Modern Vespa

Hey yall! Now that its cooled down a bit I've been riding my Stella a bit more, and I've been thinking about its temperatures, as well as some quirks that are affecting its rideability. I have a Trailtech cylinder head temperature gauge installed under the sparkplug.

A couple of people here have told me it should almost NEVER creep above 300F, and should ideally stay around 250F all the time. It goes above 300 if I cruise at 55+mph for more than 30 seconds or so. If I am really wringing it out on backroads it might hover around 295-310F. Once I stop at a stop sign or light for a few seconds it quickly cools down below 280.

Main jet 130, BE5 mixer, 140 AC. 52/140 pilot 2.5 turns rich.


I put a new sparkplug in recently and the old one definitely looked a bit rich. I believe this is partially due to me cruising around at the bottom of the powerband where it is a little rich and the pipe is not exactly matched to the cylinder. This is the main quirk that I am thinking of tackling right now.

The VMC 177 + SIP 2.0 revs high but below the powerband it is really flat, and 4-strokes and sputters a lot. A new box exhaust and a MRP intake + Keihin PWK28 have been on my wishlist for a while, but lately I have considered a SI 24/24 as an interim upgrade along with a new box. My choices so far are the Polini box, Malossi power classic, and the SIP road 3. Polini box seems to make the most power and torque and is pretty quiet but last I checked no one sells them locally in America; need to ship from SIP or something which is more expensive. Malossi box is close in performance and is sold locally by scootermercato but I have heard it is pretty loud. SIP 3 is also close in performance and is sold locally but I have heard complaints about its strange shape causing ground clearance issues.

With the MRP intake I would definitely be forced to finally get the dremel out and do some case work to match the cylinder, smooth out the crankcase path, etc., and I don't have the time or courage to do that yet. The sub $100 SI 24/24 is looking pretty promising as an in-between upgrade thatll help me get that little extra oomph to grow into the 23/64 gearing I did.

Thoughts? Any other exhausts I should look into?

I dont think the temps you describe are high. And in my experience with my kit (Pinasco 177), the 24 never dialed in right. Something to do with the reed vs. rotary intake. Someone more knowledgable can pipe in. I get 60+ gps speeds with a 22t clutch and stock Stella gearing, and I weigh 230. I'm not alarmed by those temp readings. Over 7k on my kit now. Pipe is based on riding style IMO. I like different ones for different reasons and types of riding.

CHT is fine; 350 I consider life decisions.

What carb is on there now? Standard factory reed setup?

Re. exhausts. I have compared the Polini box and Malossi box back to back. On my setup at least, the Malossi makes a little more horsepower at high rpm, the Polini makes a little more torque at low rpm. The Polini is easier on the ears. It is not so much a matter of decibels as a matter of tone. The Polini has a deep note while the Malossi has a higher, harsher note. I've taken to using earplugs with the Malossi. There is a US source for the Polini: https://primoscootercompany.com/products/146-62?variant=32979998965900&currency=USD&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&utm_campaign=gs-2020-03-29&utm_source=google&utm_medium=smart_campaign.

Re. jetting. I expect you are too rich across the board. Does the AC140/MJ130 combo rev out in third gear? Second gear? Mixture screw at 2.5 turns is pretty rich for a 52/140 idle jet.

Re: jetting. Is the float bowl drilled to 2.5mm? If not you effectively have a 125 main. Waste if time going 128, 126 etc. if you need to lean the stack as Kowalski suggested. I run 140/BE5, 125 main and a 45/120 (2.66) idle with a SIP Road 3.
Re: exhaust, never had a clearance issue, and I ride a lot on twisty rural 2 lanes. I dont find any of the pipes "too loud" to be honest. No complaints from neighbors yet anyway.

Standard spaco SI 20/20. I have a GGR carbon reed instead of the stock one, installed in the stock LML reed block. Yes it revs out fine in all gears no sputtering. I have done WOT pulls to top speed no issue. Plug appears a little rich though; I've known its a little on the rich side but as I thought my temperatures were still high I was scared to lean out. I might go back to 125 main in that case. Yes it is drilled following FMP's guide. I did it immediately when I got the big bore kit.

Your 5 pts over my main on dr177 with exact same jetting , so if the vmc is significantly higher performance than a DR I'd question your carb config . 125 mj be5 ac140 on 24 si my setup with road 2

If my memory serves me correctly, the DR is on the lower end of the performance spectrum for all the kits; basically just a straight displacement upgrade from stock with similar port timings. I read that the VMC Super G is about on par with a Polini cast iron 177cc in performance. Judging by butt-dyno, dyno sheets of similar setups, and some pulls against my friends' bikes, my setup should be making around 11-13hp.

I ran 125 main for a while but it was definitely on the lean side. Was fine until I started cruising around a bit more at higher speeds, and saw the temps creep past 360f on two occasions. Thankfully no visible damage occurred. I vaguely recall a 160 air corrector led to better lower RPM performance for me, but at the discretion of a few members here I went to 140 AC for safety.

My main jet collection is kinda small for now, I have a 122, 125, 130, and 135.

Just as a point of comparison, I am running the VMC Super G with a long stroke crank, Sip Road 2, Si 26/26 carb, 22t clutch cog, Boyesen reeds on stock Stella reed block.

I had problems with high CHT's at higher speeds, but it's a lot better now. One of the issues was head leaks caused by the fasteners that came with the kit. They have a coating that tends to give false torque readings, so I changed them for normal nuts. Once a leak starts, it tends to burn the o-ring, making it worse.

Still working on jetting, but it's pretty close. 55/100, AC 100, be3, 130. Kind of an odd combo. Even with a 100 AC, the 125 sent CHT's to 350F. 135 was slightly rich, 128-130 feels about right. That's with an Si26. There is some sputter at low throttle when there is no load on the engine, but it's not flat.