference, still too rich.

High fuel consumption and lot of smoke.
Already done first full tank out. I use 3% mixture according to manual. Do I need use 3% for first 300 km or next tank can go with 2% or 2.5% ?

I already done first 100 km. Was rode very accurately not rev more than 5000, most time 3000-4500.
Here is how spark plug looks.
Can I do some changes in jetting or left all as is and ride for 300 km?

Oil at 2% now is ok.

I would change to AC120/BE3 and use a smaller main jet but one that is still too big. Some higher but short tests in 2nd gear for the main jet should be ok by now.

Try your 125 mj. Baby steps!
The ij feeds off the mj, so that circuit will also lean.

You still have a CHT there, right?

Sensor is mounted into cylinder head. I wondering that temp always keep under 100° C since I started break in. Only one time I saw 102° for very short time.
Is it can be defective sensor? Or I just not give enough load on engine?

Hotter is not faster. A really well balanced engine will run cool. Cruising around without splutter at 100C is good.

100c sounds nice and cool. I did the 2nd gear at WOT test after going through a tank of 1%. It hit 9000rpm. Maybe a bit soon, but it was a short run and it has been through a bunch of heat cycles already. I'm still in the break in period, but sometimes get close to 6000rpm for short periods. It's hard to keep it under 5000.

I understand it for sure thanks Jack. I'm still far from "without splutter"

Also few times tried WOT on 2nd with
AC 140
BE 5
130 mj
55/160 ij
Mix 2 out
Engine revs only to 6000-6200 rpm and then splutter.

Now I'll try to go to AC 120 BE 3 as you mentioned above. Same story mate. Especially that my engine now awake only after 4300-4500 and there is so plenty good punch.

So just tried
AC 120
BE 3
128 mj
55/160 ij
Mix 2 out
Feels richer than AC 140 BE5 130mj. Tried WOT while scoot staying and revs to 6200 then splutter.

Then went to 128 mj and then to 125 mj. Think that stay with 125 for some kilometers.
Tried WOT and it reaches 7400 and then splutter.

While riding temperature is also very similar as it was before.

Still feels too rich on idle jet in my opinion.
Maybe try 48/140 idle jet?
Maybe try 122 main jet?

Better. It is richer than AC140/BE5. Keep reducing the main jet, one at a time, until the WOT splutter stops.
Then we worry about the rest.

Cool, thanks! I now near scooter so I will put 122 in right now. And test it tomorrow.

So I tested 122 mj for several tens of kilometers. Obviously better than 125 but still too rich.

I also marked throttle grip.
For 1/4 is boggy, jerks and rides unevenly. Close to 1/2 and up to near 3/4 is nice and clear with plenty punch of power. From 3/4 to WOT there is no big difference.

So I decide to put in 120 mj. Bit scary but as is. Will test it tomorrow.


With 122 mj a very little brown appears on the spark plug.

When adjusting the main jet. The only thing interesting is what happens at WOT. Anywhere else is not just due to the main jet.

Should have full splutter at WOT, Then reduce the main jet until it really faintly splutters, when held at WOT with max rpm. At which point you can decide, down one more jet, or down two more jets. This is then the main jet size done.

All other issues are then something else.

Yes Jack. I absolutely understand what you mean.
I just try to describe more how all goes as I think maybe it will help to understand better the whole picture.

Anyway I heading to you advices in reducing main jet. I think that 120 will be small enough. Or no? Tomorrow will tell.
I just remember that on my DR main was 125-128 but with AC 160. And I bit scary about 120 or smaller with current cylinder…
What do you think?

Cosa (drilled) filter?
Try your 140 AC again?
Don't want to take you sideways, but it's going to be difficult to dial that 120 AC in. It is richer, as you can see. It also initiates the circuit too early.

By the way, I share your fear riding with a 120 or smaller mj. There's no science behind this, but it does feed all fuel to the engine.

Elephant in the room is what size jets you actually have, vs their stamped numbers. Spaco (SCL), Dellorto (INC), BGM and SIP jets are not the same. It's pretty wild how much the stamped number deviates from its actual size. Without measuring them it's somewhat a roll of the dice.

Promise not to tell anyone, but I don't ride to splutter to find the mj anymore.
CHT response, feel and sound.
I know, I know. Not what you're supposed to do. It works, though.

It's a shame you can't dedicate a few hours with a pouch of bits to try on the road!

I put in a very lean main jet and work up but would never suggest that to anyone who didn't already have at least 40 years experience in jetting 2 strokes. Jetting from obviously rich is safer for the beginner. 122 sounds like its already too lean. When moving the throttle from 3/4 where its already pulling to WOT, it should pull even harder.

Which size main jet gives faint splutter while holding WOT at max rpm?

The whole "find the jet that will make it sputter" sounds like the long way around, but it works.

