Modern Vespa

When people talk about revving out cleanly they are usually referring to reaching maximum rpms at wide open throttle.

Try doing the WOT testing in 2nd gear instead of 3rd. Max rpms in 2nd may be higher than max rpms in 3rd. In other words, jetting that revs out in 3rd may still be too rich at WOT in 2nd.

Without experiencing it in person, the "gurgling" you describe is ambiguous. Sounds a little like a lean bog, except that a smaller MJ would not make that go away.

Some questions that may aid us. Which scooter are you talking about? What is the current jetting? Have you made any changes to the gearing? How fast are you going (GPS only, speedometer is worthless for this) when you think you're revving out?

Edit: also, besides jetting, have you made any changes from the stock carburetor?

I'm working on a Stella with a ported DR177, SIP road 3.0, 20/20 carb. No changes to the carb. Stock gearing. Air filter has drilled holes in the cutout. Boyssen reeds.

I'm starting the tuning over with 118-BE4-140 and 45/140 idle. 118 is the largest I've got right now.

Revving out the 118 MJ in 2nd and 3rd:
On the way up both gears feel like heavy four stroking (what I called gurgling) steady machine gun wet-sounding pops. They don't rev easy but eventually clear up as the rpm increases.

Once I get to the max rpm in each gear it feels like skipping forward, a stuttering losses of power. The RPMs can stay up and screaming like this with the stutter but I feel like something is gonna blow up if I hold on for more then a few seconds like this.


Next steps for me, use GPS to calculate where the revs max out so I can share numbers.
Then drill out an old main jet so I know what a obviously rich one feels like.

I'd always suspected my setup was too lean at the top, but I was dialed in well for city travel. Too long I've been scared away from speed runs and highways. Many thanks to folks on this thread and for the wiki additions! Finally going for it.

What does the plug look like?

This is helpful info.

My first reaction is the mid-rpm gurgling is because the BE4 mixer tube is too strong for your setup. I would try a BE5, or even the stock E3, and see what you get.

My second reaction is you don't need to drill out a main jet to see what too rich feels like. It feels like the WOT stuttering you already have. In other words, the AC140/MJ118 combo is already too rich and you need to go up on the air corrector or down on the main jet.

Plug after 30 sec WOT with the 118 main jet.

This was an older plug though. I put a new one in but it needs some time to color up.


Gonna step up the AC to 160 and repeat everything.

I have a better idea of how the engine should behave when you hang at the top of a gear. I feel I'm on the right track. Thanks again!

I agree with all of this. If you have a 160 AC, switch to that first. It'll give you better atomization in the lower throttle ranges. BE4 will still probably be too rich for your setup.

Did you do any porting to the DR, or just bolt it on?

Max rev's on that setup is going to be right at 7,000 RPM's.

Kowalski, I'm going to add your description of revving cleanly to the Wiki Page

shouldn't individual jetting threads happen elsewhere, isn't this a how to in general, not for a specific bike?

^Yeah… seems like it is kinda happening with many of our "topics" just being a catchall.

Agreed...people don't want to start their own thread for whatever reason. Of course, we enable the behavior by answering their questions in the de facto catchall threadd, so it's only semi-their fault.

So not to be a "get off my lawn" old man... but is it worth pushing folks to stay between the lines? IE - Deal of the Day is for "deal of the day", Bodge thread is for bodges, etc.

Thoughts, comments, spitballs?

Yeah, anything about how to jet your particular scooter should be a separate thread.

Agreed. The sub-threads all start as one-off questions that take on a life of their own.

I think that as the Question Answerers, we'll just have to discipline ourselves to direct people to start their own threads going forward for questions specific to their bikes.

I think so. Almost to the point of starting a new topic for them! And this comment should probably be in a new topic also. Sometimes great topics like this one get so thick with other fluff that the point of the thread is hard to grasp.

Solutions? gentle nudges to creating new topics to removing the comments. I'm for it all. And for the record, I'm guilty as the next sometimes in doing this, so reminding me, would work great and I'd gladly start a new topic or delete my post.

Thanks for taking a look at this....

I added a comment to the top of the first post: Who knows. Maybe it'll even help.

Sure Jess could fix it so a new thread automatically starts if a random tries to post on this thread. Probably.

That's a tricky ask for a number of reasons, starting with the fact that there are legitimate reasons a person might want to contribute to this thread, rather than start a new one.

It's also adding thread-level access control, and even limited changes like that are tricky to add, because it's not just the access control logic, but also the ability to manage it, which makes is a much larger and more painful task than it might seem.

I think that cultural pressure combined with some lightweight moderation is the best path forward.

And let's be honest...someone who can't/won't follow the direction of "Please create a separate thread with your question" probably isn't going to follow the advice we give anyway.

Admins have the ability to split a thread but I've not done it in so long I don't recall how much of a pain it is.

Thought I would throw in a quick video of a second gear WOT test. In this example I was getting closer with the main jet, so it wasn't crazy rich at this point. In the video, I was able to rev out to 8600 rpm. I couldn't hear the sputter at first, but Jack could. Listening more carefully, I can hear it breaking up. This was a 135 MJ. 138 sputtered obviously rich. 128 revved out cleanly.

These differences can be hard to hear for newbies or at least it has been for me. I have read everything I can find on jetting, but there's no substitute for experience. Hope this helps someone.

https://youtube.com/shorts/yKURucJ3-UI?si=Z8Tg4WvVaWiNfIOb