Modern Vespa

Still trying to wrap my head around jetting and what the symptoms of those changes feel and sound like. I believe I have jetting dialed in correctly and plug always looks good after running at different throttle positions. There are no flat spots or bogging. I have been riding on a smaller highway 2 mile connector between the main streets I normally ride on, to get used to highway speeds and to see if I am comfortable with the top speeds of my scooter in light traffic on a highway (which I am not). What I've noticed on my stella is that when I am at WOT and max out on speed (50-55mph) in 4th gear on this stretch of highway, my scooter starts to slowly pulse higher rpm lower rpm on about a 1 second pulse up/down cycle but rear wheel never locks up. I don't believe it is a flat spot or bogging from incorrect jetting. Its like the scooter is hitting a wall of max power and when it hits that wall, it goes back down a bit then goes back up and hits it again. This is on about 2 mile stretch of less populated highway, WOT for the whole length. If I back off WOT slightly it stops pulsing RPM and holds steady.

I have temp and rpm gauges on. Scoot never gets over 225F at WOT, granted it is only like 45-50F out. RPMs when in neutral and in 3rd gear can rev up to 6500, but at WOT under load in 4th gear max out around 5500. Everything is stock on the scoot except sip road xl and boyesen reeds.

I don't think this is a soft seize as the rear wheel never locks up and the bike engine doesn't seem to be locking up at all. I want to think that it is fuel starvation at WOT? I replaced the LML stock petcock with a PX petcock from scooterworks, which seems to not have reserve setting, just off and on. Also replaced the fuel line recently. Carb is stock 20/20.

I had a similar issue on my PX, always on the same bit of road, slightly uphill, I thought it was fuel starvation, long un short, in the end I changed my HT lead & suppressor cap and bingo. it must have been arcing through at high revs.

Well, I just replaced the CDI, HT coil and cap under 1000 miles ago as it wouldn't start or idle when I first got this scooter. I'm hesitant to buying all of that again as I think they are working fine now. But your issue sounds a lot like mine. I'll have to try a new coil and cap just to be sure.

A stock Stella with a SIP Road will need to go up a few sizes on the main.

I am sorry I did not clarify, scoot is stock except for road xl, boysen reeds and jetting. I am at 45/140 idle, 140/be1/116 main stack.

A photo of your plug after one of these long runs will clear this question up

What size main jet?

Silly question, but have you tested this stretch of road in both directions or used a GPS to look at elevation changes. A ballistical side note that may not be applicable here, the faster you go, the more you are affected by cross wind until exceeding about 1400 fps.

140 air be1 atomizer 116 main

Last edited by swiss1939 on Thu Feb 28, 2019 8:16 pm; edited 1 time in total

Yes, It was definitely worse in one direction over the other, but still present in both directions. slight incline at opposite ends depending on which direction you are travelling. Also, If I remember, there was slight wind, but it was not an extremely breezy day.

It's been too cold, and family in town visiting the past week, so I have not had a chance to get out there and test further.

Still haven't had a chance to really debug this yet either the weather is too cold on days off or I haven't had enough time before work on warmer days to really run some tests. I did try to use the GSF Dyno app today briefly to see what it would come out with. Not sure if I'm using the app correctly and/or recording the audio clear enough, but I feel like the results of this first usage kindof show the bouncing at wot that happens. It not only happens on 4th gear but also at 3rd gear wot on a flat road (granted it was windy today, I don't think my issue is wind related). Someone else suggested maybe too rich main jet, to try walking it down little by little.


who knows how accurate this first test of dyno app is. This is 3rd gear.

Timing problem ?

Tried to do a full plug chop today quickly. Normally I just run a new plug at full throttle or 1/4 throttle pull it and check it without chopping it. Those times the plug always looks chocolate. This is the image of the chop full throttle plug running 3rd gear straight road for 10 seconds of max rpm. I'm probably doing something wrong with this but here it is. I've been working on fixing a 4T stella automatic at the same time so I have really been distracted on this 2T (doing too much) so I will try to give it more focused attention once I get the 4T out of the way. Needed to get the automatic fixed cause it was in pieces taking up my whole (small) garage.

Turns out I had 114 main jet in, forgot that I changed it from 116/118. Ran some tests today with the suggestion of Lee Lux of @thatscooterthing and came to conclusion I was way too rich on the main jet so it was killing the RPMs at the top end. Went down to 105 main jet and it accelerates fine up to 6500 rpm and maxes out at 58 gps 65 speedo on that stretch of highway. Gonna try to walk the main jet down to 102 or 100 and see if it gets any better. I would have thought being that rich would have created bogging or dead spots in the middle of the range but it ran fine up until wot.

Lee is also suggesting changing the timing from A to IT with this sip road xl pipe.

Can't wait to see if my fuel economy goes way up from current 42 mpg now that im not so rich.

Here is the GSF Dyno report from a 105 main jet. at wot hit some pot holes and manholes so it got bumpy.


13.14 PS / 17.98 Nm / max 6782 RPM

Pardon the question what is the GSF Dyno ap?

No problem. German Scooter Forum Dyno App. http://atom007.heimat.eu/tmt/gsf_dyno.html

Record audio on your phone of a run in any gear then use this app to get a dyno graph from it. You have to modify all these settings in app for gearing ratio, tire circumference, etc.