When I went through the process, I was sure it was overly rich and bogging down…until I went to a much bigger jet. Woke it right up. Once you find that puke rich, runs awful jet, it will be obvious. Then there is only one direction to go. It's really hard to tell otherwise.

You might be holding off on revving it out at full throttle until you put more miles on it. I feel the same way. I jetted mine overly rich, but rideable for breaking in. And taking it easy.

I didn't suggest that

".../Big mj"

Original Piaggio air filter which was on scooter when I got it. I drilled it above the jets. Filter also have no "walls" in front and back.
All main jets that I have is SIP. Idle jets and AC's is Dellorto. Mixer tubes is two Dellorto and two KMT. I think 122 or 125.

If 122 has faint RICH splutter, when held at WOT while maintaining maximum rpm, then it is defiantly one jet size too big (120). If you want city quick and do few high speed runs then, two jets down (118). If you want race track quick and don't care about wear rate, then an even smaller jet.

Once you've decided on your AC/main jet size, then the other positions can be fixed.

All jets are +/- a few % at least. Measuring them on a needle and using a single box set is the best way. Exact to stamped size is irrelevant. Incrementally bigger and smaller to each other matters. KMT jets are the best for accuracy. Dellorto have too many generations and factories to be anywhere close.

Soon I will go to test 120.

Lets assume that I ended with
AC 120
BE 3
122
55/160
Mix 2 out.

What is next steps? Idle jet?

I don't need racing machine and to beat out all sh*t from it. Want to have good reliable engine on which I can easily ride for long distances on the highways or out of town.

I just come back from 40km ride out of town. Gave it some good short blasts and with 120 mj it seems a very bit splutter on WOT. Also now it good when cruising on 1/4 throttle - it is smooth and all jerks dissapeared

I think I will stay with 120 mj at least until I hit 300 km of breaking in.

So it is time to work with idle jet and mixer tube?


This is how spark plug look now.

I'm with you there…setting it up for the same purpose; long rides into the hills to the middle of nowhere. Getting back home again without having to fix stuff on the way are my biggest priorities. I will be keeping it on the rich side too. So far, these kits have given me more top speed than I have had the nerve to try, so no need to squeeze max power out of it.

Yes mate. DR 177 given me 80-85 km/h cruising without problems for a 80-100 km distance.
I'll be happy if VMC will give me more than 85 km/h cruising.
Now I have 22/68 primary and it seems bit short. After break in I will try 23/68.

Go for one jet under splutter once run in. What issues are there now?
Maybe post a sound recording.
Plug is a bit too black. What number is it?

I can't say that there is some major issues. Maybe feel rich on the idle jet.
Spark plug is NGK BR9ES(according to manual) so maybe it is too black because idle jet is too rich? When I was riding yesterday most time I was on 1/4-1/2 throttle.

Now I go to work and will try to notice more details.
And I will record sound in the evening.

Hi there.
Hope everyone is good.
Just figured I'd let you know my experience of trying to tune the vmc. I'm using the cast iron super g on a sprint engine case (casing is matched to cylinder) also the standard sprint airbox and hose set up with the drilled filter.
Bgm big box tourer exhaust, px 12v static ignition. 18°btc

I've tried more than a few jets over the last year. And the best I've found is my current set up. (Maybe a tad rich but not bad fouling like In your pics)

be9 spark plug,
stack is 160ac,be3, 127mj (125 possibly is where I may go down to but I'd rather a touch rich than too lean)
Most importantly 45/150 pilot jet. 55/160 is way too rich and was getting very fouled plugs.

Also, make sure it's very accurately timed at 18° btc

I'm at sea level, mixture screw is 1 1/4 turns out (dellorto coarse pitch)

All the best.

Thanks mate great tip!

When properly set up a 9 plug will run much darker than a 7. Best you can hope for is chocolate brown.

If you get through 3 to 4k with no bog issues the atomiser is not so bad. See how it sounds and re do the pilot jet.

No worries.

Re: the pilot jet. The clear indication was the mixture screw was only crisp one turn away from standard. Swapping that out cleared that issue right away.

I have found that my Polini kit would hit 90kph pretty easily once the main jet was better sorted out running a 22t clutch cog. It didn't feel wound out or like it was revving its nuts off. The VMC seems to have even more oomph though I have kept it at 80 or below for break in.

The roads I have been riding on as well as my tame riding style and crappy stock suspension limit comfortable cruising to about 80. On small 2 lane highways, I find myself going faster. I'm hoping to be able to cruise comfortably at 90-100 with reasonable temps as some of the bigger roads are unavoidable if I want to get from here to there.

Yet it turned out that 120 mj is lean. I rode more and it turned out that temperature is increase faster and higher than on any other jets I try. Easily reached 121º C without any noticeable loading. I also don't like how rpms returns down after blipping the throttle.

I went back to 122 mj and here is video. Still lean as for me. Right?
https://youtube.com/shorts/pUlQItkV3Q4?feature=share


AC 120, BE 3, 120, 55/160, mix 2.5 out.