Having the biggest main jet that runs clean, with a tan coloured plug of the correct temperature rating, will give the most power. It's possible to get SI main jets in 1's now.
Looks like good progress and some healthy power even if you don't fine tune any more.
The power curve does say the exhaust port is too small but that's just by the way.

How can you tell that from the curve? Always interested in understanding more about how all this stuff works.

I will be fine tuning it more using fmp's choke test. I want to get it dialed in to get a good baseline of this stock engine with road xl pipe. I bought a malossi 166 to slap on it the next month or so as my goal is to get top speed up to the point I can drive what feels safe on highway in order to commute to manhattan for work instead of public transportation which i'm sick of. current 58mph straining to stay there is just not safe enough with merging traffic. months later the plan is to crack case and upgrade crankshaft, match cases and basic port work for optimal flow.

I am down to 102 main jet now and it is night and day, with way more acceleration and just feels cleaner and stronger running all over. Still gonna test to see if i need to go down to 101 or 100 main jet cause there is no increase in max rpm or top speed from the aggressively rich main jet I had till now.

Im really curious how much the air mix screw plug affects the main jet sizing if anyone knows? I think that may have been my problem as when I got this bike off my uncle, he had a 105 main jet in. Shortly after when I was brand new to working on them by myself, the ancient plastic screw plug that closes the rear of the air box broke off when I unscrewed it to make adjustments to that air mix screw. The scoot then ran like crap, but I was dealing with figuring out many other issues at the same time (bad CDI and HT Coil/crud in the tank) so I didn't realize that plastic screw plug made a difference being missing.

I finally got those other issues fixed and scoot running relatively well (what I thought was good) by jetted up to 114-118 main jet 140 air corrector, and I feel like it was close to spot on then. Since then, having run this scoot for almost the past year with no plug on that airbox hole in the back.

About a month and a half ago I finally found those plastic screw plugs on SIP and ordered a bunch of them to have some spares. Then when I re-installed the plug, the scoot started this issue of losing rpms at full throttle. I now know that my main jet was way too rich at 116. So I guess my question or statement is can that plug being out really change the main jet 14 sizes to get it running well at either situation?

If you think that plug makes a difference you should check there is nothing under your seat blocking the air intake hole.

Remember when reducing the main jet it will get better and faster at every step, until disaster. A perfectly set up main jet is only two sizes over one that seizes at sustained WOT.

Check your WOT plug colour after a long run. No need to cut it open just look at the insulator.

I think it was just me jetting it way too rich and nothing really to do with that plug, cause I never really ran it WOT much until the first post in this thread a month or so ago. Im mostly driving around 30-40mph on city streets, so I don't think I ever ran it high enough to accurately check the plug for the main jet. Whats strange is the plug looked good to me and wasn't wet when I had it jetted really rich. But I guess it's cause I never pushed it high enough to test the main jet independently. I was so paranoid of seizing it, that I was way too cautiously rich.

I got jet sizes 100-104 today, so was able to bring it down from 105 to check at 103 main jet after a WOT run with a little bit of bogging at full throttle, and it was just damp on the bend of the electrode and threads, but just starting to look good at the tip. So I went down to 102 as originally suggested by Lee @thatscooterthing and that's when it felt like it was a million times better. I'm gonna run another WOT test at that same jetting tomorrow to check plug and see if I need to go down 1 more size. I know its close, but hoping one more size down would be the sweet spot. I will not be thrashing it beyond the plug tests at any sizes smaller than 102 until I feel confident it is jetted correctly.

Btw, this is great information! Appreciate the help and will definitely store this info in my brain right next to fmp's main jet choke test which i used when still on the 105 main jet.

Plug at 102 main jet. Finally starting to see the chocolate and similar look to everyone plugs. I am realizing that what i thought was chocolate before was just black plug. I was misinterpreting it as dark chocolate, when it's more a strongly saturated brown color with some slight red and yellow in it.


Still think it's a little rich at 102

Down to 100 main jet and could maybe go to 99 or 98. It's like a whole new bike. Screams all the way up to Max speed 55-58 now. Max rpm 7600. Smile on my face the whole way.

None to minimal four stroking on throttle changes now. Strong acceleration the whole range of throttle.

Running temps used to be around 180-200f on streets 30-40mph with long wot run up to 225f.

Now it is more normal range of 200-230f on streets 30-40mph. Highest i saw it go for short run wot was 250f. Tomorrow will check a longer run.


101 main jet full throttle

100 main jet plug checked at mid to low throttle. Ran out of time before work today to check plug at full throttle

Plug looking more like it. One issue to deal with before going any smaller on the main jet, is the plug itself. I'm guessing that is a B7 or B8 plug? It's running to cold. Try a B6 plug. Run about 50 miles and check the WOT again.
Running a higher numbered plug when not needed makes the plug darker than it should be. This can make you believe the jetting is rich, when actually it is closer to correct than you think.

Good info! Im assuming that will clear up the darker edges to the plug on the bend of the electrode and the screw base, making it more uniform color throughout, which is why I felt it may still be too rich. I am using BR7ES plug, so I will switch to BR6ES tomorrow and check the plug Monday or Tuesday.